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Offline Len Moskowitz (Core Sound)

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The World's Finest Blumlein Array
« on: July 20, 2020, 04:57:43 PM »
One of the most popular microphone techniques for recording vibrant. precise and accurate stereo images is the Blumlein array.

It's two dipole (figure-8) microphones crossed at 90-degrees and centered with one at +45-degrees, and the other at -45-degrees.

We say that a single Core Sound's OctoMic (a second-order ambisonic microphone) is the world's finest Blumlein array.

What makes it so? The OctoMic's virtual dipole microphones' polar directivity patterns make it so.

Here they are:

 
 

Please compare them to any world class dipole microphone from any manufacturer in the world. You'll see that OctoMic preserves the dipole pattern up to higher frequencies, down to lower frequencies, and with smaller deviations from a perfect dipole.

(Note that these graphs uses a linear scale to show off how consistent the OctoMic response is. Comparing to log scales, 0.5 on these graphs is the same as 6 dB down on the log graphs. 0.25 is 12 dB down.)

Rotate the two patterns 45-degrees in post-production and you have the world's finest Blumlein array.

Hear an OctoMic Blumlein array recording of Jefferson Airplane's song "Somebody to Love" played by Back To The Garden 1969: https://u.pcloud.link/publink/show?code=XZ0coNkZPaeBJB24WkYeXnjPuyTpMXb9PA6V

Hear an OctoMic Blumlein array recording of Stile Antico at NYC's Music Before 1800, recorded by Rob Anderson, here: https://u.pcloud.link/publink/show?code=XZP7yakZXjfMVKMBIAysJHbFhlmWc8NiYQh7

Hear an OctoMic Blumlein array recording of the Allman Brother's "Midnight Rider" played by The Friends of the Brothers here: https://u.pcloud.link/publink/show?code=XZ2yyakZrEzyjXFa2DkdUdsXUgWyPzUyJId7

Learn how to use OctoMic as a Blumlein array here: https://www.core-sound.com/OctoMic/4.php#Blumlein

Learn more about OctoMic here: www.core-sound.com/OctoMic/1.php

See more OctoMic polar directivity patterns here: www.core-sound.com/OctoMic/2.php
« Last Edit: July 22, 2020, 01:10:11 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #1 on: July 21, 2020, 01:28:56 PM »
Very interesting, particularly that 10kHz gets more sensitive towards 90º where most get less sensitive with increasing frequency.  I suppose the 10kHz variation comes down to the head shape and spacing (as in every mic), with a result differing from traditional small figure 8's.  I'm curious how it (or any other) looks even lower, say 50Hz, though pretty much no company show data down in that range.  Not to detract in any way, this looks great.  Hope to try one someday.   
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Re: The World's Finest Blumlein Array
« Reply #2 on: July 21, 2020, 02:56:19 PM »
Noticed the same trend up top.  As I'm sure you are aware, rather than being native the bidirectional pattern (as well as all others) is derived, so there is significantly different stuff going on with respect to the acoustic capsule geometry relationships in comparison to standard single and dual diaphragm fig-8's.

Len, as a TetraMic user (for those following along, that's CoreSound's 1st order, 4 channel ambisonic microphone which proceeded the 2nd order 8ch OctoMic), I'm curious how much the TetraMic bi-directional pattern differs. The TetraMic is capable of a well behaved Blumlein pattern itself. Are the two similar with regards to the behavior of all 1st order patterns? Or is the 2nd order capability able to be used to demonstrably improve upon the 1st order pattern behavior of the TetraMic? 

What most interests me about the OctoMic is potential for deriving directional pickup patterns unavailable to 1st order microphones.  Specifically cardioid-like patterns with a narrow front lobe similar to that of a 1st order figure-8, yet with minimal back lobe sensitivity (and acceptably minimal side lobe behavior).  Can you tell us more about those aspects?

With the TetraMic in typical taper use, I generally found myself gravitating toward dialing in crossed supercardioid to hypercardioid patterns with a somewhat wider included angle rather than Blumlein.  That's probably not surprising to folks here intimately familiar with the use of coincident pair techniques in taper situations. It is in that pattern range where I expect the advantage of a higher order system to offer some interesting possibilities and the potential of significant improvement.


BTW, that Stile Antico recording is very nice.
« Last Edit: July 22, 2020, 05:36:03 PM by Gutbucket »
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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #3 on: July 22, 2020, 12:10:57 PM »
You can see more OctoMic polar patterns here:

www.core-sound.com/OctoMic/2.php

As you can see, while a first-order cardioid is down 6 dB at 90 degrees, a second-order cardioid is down 12 dB. Look at the depth of the nulls at 180 degrees.

And OctoMic's patterns' stability over frequency is phenomenal.

You can get much tighter patterns, if that's what you want. Second-order mics will give you patterns not available from mono mics. You can see some of them on the page noted above.

While TetraMic's polar patterns are probably the best of the first-order microphones - because we calibrate and correct each one of them - they don't compare to OctoMic's.
« Last Edit: July 22, 2020, 01:13:13 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #4 on: July 22, 2020, 06:15:51 PM »
While TetraMic's polar patterns are probably the best of the first-order microphones - because we calibrate and correct each one of them - they don't compare to OctoMic's.

More specifically, I'm especially curious how much difference there is between the two if restricting the comparison to 1st order patterns in the super/hyperish-cardioid range.
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Re: The World's Finest Blumlein Array
« Reply #5 on: July 22, 2020, 09:20:40 PM »
With regards to 2nd order patterns, are there any recordings available using them?  Coincident stereo configurations in particular where the behavior of 2nd order pattern interactions are less than intuitive?

Will be interesting to listen for how 2nd order hypercardioid patterns interact in a coincident stereo pair with regards to the negative-polarity side-lobes and the positive-polarity rear lobe. 


Looking at the plot above it appears a pair of 2nd order hypercardioids with something like a 75 degree inclusive angle may achieve a panning law similar to a crossed pair of fig-8s with a narrow 45 degree inclusive angle, and overlapping out of polarity information outside of it.  A PA/stage width "orchestra angle" of 75 degrees or less is common for audience recording positions, but I'm not sure at all how that would translate or how useful it might be.

This gets me thinking about the possibility of tweaking intermediate pattern shapes between 1st and 2nd order (selecting an optimal balance by ear), is such a thing possible with OctoMic?
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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #6 on: July 23, 2020, 12:08:33 AM »
More specifically, I'm especially curious how much difference there is between the two if restricting the comparison to 1st order patterns in the super/hyperish-cardioid range.

There's a lot of difference. The OctoMic's higher order spatial sampling results in the consistency of the patterns.

You can play with the various polar patterns yourself, at both first-and second-orders. Download some of the B-format files from the OctoMic recording page, and decode them yourself with the SPARTA beamformer decoder.

SPARTA plugins: research.spa.aalto.fi/projects/sparta_vsts/plugins.html
http://research.spa.aalto.fi/projects/sparta_vsts/img/Beamformer_GUI.png

OctoMic recordings: www.core-sound.com/OctoMic/13.php

The SPARTA beamformer has just a few selections, but they're quite useful. There are more interesting and flexible beamformers being developed.
« Last Edit: July 23, 2020, 03:52:35 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #7 on: July 25, 2020, 10:53:11 PM »
With regards to 2nd order patterns, are there any recordings available using them?  Coincident stereo configurations in particular where the behavior of 2nd order pattern interactions are less than intuitive?

Will be interesting to listen for how 2nd order hypercardioid patterns interact in a coincident stereo pair with regards to the negative-polarity side-lobes and the positive-polarity rear lobe. 

Here's an interesting comparison of four different stereo virtual microphone decodes of a recording made with one OctoMic ambisonic microphone. They are all decodes of a recording done with a single OctoMic.

(OctoMic is our second-order ambisonic microphone. Each one is individually calibrated, and can be decoded to multiple virtual microphones having very precise and consistent polar patterns. It can also be decoded to full 360 surround and headtracked binaural )

The recording is of Angelica Women's Chamber Choir (angelicavoices.org) singing "Hic vir despiciens mundum" at Church of  St. John Nepomucene in NYC (May 19, 2019).

The chorus was arranged in a semi-circle. The church has a reverb period of longer than six seconds, and is situated on a very busy (and noisy) NYC street.

The decodes are:

1. Blumlein (two figure-8 microphones pointed at +45 and -45 degrees.

www.core-sound.com/temp/Hic-vir-despiciens-mundum-Blumlein.wav

2. Two second-order hypercardioids pointed at +45 and -45 degrees

www.core-sound.com/temp/Hic-vir-despiciens-mundum-Two-Hypercardioids.wav

3. Three second-order hypercardioids pointed at +45, 0 and -45 degrees, with the center microphone mixed at -3 dB

www.core-sound.com/temp/Hic-vir-despiciens-mundum-Three-Hypercards-to-Stereo-center-3.wav

4. Three second-order hypercardioids pointed at +45, 0 and -45 degrees with the center microphone mixed at +6 dB

www.core-sound.com/temp/Hic-vir-despiciens-mundum-Three-Hypercards-to-Stereo-CENTER+6.wav

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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #8 on: July 26, 2020, 09:04:19 PM »
Here's a recording made with an OctoMic and decoded to Blumlein.

The performers are Stile Antico at Music Before 1800 in NYC.

https://u.pcloud.link/publink/show?code=XZP7yakZXjfMVKMBIAysJHbFhlmWc8NiYQh7

Listen to the incredibly sharp imaging of the choir in their semi-circle.

Thanks to Rob Anderson for the recording, and to all involved for permission to post this recording. Photo courtesy of The NY Times.
« Last Edit: July 26, 2020, 09:06:24 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #9 on: July 27, 2020, 09:59:45 AM »
Thanks for the links to these samples.
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Offline voltronic

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Re: The World's Finest Blumlein Array
« Reply #10 on: July 27, 2020, 05:37:03 PM »
Very nice samples, Len.  I'm a choir director, and this music could not be more in my wheelhouse.  I also love Stile Antico - own several of their albums.

The most convincing for me was the straight-up Blumlein of the Anglica Women's Choir, as it captured the acoustic the best while still giving very good balance to the choir.  That's really impressive - the best Blumlein recordings I have heard usually use single-diaphragm fig8s like MKH 30 or MK 8, and there are some that it's verboten to do it any other way (I'm not one of them).

Second place goes to the two second-order hypers.

For a recording such as this, I always like to try spaced omnis 40-60 cm, but that's the one thing a single ambisonic mic cannot give me, I suppose!  ;)


For the Stile Antico selection: was this room extremely dry and/or with a low ceiling?  The imaging is indeed pinpoint, but sounds like X/Y cardioids in that I am not hearing any room reflections (if there were any to be had).

Thanks for sharing this.
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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #11 on: July 27, 2020, 07:55:23 PM »
Stile Antico was recorded at Corpus Christi Church in NYC.

You can see photos of the sanctuary here:

https://www.corpus-christi-nyc.org

You're hearing the room, but it doesn't add much. It's a medium-sized colonial-style wood church, not a large stone church like St. John Nepomucene. The ground floor pews were full. The balconies are on three sides - they were empty. The room is pretty much a cube with the chancel on the north side of the room.

I probably have a recording of a loud hand clap in the empty hall somewhere. Of course, the acoustics were much dampened by the audience.


« Last Edit: July 28, 2020, 02:53:17 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #12 on: July 28, 2020, 10:38:26 AM »
Thanks again.  I had a feeling it was just a dry acoustic.


Regarding the two omni samples:

These are the least successful to my ears.  I admit I don't really have the first idea of what goes on with Harpex, but on the Angelica example, the image width has collapsed and become much more distant, as though a directional stereo array were rotated around so the rear of the mics were facing the choir.  In the Stile Antico example, the L/R placement of the choir sections has been flipped and there is also a persistent buzz not present before, so I wonder if something went amiss in the decoding of that track.  I hope you don't mind the criticism; I mean it to be constructive, but I don't think those two examples are indicative of the performance of your mic.


I continue to be very impressed with the other examples, especially the Blumlein decodes of both choirs.
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Re: The World's Finest Blumlein Array
« Reply #13 on: July 28, 2020, 11:53:54 AM »
Looking forward to listening to all of these.

Volt- I've been long curious about Harpex's virtual patterns and stereo output derivations but have never gotten around to demoing it using any of my own recordings.  Especially curious about it's synthesized near-spaced and A-B configurations, which I presume use the plane wave decomposition function Len mentions, yet I remain skeptical about how well real-world phase/spacing interactions can be simulated using sample data from only a single point in space.
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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #14 on: July 28, 2020, 02:52:22 PM »
I too am not very happy with Harpex's synthesized spaced mic decodes. It does a good job with first-order recordings that are not complex, though if I need a parametric decoder, I use COMPASS.

I've removed those two recording links.
« Last Edit: July 28, 2020, 02:55:29 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #15 on: July 28, 2020, 03:24:29 PM »
Looking forward to listening to all of these.

Volt- I've been long curious about Harpex's virtual patterns and stereo output derivations but have never gotten around to demoing it using any of my own recordings.  Especially curious about it's synthesized near-spaced and A-B configurations, which I presume use the plane wave decomposition function Len mentions, yet I remain skeptical about how well real-world phase/spacing interactions can be simulated using sample data from only a single point in space.

The bold part is what has me baffled.  I don't understand how you can take a coincident mic array where there are effectively no time-arrival differences between mics, and make it sound like it was actually in two physical spaces without altering the sound significantly due to new phase interactions between the individual tracks.  If it can be done convincingly, I will happily eat my words.

For now, it is pretty incredible what can be done with post-processing of ambisonic and other coincident arrays.
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Re: The World's Finest Blumlein Array
« Reply #16 on: July 31, 2020, 10:57:51 AM »
Len, thanks again for the links to the various virtual microphone renderings of the Angelica Women's Chamber Choir recording.  They are very helpful.

It's very interesting to hear those including a center microphone at different levels, as a stronger center emphasis is something I frequently find myself desiring in Blumlein recordings (some, not all).  Generally I've achieved this myself by setting up for a recording using a somewhat more narrow inclusive angle than 90 degrees, or by making a Mid/Side ratio adjustment afterward, both of which increase center level at the loss of some overall width.  The use of second order patterns may allow one to set things up so at to retain a bit more width.  Note to those listening: I find the biggest difference to be the difference in reverberant balance between the Blumlein and all three variants using the 2nd order hypercardioids, which is to be expected as all three hypercardioid variants share less collective sensitivity to the rear.  The difference in level through the center I find to be the second most noticeable difference, and the difference in width impression somewhat more subtle.

If I were searching for the most optimal stereo output for that recording, I'd probably first try Blumlein along with some center-pointed 2nd order hypercardioid added to it, and compare that against the three 2nd order hypers with the inclusion of a rear-facing pair.. both in search of an optimal reverberant balance without overly compromising the other aspects.

The reason I'm posting about this is that I'm realizing it reflects what I've come to recognize as the core aspect of my own approach to live music taping, which is gaining a useful degree of increased control over the directional sensitivity of the recording array in an overall sense.  Not simply at the level of single microphones and stereo-pairs but across the entire horizontal plane.  Blumlein is fantastic when the situation allows for it to work well, but as tapers we are rarely presented with the perfect situation for using it.  I find it is far more useful to have a useful ability to adjust and balance the contribution from each direction individually, collectively producing an overall balance which is otherwise unachievable.

This ability to optimally balance the contribution from all directions is incredibly valuable in the odd world of concert taping where the freedoms of the recordist are so greatly constrained, and it is this aspect in particular I want to emphasize to other tapers as being of special interest with regards to OctoMic.. even more than its especially well behaved Blumlein capability.  It is what I see as the biggest difference between the situations in which tapers find themselves in comparison with most every other form of music recording.

I currently achieve such functionality by either using four baffled omnis facing in the cardinal directions, or five supercardioids (center one a M/S pair) in combination with two wide omnis.  A good ambisonic coincident mic offers similar horizontal plane directional balance flexibility, and a higher order one has potential for increased performance that may serve to offset the absence of spacing between elements.  Although not as daunting as once was, running eight channels is still a lot for tapers, yet a number of tapers do that now.  I do with far greater setup complexity than is required for OctoMic.

This post is an strong endorsement of the value of multi-directional pickup in general and OctoMic represents a compact, simple to setup, single unit way of achieving that. I credit TetraMic with being one of the especially useful tools which helped me develop a deep, intuitive understanding of coincident mic'ing, and helped further the deep recognition of what I now think of as a fundamentally important aspect of control over differentiated pickup across all directions.

Len, big thanks for developing these tools and making them available to the audio community.
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #17 on: July 31, 2020, 01:14:53 PM »
You're very welcome.

> If I were searching for the most optimal stereo output for that recording, I'd probably first try Blumlein along with some center-pointed 2nd order hypercardioid added to it, and compare that against the three 2nd order hypers with the inclusion of a rear-facing pair.. both in search of an optimal reverberant balance without overly compromising the other aspects.

With OctoMic and the SPARTA beamformer plug-in, that's ridiculously easy. And all done in post.

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Re: The World's Finest Blumlein Array
« Reply #18 on: August 02, 2020, 02:34:16 PM »
Thank you all for very interesting posts. I know very little about ambisonic recording. And I would like to ask a few questions.

Please, is it possible to decompose a directional pattern of the 1st order to an omni and a fig. eight component? For example:
1st order Cardiod =  0.5 x (1st order omni + 1st order fig. eight)?

Directional patterns of 2nd order look complicated. Is there an easy way to decompose it to the components?

I was playing a little bit with Len's recording and Sparta plugin. There is an audible difference between the 1st and 2nd order. I also tested the 2nd order beams at +-45. When I changed the beam type from Card to HyperCard, I got a wider stereo image, a less reverberant enviroment, but also less low frequencies. I did not notice this decrease in low frequencies in the 1st order when switching between cards and hyper. I don't know if I setup the plugin properly. I would like to ask if an ambisonic mic generally behaves as an ordinary mic - the more directional pattern, the less sensitivity for bass? Has an ambisonic mic a proximity effect when recording close to a source and decoding to the directional patterns?

And the last question. I'm curious about the artificial simulation of a spacing between the microphones. Len is sharing the recordings of his ambisonic mic and a pair of a regular mics in ortf. I wanted to generate ortf from Len's ambisonic mic and compare it with regular pair. Len is recommending Compass plugin. Unfortunately, I was not able to set it up correctly. Has anyone done it? (Ha ha, if this works, it would completely change our hobby. Maybe we will be pleasantly surprised one day)
« Last Edit: August 02, 2020, 02:41:12 PM by kuba e »

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Re: The World's Finest Blumlein Array
« Reply #19 on: August 02, 2020, 06:31:37 PM »
Please, is it possible to decompose a directional pattern of the 1st order to an omni and a fig. eight component? For example:
1st order Cardiod =  0.5 x (1st order omni + 1st order fig. eight)?

Yes, very easily.

Three of the four first-order B-format channels (X, Y and Z) are figure-8s. One of them (W) is an omni.

And with a beamformer plugin, you can create any combination of them and output any first-order microphone directivity pattern, including sub-cardioid, cardioid, super-cardioid and hyper-cardioid. As many you want, in any combination. Pointed in any directions that you want. Simultaneously.

And all of them are equal to or better than any of the world-class mono mics.

Quote
Directional patterns of 2nd order look complicated. Is there an easy way to decompose it to the components?

They are not decomposable - they are the second-order spherical harmonics, just as the first-order channels are the first-order spherical harmonics.

The second-order B-format channels are combinable with each other and with the first order channels to produce patterns like the second-order cardioid and hyper-cardioid directivity patterns you can see on our web site: www.core-sound.com/OctoMic/2.php

And there are many more, including ones with very, very low side- and back-lobe levels, and very tight directivity.

Quote
I would like to ask if an ambisonic mic generally behaves as an ordinary mic - the more directional pattern, the less sensitivity for bass?

No. What you're hearing may be an artifact of the plugin. Or an interaction of the room's bass response and the directivity pattern.

OctoMic goes down to around 30 Hz @ -2 dB. That's for everything OctoMic records.

Quote
Has an ambisonic mic a proximity effect when recording close to a source and decoding to the directional patterns?

There's no proximity effect if the target directivity is a B-format channel. There will be a proximity effect if the target is a combination of the channels. In that way it's pretty much the same as mono mics.

Quote
And the last question. I'm curious about the artificial simulation of a spacing between the microphones. Len is sharing the recordings of his ambisonic mic and a pair of a regular mics in ortf. I wanted to generate ortf from Len's ambisonic mic and compare it with regular pair. Len is recommending Compass plugin. Unfortunately, I was not able to set it up correctly. Has anyone done it? (Ha ha, if this works, it would completely change our hobby. Maybe we will be pleasantly surprised one day)

Spaced arrays with first-order directivity patterns can be decoded from a single ambisonic mic - not very well with a first-order ambisonic mic, and somewhat better with the first-order components of a high-quality second-order ambisonic mic. The process uses plane wave decomposition, which - as I understand it - works only at first-order.

The Harpex plug-in provides for it, but I'm not satisfied with its results.

That said, it's easy to do all of the ORTF arrays (and other spaced arrays) with OctoMic. In fact, you get the world's best behaved (and IMO sounding) ORTF, ORTF-surround and ORTF-3D arrays. How? Use multiple OctoMics.

For the ORTF-surround and -3D arrays each of the OctoMics plays the role of between two and six mono mics. All of the pointing angles are set in post. And the OctoMics give better results than the mono mics.

See the photos below.
« Last Edit: August 02, 2020, 10:37:53 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #20 on: August 03, 2020, 06:04:49 AM »
Len, thank you very much for the explanation.

It is great that your mic goes around 30 Hz, -2dB. I am also starting to understand that when we are decoding from B-format to virtual polar pattern, there is no proximity effect. (I imagine that the virtual directional pattern is based on the differences of all microphones and therefore the microphone has no proximity effect and has a flat characteristic up to 30 Hz.) - this is not correct

Please, I have one more question about decomposing virtual directional pattern. Sparta beamformer plugin can make only Card and HyperCard pattern. So for example, If I make two 2nd order Cards at 90° and -90° and subtract them, do I get 2nd order Fig 8?

Yes, multiple OctoMics must give a great freedom when recording. It is a lot of channels for hobby tapers now. But this can change in a near future. I can imagine that one OctoMic will be connected with one connector and we will be able to record several of them with simple multi channel recorder. And one OctoMic would be also great in a Gutbucket's Oddball mics array. It's also nice for me that with OctoMic it's easy to learn the basics of recording. With one click, you can change the pattern or angle and hear the result immediately.

I appreciate how much work you've done in developing Ambisonic mics. Making development in a multinational company is easier in terms of money, time and capacity. You are a smaller company and it is even more valuable.
« Last Edit: August 06, 2020, 02:12:57 PM by kuba e »

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Re: The World's Finest Blumlein Array
« Reply #21 on: August 04, 2020, 04:07:08 AM »
I noticed now that the polar patterns are only up to 10kHz. I found in the specification that a calibrated OctoMic works in the range of 30Hz to 18kHz + -2dB. Just out of curiosity, what's going on above 10 kHz?

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Re: The World's Finest Blumlein Array
« Reply #22 on: August 04, 2020, 11:20:55 AM »
Still a very flat frequency response on-axis, but the patterns are not as clean.
« Last Edit: August 05, 2020, 05:25:23 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #23 on: August 04, 2020, 02:49:19 PM »
It is great that your mic goes around 30 Hz, -2dB. I am also starting to understand that when we are decoding from B-format to virtual polar pattern, there is no proximity effect. (I imagine that the virtual directional pattern is based on the differences of all microphones and therefore the microphone has no proximity effect and has a flat characteristic up to 30 Hz.)

Close, but not quite correct.

When using the native B-format patterns, there is no proximity effect.

Patterns that derive from the B-format patterns - like cardioid, derived from omni and dipole - will have a proximity effect.
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Re: The World's Finest Blumlein Array
« Reply #24 on: August 05, 2020, 07:56:19 AM »
Thank you Len for the answers, it is nice from you. I like the theory about recording a lot. But I am hobby taper so some of my questions may not have been relevant.

The proximity effect of virtual patterns is not easy to understand intuitively. I have to read more about ambisonic mics.

But I guess I'm starting to understand the ambisonic mic frequency response and polar patterns. The ambisonic mics have very tight polar patterns up to 10 kHz. So the frequency response up to 10 kHz is measured on the microphone axis in a free field. Above 10 kHz, the comb filtering is starting to be dominant (because there is a distance between capsules) and the polar patterns are becoming beam polar patterns. And for this reason, the frequency response above 10 kHz is measured only in a diffusion field.
« Last Edit: August 05, 2020, 07:58:38 AM by kuba e »

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Re: The World's Finest Blumlein Array
« Reply #25 on: August 05, 2020, 10:02:18 AM »
Len, hi. I just don't see how any pressure gradient transducer can fail to have proximity effect. Once the capsules have delivered their signals, with components from distant and close-up sources combined, how can any form of processing separate them out again?

Actually as I'm sure you know, it's the shapes of the wavefronts that matter; a plane wave won't provoke a bass boost if you manage to generate it an inch away from the capsule somehow. But any curved wavefront--such as one that emanates from a point-ish source in the near-ish field of a pressure gradient capsule--will be "read" by that capsule like a plane wave with a bunch more low-frequency energy in it. And once the cream has already been stirred into the coffee, [simplification here, but both experientially and experimentally valid:] no amount of further stirring will separate it back out.

--best regards

edited later to add: The same thing applies to breath noise (popping) and handling noise/solid-borne sound. If there's any way to process those things out of a microphone's signal without affecting the rest of the program material, I'll carry you on my own aging shoulders down to the Patent Office.
« Last Edit: August 05, 2020, 10:10:18 AM by DSatz »
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Re: The World's Finest Blumlein Array
« Reply #26 on: August 05, 2020, 11:07:09 AM »
Thanks D, was going to question that myself.  Kuba e, the proximity effect of virtual patterns shouldn't be overly mysterious in practice, having the same behavior as non-virtual patterns. The mathmatics of basic ambisonics assumes plane waves, but real world microphones used for ambisonics do have proximity effect, and it is adapted for in various ways as necessary in playback decoding.

As for high frequency pattern behavior, the degree to which single-point coincidence is achievable in real-world ambisonic microphones is determined by the size of the microphone elements and how closely they can be positioned to each other while maintaining the appropriate geometric arrangement.  The closer together they can be positioned, the higher the frequency the virtual polar patterns can be maintained, resulting from various combinations of  the individual microphone elements. Above a certain frequency determined by the relationship between the wavelength at that frequency and spacing between microphone elements, spatial aliasing begins to occur.  The patterns start to loose their smooth curves and intended shape and grow increasingly "spikey" in terms of small scale deviations resulting from adjacent regions of constructive and destructive interference (forming 3-dimensional comb-filter moire patterns) while the average spatial sensitivity tends toward omnidirectional overall.  The pattern increasingly degrades, while the overall response may or may not.

Such behavior is unavoidable but can be made non-problematic for the intended applications of the microphone by appropriate design. As long as that spatial aliasing transition can be pushed high enough in frequency, it doesn't pose an audible problem for music recording intended for human listeners.  No traditional ambisonic microphone is appropriate for recording ultrasonic sources such as bats and insects, etc.

This is the same acoustic effect one can sometimes hear at somewhat lower frequencies where it is audible and objectionable in some older and less well aligned PA setups, particularly earlier less well-designed line array PAs, when the sound gets audibly "swishy" with rapid regions of response variation as one listens while walking around in relation to the speaker array, while not being especially noticeable if standing still or sitting.  In better aligned and well-designed systems the effect is similarly pushed high enough in frequency to become unnoticeable, yet still occurs at frequencies above general human perception.
« Last Edit: August 05, 2020, 11:08:52 AM by Gutbucket »
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Re: The World's Finest Blumlein Array
« Reply #27 on: August 05, 2020, 12:00:54 PM »
It's definitely counterintuitive, but according to our DSP gurus, it's really so.

Take, for example, the B-format W-channel. It's omni, and for OctoMic it's a near-perfect omni across frequency.

It's really, truly an omni and has no proximity effect, despite the capsules in the array being cardioids.

I just ran a quick & dirty test that confirmed it.
« Last Edit: August 05, 2020, 12:41:29 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #28 on: August 05, 2020, 01:19:12 PM »
I think that would be expected given the pressure omni component in each of the A-format cardioid capsules which are collectively summed to produce the W-channel, while the bi-directional component of each A-format cardioid in which proximity effect would be expected to manifest is differentially cancelled out.

Can you similarly test X, Y or Z channel?


This would seem like it might be related to Near-Field / Distance Compensation in ambisonic decoding schemes, so I did a quick search and found the paper linked below. The last lines in the foot note quoted below about a properly aligned ambisonic microphone "encoding distance by virtue of accurate transduction of the incident wavefronts" may be indicative of the behavior Len is describing.

From: https://pdfs.semanticscholar.org/e101/9d172e9263296925f6b528959bfa1236ad4c.pdf

Excerpt of note from that paper:
"The exact decoder matrix recreates the pressure and velocity at the central position under the assumption that the wavefronts are planar, i.e., sources and loudspeakers at an infinite distance. Sources and loudspeakers at finite distances produce wavefronts with a “reactive” (or imaginary) component, which is perpendicular to the direction of propagation, in addition to the “real” component, which is parallel to the direction of propagation. This results in the wellknown bass-boosting proximity effect in directional microphones. It is important to understand that this is an actual physical effect, not a design flaw in the microphone or loudspeaker.4"

footnote 4-
"The implication for B-format signal encoding is that the X, Y, and Z signals must have a low-frequency boost and phase shift relative to the W signal, the amount of which is a function of the source distance. For natural acoustic sources, a properly aligned Soundfield-type microphone does this by virtue of accurate transduction of the incident wavefronts, and thereby encodes distance. For synthetic sources, this must be included in the encoding equations. Further discussion of this is in AppendixB."
« Last Edit: August 05, 2020, 01:22:26 PM by Gutbucket »
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Re: The World's Finest Blumlein Array
« Reply #29 on: August 05, 2020, 01:27:49 PM »
Len, when you synthesize an omni pattern, it's no surprise that proximity effect will be minimal. The same is true of conventional, dual-diaphragm studio microphones when set to "omni" even though their output is then the sum of two back-to-back cardioids. Each cardioid is a ~50/50 mix of pressure and pressure gradient, and in the omni setting the pressure gradient components are summed in opposite polarity, so they mostly cancel. Not completely, though, because they're not at the same point in space. So even in the omni setting there's still some small amount of proximity effect which is measurable--but of course the lower you go in frequency, the smaller the fraction of a wavelength that's represented by the distance between the two "half-capsules."

Would you please try the same experiment with the Blumlein pair that this thread is about and let us know what you hear and see?

--best regards
« Last Edit: August 05, 2020, 01:34:54 PM by DSatz »
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Re: The World's Finest Blumlein Array
« Reply #30 on: August 05, 2020, 02:14:07 PM »
Ambisonics doesn't act the way you'd intuitively think that a group of mono mics, combined in a simple way, would work.

And distance compensation is something very different than proximity effect.

I'll run a recording test on a dipole channel later today and see if the result conforms to the theory.
« Last Edit: August 05, 2020, 02:16:58 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #31 on: August 05, 2020, 03:44:35 PM »
Nice! I love Taperssection. I can't explain to myself how it is possible that ambisonic fig. 8 is better at low frequencies than ordinary fig. 8. That's why I also asked about the proximity effect. I thought it would help me with a better understanding.

I know (but I'm not 100% sure) that SN ratio is improved when using multiple capsules. Is it also possible to improve bass response and bass polar pattern of the virtual microphone to be better than parameters of a real capsule (it's pressure and pressure gradient components)?

I will not interrupt the discussion and, in the end, I will delete my erroneous considerations.
« Last Edit: August 05, 2020, 03:50:52 PM by kuba e »

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Re: The World's Finest Blumlein Array
« Reply #32 on: August 05, 2020, 05:21:32 PM »
I can't explain to myself how it is possible that ambisonic fig. 8 is better at low frequencies than ordinary fig. 8.

A higher order ambisonic mic that's properly calibrated characterizes everything going on acoustically from the vantage point of the center point of the array. If it's accurate and has sufficient spatial resolution, essentially any directivity pattern can be accurately derived. If the calibration is precise, the low frequency response can be corrected down pretty much as far as you'd want to go. We limit it to 30 Hz so as not to have it be an earthquakle detector.

In comparison, an ambisonic mic that is not calibrated (or generically calibrated) typically only goes down to between 70 and 90 Hz.

Quote
I know (but I'm not 100% sure) that SN ratio is improved when using multiple capsules.

Every time the number of capsules is doubled, the output goes up 6 dB while the noise only goes up 3 dB. So the noise spec improves 3 dB. That's true for an omni decode. (It varies with the decode.)

Quote
Is it also possible to improve bass response and bass polar pattern of the virtual microphone to be better than parameters of a real capsule (it's pressure and pressure gradient components)?

Yes. Please compare OctoMic's specs and polar directivity graphs with the world's finest mono mics. You'll see that in almost all cases it betters the mono mics.
« Last Edit: August 05, 2020, 05:30:52 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #33 on: August 05, 2020, 10:32:20 PM »
Just for reference, there's no particular inherent limit to how good (= how close to a cosine function at all frequencies within the given range) the pattern of a conventional figure-8 microphone can be, particularly with single-diaphragm construction. Even at low frequencies where such figure-8s roll off sooner than I personally would like, that rolloff is an even and predictable 6 dB/octave, and the polar pattern is unaffected by the rolloff.

Thus you can easily apply a compensating boost and get response that's as flat as you want down to (in principle) any frequency that you choose, while obtaining a polar pattern "down there" that's as accurate as you please. In other words, this particular strong point of an Ambisonics system is already the strong point of any well-designed condenser figure-8. In no way does that detract from Ambisonics systems--but it's curious territory to be staking a claim of superiority in, is all.
« Last Edit: August 05, 2020, 11:06:12 PM by DSatz »
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Re: The World's Finest Blumlein Array
« Reply #34 on: August 06, 2020, 07:39:57 AM »
Thank you Len and David, now all is clear for me. The bass response is not improved by ambisonic principle but by eq. And Ambisonic can change/rotate polar patterns in space, but not to improve them (make them tighter or smoother). So when Len measured great microphone values, it means he uses great cardiod caps and also the capsules are well matched and eq with each other.

I am sorry for so many questions. I am now remembering the basic lesson from Taperssection that it is better to listen than to think too much.

(Thanks to this thread, I understood now how ambisonic works - it is the same as Mid/Side manipulation of coincident pair of mics, but this is in 3d space and with the possibility of more combinations)
« Last Edit: August 06, 2020, 10:12:04 AM by kuba e »

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Re: The World's Finest Blumlein Array
« Reply #35 on: August 06, 2020, 10:02:36 AM »
It is the intermediate patterns that range between figure of 8 and omni where I would expect pattern improvements across the operating band of frequencies to be most notable.

Standard microphones with wide-band figure-8 response (achieved using a single diaphragm design) and wide-band omnidirectional response (achieved using a small capsule design) are relatively simple in mechanical design in comparison to patterns where the balance between bidirectional and omnidirectional components are being carefully manipulated by way of the design and manufacture of the pathways from the back side of the diaphragm to the rear vents.  Based on that I think of intermediate patterns as being something of a "cardioid compromise".  That's not to say its easy to design and build a single diaphragm figure 8, only that I see additional complexity and engineering trade offs required to achieve intermediate patterns which maintain pattern consistency over the full frequency range of operation.

That said, I have a better understanding of the basic principles than the design and engineering specifics required to implement them and the specific trade-offs involved.

David, as someone with far deeper experience and insight into that side of things, would you consider this an accurate assessment?

« Last Edit: August 06, 2020, 10:07:11 AM by Gutbucket »
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Re: The World's Finest Blumlein Array
« Reply #36 on: August 06, 2020, 11:37:00 AM »
Gutbucket, please don't overestimate how much I understand about the physics of microphones--I'm just treading water like many of the rest of us here. But I do look at things much the same way as you seem to, including what you say about cardioids. There's nothing special about that pattern other than that it's midway between (and a superposition of) omni and figure-8. In some ways it offers the worst of both, and I never think of it as deserving to be the default choice by any means. Often just a small step to the left or right is preferable (I seem to think of omni on the left and figure-8 on the right; that's arbitrary, but it also seems to be the way that pattern switches or knobs are generally arranged).

Also, we should keep in mind that the "compromise" aspects of conventional microphones are deeply embedded in our recording practice. This includes their pattern deviations as well as non-linear distortions, which are quite considerable in many popular microphone types. The most highly revered "vintage" studio mikes, for example, generally have both types of problem--and the ways in which they're used have grown up around their characteristics as a whole, including those problems. Of course the particulars of those characteristics are important, too--but in general they tend to make the microphones suitable only for certain specific applications and usage patterns. For example most of us, I think, are way better off due to the fact that an omni of normal size that's relatively flat on axis (at working distance) will have rolled-off treble response off-axis, because of the absorption characteristics typical of building materials in most places where music is performed.

I'm all for making improvements, including ones that may seem small; they add up over time as more and more such improvements are made. But if you know your existing microphones well, and have struggled for years and found ways of working with and/or against their characteristics, then if Athena descended from Mt. Olympus and handed you a pair of perfect microphones from Her own private locker, your next few recordings would almost certainly be worse rather than better.

Also, sometimes the truth hurts. Most rooms suck! When you travel, sometimes you set foot in a place and say to yourself (or, obnoxiously, to your travel companions who aren't recording engineers), "Damn, I wish I could record in here." But that doesn't happen very often. Collectively what this hobby is about, and its professional counterpart to an alarming extent as well, is making the best we can out of very non-ideal recording situations--sometimes very VERY non-ideal--and fooling the ears of our listeners without getting caught at it too often.

--best regards
« Last Edit: August 06, 2020, 12:07:38 PM by DSatz »
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Re: The World's Finest Blumlein Array
« Reply #37 on: August 06, 2020, 12:23:37 PM »
Small observation. 

After I got an MKH 800 Twin and started assessing patterns in post, I found I rarely land on cardioid.  Many times something slightly in the direction of omni instead, with relatively close work.    I hacked a Neumann TLM67 into a dual output mic, and it's the same there, I usually go with wide cardioid instead, which isn't a pattern that's available on that mic, or a U87. 

Of course in this context we're talking about distant work.

Someone in the remote section at Gearslutz, in a dual output mic thread (specifically Pearl ELM-A, a mic with varying pickup pattern vertical versus horizontal), mentioned for their work they thought a toroidal pickup pattern seemed ideal, to maximize horizontal ambience while rejecting vertical ambience.  They reasoned an omni combined with a hypercardioid looking up and another looking down, both hypers polarity reversed, would create that pattern.  The OctoMic is ideally suited for that job.  You could likewise make it point to a lesser part of the horizontal spectrum. 
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Re: The World's Finest Blumlein Array
« Reply #38 on: August 06, 2020, 05:23:54 PM »
Similarly in my experience with TetraMic when listening carefully while adjusting the pattern parameter, I found myself gravitating toward the sound of intermediate patterns to either side of straight cardioid.  However, because I was using it to derive a coincident stereo output rather than a mono output I almost always ended up using crossed super/hypercardioid rather than crossed subcardioid patterns in order to achieve sufficient stereo difference information for good stereo width.  Ignoring stereo aspects, the overall sound was typically very attractive with subcardioid patterns, very smooth and with somewhat improved bass response.

That preference for individual patterns to either side of cardioid, along with the overall shape of the stereo patterns in combination (the total horizontal coverage of both overlapping shapes combined, irrespective of polarity) are partly what inform my comments earlier in this thread about the value of gaining control over the directional sensitivity of the recording array in an overall sense.  Not simply at the single microphone or stereo-pair interaction level but across the entire horizontal plane.
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Re: The World's Finest Blumlein Array
« Reply #39 on: August 06, 2020, 06:32:27 PM »
Someone in the remote section at Gearslutz, in a dual output mic thread (specifically Pearl ELM-A, a mic with varying pickup pattern vertical versus horizontal), mentioned for their work they thought a toroidal pickup pattern seemed ideal, to maximize horizontal ambience while rejecting vertical ambience. 

In the overall combined pattern sense mentioned in the posts above, Blumlein presents a horizontal toroid (donut/fat-inertube) shape, and crossed hypers a toroid which is ballooned out on one side more than the other, sensitive across the full horizontal plane but with increased bias toward the front.  We've discussed in other threads how blumlein has reduced sensitivity to sound arriving from both directly above and below, particularly how it might produce a somewhat more even-sounding audience portrayal by reducing sensitivity somewhat for nearby audience noise immediately surrounding the recording position when the mics are on a tall stand.

Quote
[..] They reasoned an omni combined with a hypercardioid looking up and another looking down, both hypers polarity reversed, would create that pattern.  The OctoMic is ideally suited for that job.  You could likewise make it point to a lesser part of the horizontal spectrum. 

I don't think a monopolar toroid is possible to construct that way.  Thinking through it..  Ignoring the omni microphone for the moment, if done perfectly, two summed hypers of opposite polarity pointing in opposite directions would produces a figure 8 response.  The omni component of the hypers is subtractive and cancels out, while the bi-directional component is additive, producing just a figure 8 oriented in the same direction as the positive polarity hyper.  Two summed hypers of the same polarity pointing in opposite directions would produce an omnidirectional response.  The omni components then sum, and the figure 8 components cancel out.   That remaining omni component from both hypers would have inverted polarity in comparison to the omni microphone, so the result of the full combination of all three microphone signals is theoretically a complete null.

Being 2nd order, OctoMic should actually be able to provide a toroidal monopole response, possibly by using the advanced plugins Len mentions.

Below is a visualization of spherical harmonics, with order increasing by row from the top down.  Alone in the top row is the zero-order monopole representing fundamental omnidirectional response (the W b-format channel).  Second row shows the basic 1st order bi-directional response oriented in three ways corresponding with the 3 directional dimensions: left/right, up/down, front/back, as displayed above (Y, Z, and X channels).  By manipulation of the combination of the top and second row patterns we can derive any 1st order pattern pointing any direction.  By including the 3rd row patterns as well we achieve 2nd order response patterns.  If 0-order omni is summed with the center-most 2nd order harmonic (I don't know the b-format channel name assignment for those), the inverted polarity "belt" lobe cancels the equatorial region of the omni while the north and south poles are reinforced, resulting in a vertically-orinented figure 8 shape with both lobes having identical polarity.  If summed with a polarity inverted version of the center-most 2nd order harmonic, a toroidal monopole donut shaped response should result.



« Last Edit: August 06, 2020, 06:40:59 PM by Gutbucket »
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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #40 on: August 06, 2020, 09:32:52 PM »
The B-format X channel, which is a dipole, has no obvious proximity effect. So the Blumlein decode will also have no proximity effect.
« Last Edit: August 07, 2020, 10:21:12 AM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #41 on: August 06, 2020, 09:35:04 PM »
Thus you can easily apply a compensating boost and get response that's as flat as you want down to (in principle) any frequency that you choose, while obtaining a polar pattern "down there" that's as accurate as you please.

If that mono dipole mic hasn't been calibrated, and isn't re-calibrated over time as it drifts, then any compensating boost that you add will be, at best, an educated guess.

OctoMic is precisely calibrated and is flat down to below 30 Hz.
« Last Edit: August 06, 2020, 11:10:28 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #42 on: August 06, 2020, 09:39:19 PM »
And Ambisonic can change/rotate polar patterns in space, but not to improve them (make them tighter or smoother).

This is quite wrong.

The stable and precise patterns are completely due to the Ambisonic process. The precision over angle is due to the spatial sampling, precise calibration and a lot of DSP know-how.

And the higher the order of the ambisonic microphone, the more stable over frequency and precise (provided the mic calibration is well done).

It literally has nothing to do with the pattern of the microphone array's capsules. The capsules could just as well be omnis as the directional capsules we use in OctoMic.

Quote
...it is the same as Mid/Side manipulation of coincident pair of mics

Mid/Side has two patterns created by mono mics. You will not find any such patterns in the capsules used in an ambisonic mic. All of the patterns have to be synthesized from the B-format channels. At that point you could do the Mid/Side thing if that's what you want. But there's so much more you could do.


« Last Edit: August 06, 2020, 09:50:18 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #43 on: August 06, 2020, 09:57:26 PM »
being 2nd order, OctoMic should actually be able to provide a toroidal monopole response, possibly by using the advanced plugins Len mentions.

Why don't you try that? We offer OctoMic B-format recordings for download on our web site. The SPARTA beamformer plug-in is free. Reaper is cheap, and Plogue Bidule is inexpensive.

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Re: The World's Finest Blumlein Array
« Reply #44 on: August 06, 2020, 10:42:50 PM »
what would be a recommended calibration interval for octomic, and at what cost?

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Re: The World's Finest Blumlein Array
« Reply #45 on: August 06, 2020, 10:50:19 PM »
what would be a recommended calibration interval for octomic, and at what cost?

We recommend re-calibrating every three years. Cost is $250.
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Re: The World's Finest Blumlein Array
« Reply #46 on: August 07, 2020, 07:42:19 AM »
It literally has nothing to do with the pattern of the microphone array's capsules. The capsules could just as well be omnis as the directional capsules we use in OctoMic.

Len, I'm sorry we don't have the same view. I'm out of the internet now. But I'm sure everything will be explained. If OctoMic can be assembled with omni capsules, then you use a different principle than I was thinking. I am not an expert on this, I hope this will help someone clarify.
« Last Edit: August 07, 2020, 07:50:58 AM by kuba e »

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Re: The World's Finest Blumlein Array
« Reply #47 on: August 07, 2020, 12:03:58 PM »

In the overall combined pattern sense mentioned in the posts above, Blumlein presents a horizontal toroid (donut/fat-inertube) shape, and crossed hypers a toroid which is ballooned out on one side more than the other, sensitive across the full horizontal plane but with increased bias toward the front.  We've discussed in other threads how blumlein has reduced sensitivity to sound arriving from both directly above and below, particularly how it might produce a somewhat more even-sounding audience portrayal by reducing sensitivity somewhat for nearby audience noise immediately surrounding the recording position when the mics are on a tall stand.

Quote
[..] They reasoned an omni combined with a hypercardioid looking up and another looking down, both hypers polarity reversed, would create that pattern.  The OctoMic is ideally suited for that job.  You could likewise make it point to a lesser part of the horizontal spectrum. 

I don't think a monopolar toroid is possible to construct that way.  Thinking through it..  Ignoring the omni microphone for the moment, if done perfectly, two summed hypers of opposite polarity pointing in opposite directions would produces a figure 8 response.  The omni component of the hypers is subtractive and cancels out, while the bi-directional component is additive, producing just a figure 8 oriented in the same direction as the positive polarity hyper.  Two summed hypers of the same polarity pointing in opposite directions would produce an omnidirectional response.  The omni components then sum, and the figure 8 components cancel out.   That remaining omni component from both hypers would have inverted polarity in comparison to the omni microphone, so the result of the full combination of all three microphone signals is theoretically a complete null.

Being 2nd order, OctoMic should actually be able to provide a toroidal monopole response, possibly by using the advanced plugins Len mentions.

Sure, Blumlein makes a bumpy toroid of sorts, but in stereo.

The hypers in my example are in polarity relative to each other, opposite the omni.  At the right level and placement, the hypers should not make omni, they should leave a toroidal hole at 90/270º when mixed with an omni.   Tweaking between hyper and super is probably the ticket.   Second order patterns from an OctoMic should do a better job of it. 

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Re: The World's Finest Blumlein Array
« Reply #48 on: August 07, 2020, 01:41:58 PM »
The hypers in my example are in polarity relative to each other, opposite the omni.  At the right level and placement, the hypers should not make omni, they should leave a toroidal hole at 90/270º when mixed with an omni.

Unfortunately no, I had to think about it yesterday to get it myself (it's the second example in my post quoted above).

Two coincident microphones of any pattern with the same polarity but opposite orientation will sum to omni (except two figure-8's which sum to null).

Regardless of the pattern used, the omni compnents are of the same polarity and add together, while the bi-directional components are of opposite polarity and subtract from each other, cancelling out.  The symmetry of identical patterns facing opposite directions means all the bidirectional component cancels out regardless of the proportion of bidirectional in the nominal patterns of the mics used.

[edited to add- using two hypercardioids rather than two cardioids just means you get a less sensitive omni out of the result]

Summing the resulting omni with a polarity inverted omni 3rd microphone results in null (after adjusting levels to match).



It may help to imagine taking it on step further by varying the pattern of one of the two opposed directional microphones.  What then results is some 1st order directional pattern other than omni, yet it always remains a familiar 1st order pattern.  Flipping polarity on one of the mics flips the orientation of the resulting 1st order pattern.
« Last Edit: August 07, 2020, 01:48:53 PM by Gutbucket »
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Re: The World's Finest Blumlein Array
« Reply #49 on: August 07, 2020, 01:53:25 PM »
Here's another way of thinking about it-  Imagine setting up a "native" ambisonic b-format array made using three fig-8s and one omni.  You can use that to derive a 1st order pattern pointing in any direction, but you can't derive a higher order pattern like a toroid shape from it.   The three mic example of two hypers and and one omni are just a subset of that.
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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #50 on: August 07, 2020, 02:00:08 PM »
Here's another way of thinking about it-  Imagine setting up a "native" ambisonic b-format array made using three fig-8s and one omni.  You can use that to derive a 1st order pattern pointing in any direction, but you can't derive a higher order pattern like a toroid shape from it.   The three mic example of two hypers and and one omni are just a subset of that.

And because of the separation in space of the mono mics, the results will be quite poor.

If we then go to a true ambisonic microphone (instead of a native B-format made with mono mics), the first-order patterns you'll get from a first-order ambisonic microphones are much, much less stable and precise than the ones you'll get from a well-calibrated second-order ambisonic microphones like OctoMic.
« Last Edit: August 09, 2020, 08:34:34 AM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #51 on: August 07, 2020, 02:05:08 PM »
Two coincident microphones of any pattern with the same polarity but opposite orientation will sum to omni (except two figure-8's which sum to null).

Regardless of the pattern used, the omni compnents are of the same polarity and add together, while the bi-directional components are of opposite polarity and subtract from each other, cancelling out.  The symmetry of identical patterns facing opposite directions means all the bidirectional component cancels out regardless of the proportion of bidirectional in the nominal patterns of the mics used.

[edited to add- using two hypercardioids rather than two cardioids just means you get a less sensitive omni out of the result]

I would think hypercardioids would have other problems besides being less sensitive in this arrangement.  Hypers have that pronounced rear lobe, which will overlap the front lobe of the other coincident mic oriented 180 deg opposite and vice versa.  A sound source directly on-axis to the array would be picked up strongly by the front lobe of the forward-facing mic (duh), but also somewhat well from the rear lobe of the rear-facing mic.  Would that overlap not cause the bass to be overly bloated for sources on-axis to the front of the array, since that is where that rear lobe is most sensitive?
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Re: The World's Finest Blumlein Array
« Reply #52 on: August 07, 2020, 03:25:51 PM »
What I outline above is the basic phenomena, without any of the messy complications of actual implementation.  It helps to ignore real world implementation complexities when seeking to understand the basis of how things work at a fundamental level. Things get very complected quick when moving on to actual implementation.

It's important to recognize that difference, especially in this type of discussion where both aspects are being discussed in the same thread.

Real world implementations can't work at all unless the basic underlying phenomena is correct, but a bad real world implementation can screw up a fundamentally correct underlying basis.


Len, what you say is not incorrect, but you are describing real-world implementation complications.  Voltronic I think you are mixing up the two.  The omni resulting from summed hypercards in my theoretically perfect example has less sensitivity because hypercardioids don't have much omni component in them to begin with.  They are mostly bi-directional component which is cancelled out.  Not sure what you are getting at with the bloated bass stuff, but since the proposed scheme won't work fundamentally, its pointless to speculate on the real-world problems of an attempted implementation.

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Re: The World's Finest Blumlein Array
« Reply #53 on: August 07, 2020, 04:11:31 PM »
Going back to this-
(Thanks to this thread, I understood now how ambisonic works - it is the same as Mid/Side manipulation of coincident pair of mics, but this is in 3d space and with the possibility of more combinations)

In terms of the fundamentals that's actually a pretty good basis for understanding basic 1st order ambisonics. At it's core ambisonics can be thought of as a further extension of basic Mid/Side.. or more specifically, of sum/difference processing of the basic omni and bidirectional components which in varying combinations form the basis of all 1st order sensitivity patterns.

I don't mean to differ with Len in his reply to that post, and I don't think we actually disagree on that point as I believe was replying with the specifics of implementation in mind rather than the fundamentals.

To clarify, not simply the manipulation of a pair of "microphones" but of three (or four) at minimum.  Specifically two crossed figure-8s and one omni for a 2-dimensional single plane 1st order ambisonics (horizontal only), and an additional figure-8 required to extend that to the 3rd dimension (height).  Those "microphones" correspond to the 4 channels of ambisonic B-format (W = omni, X = front facing 8, Y = left facing 8, Z= up facing 8 ). 

I put "microphones" in quotes above because I'm describing the fundamentals more than actual implementation.  Although it can be done using actual microphones of those patterns (doing so is referred to as a "native" B-format recording array), actual ambisonic microphones such those from Core Sound's do not actually use three figure-8s and an omni.  Instead they use a different, more advantageous arrangement of microphones which significantly helps improve a number of implementation issues, in combination with some special sauce processing.  The actual output of the microphone elements of such an array prior to the application of the "special sauce" is referred to as A-format, and is specific to each microphone design.  In typical use, that A format output is what gets recorded to the machine, yet is unusable until the "special sauce" is applied resulting in "universal" B-Format (actual output is either B-format or direct output of some form of mono, stereo or multichannel output converted internally from B-format).  Only at that point does it cross from the real-world-implementation-specific realm into the realm of fundamental ambisonic B-format signals which can be be manipulated with sum/difference techniques in the same way as Mid/Side, only with far greater flexibility.  More advanced tools and plugins can do stuff beyond simple sum/difference processing, but sum/difference manipulation of the fundamental omni and bi-directional components is the underlying basis of ambisonics.
« Last Edit: August 07, 2020, 06:15:11 PM by Gutbucket »
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Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #54 on: August 07, 2020, 05:46:01 PM »
I want to again emphasize that the process of going from A-format (the raw capsule audio) to B-format (what is called "encoding") has almost nothing to do with the directivity of the microphone array capsules.

There's no way that you can combine - for example - cardioid capsule directivities in the simple way Gutbucket has been discussing to get the directivity of an ambisonic second-order channels. Or omni capsules. Or any other mono mic patterns.

Yet, those capsules can be used in ambisonic arrays.

---

That said, you can combine virtual mics decoded from B-format in the way Gutbucket describes. The first-order virtual mics you'll get from OctoMic (the continuum from omni to sub-cardioid to cardioid to super-cardioid to hyper-cardioid to figure-8 dipole) are among the best (if not the best) in the world. (See the polar patterns on https://www.core-sound.com/OctoMic/2.php .) So the combination of those patterns are similarly excellent.

And of course, with a good second-order ambisonic microphone you get many more useful patterns than first-order mics (ambisonic or mono) can ever offer.

For Blumlein though, to get the crossed dipoles, we don't need to decode B-format to two dipoles, because the first-order B-format X and Y channels are already azimuth-plane dipoles. And being B-format, they don't have proximity effects.
« Last Edit: August 07, 2020, 05:58:23 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #55 on: August 07, 2020, 07:27:17 PM »
The hypers in my example are in polarity relative to each other, opposite the omni.  At the right level and placement, the hypers should not make omni, they should leave a toroidal hole at 90/270º when mixed with an omni.

Unfortunately no,


Well, I just tried it with an MKH 800 Twin and a 4060, it does work, crudely.    I put a metronome on the end of 18" of string and ran a circle slowly around the mic pair to maintain distance and level as best as possible. 

Best set supercardioid in my test.  When you are at 90/270º relative to the hypers, you get maximum output from the mono array of 3. It may not be a toroid, but it's at least a greatly flattened sphere, less output up and down, more side to side.    It's not purely with respect to frequency, but that has value in practice as well.  The opposite facing hypers or supers by themselves definitely have less output at 90/270º, it does not make an omni in the same way that the two capsules combine to omni: that has no drop off in level.

As visual example, go to

http://www.sengpielaudio.com/Visualization-AB60-E.htm

set it XY, supercardioid, point them 90/270º.   Sure looks like it's on the way to the void of a toroid. 


Should be able to do a refined version with second order patterns. 
« Last Edit: August 07, 2020, 07:32:33 PM by EmRR »
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Re: The World's Finest Blumlein Array
« Reply #56 on: August 08, 2020, 10:15:57 PM »
If you're looking for a toroid in the azimuth plane, combine the V and U second-order B-format channels.
« Last Edit: August 08, 2020, 10:30:10 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #57 on: August 10, 2020, 09:54:41 AM »
As visual example, go to

http://www.sengpielaudio.com/Visualization-AB60-E.htm

set it XY, supercardioid, point them 90/270º.   Sure looks like it's on the way to the void of a toroid.

Keep in mind that Sengpiel visualizises the stereo interaction, not the electrically summed behavior of the two channels.

So you had the MKH800 twin set to output two opposed supercardioids of the same polarity, then summed those resulting outputs together?
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Re: The World's Finest Blumlein Array
« Reply #58 on: August 10, 2020, 10:19:45 AM »
Why would you bother with summing first-order patterns when you'll get much, much better results in both the spatial and frequency domains with the simple sum of the V and U second-order B-format channels?

The summing is done incredibly simply: route the two  channels to a single recording output.

If you want to hear it on one of our B-format recordings, pick one and I'll decode & record it for you.
« Last Edit: August 10, 2020, 10:41:48 AM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #59 on: August 10, 2020, 11:43:25 AM »
Because he is using the microphones he has available, and the primary intent at this point was to test the Gearslutz hypothesis about the ability of producing such a pattern, more than testing presumed usefulness of it.

Thanks for your offer to make such a decode of the B-format recording.  I do intend to play around with those recordings at some point with all this in mind, just need to make some time to do so.  Are you sure a toroid shaped pattern is the result of summing channels V and U?  Given the interaction of their inverted polarity lobes in the sum, I can't see how that works if the 2nd order channel assignments the same as those shown here:



Which instead seems to imply a sum of (R and -W)
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Re: The World's Finest Blumlein Array
« Reply #60 on: August 10, 2020, 12:09:15 PM »
Yes, you're correct, if that's what you're seeking. I neglected polarity.
« Last Edit: August 10, 2020, 12:12:11 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #61 on: August 19, 2020, 12:34:18 PM »
I read more about ambisonic microphones. I'll be happy if someone corrects me. Maybe it will also be useful for someone else.

The easiest way to understand the ambisonic principle is a 1st order microphone (eg TetraMic). If someone is starting to read about ambisonic, it is best to first thoroughly read how Mid/Side of a coincident stereo pair works. When we understand Mid/Side, then understanding  1st order ambisonic is easy.

The 1st order microphone is made of four coincident directional capsules evenly angled in space. From their signals, it is possible to get any pattern of the 1st order (omni, fig.8 or their combination  - cards, hypercards, ...) and we can angle this pattern arbitrarily in space. Everything is built on the basis of summing and subtracting coincident directional microphones (eg cardiode):
-when we sum the signals from two coincident directional microphones rotated 180 °, we get omni.
-when we subtract these signals from each other, we get fig. 8.
-when we sum two coincident directional microphones that have an angle of less than 180 °, the result is a less directional microphone in their axis.
(explanation - each directional pattern contains a certain ratio of the components omni and fig. 8., it's just composing fig.8s and summing the omnis.)

By summing and subtracting the signals of the four coincident cardiodes evenly angled in space, we obtain one omni and three figs. 8 in the perpendicular directions X, Y, Z. This is the 1st order B-format. And to get the final pattern from the B-format, we use the following manipulation. The sum of the signals of three fig. 8 (X, Y, Z) in the corresponding ratio is resulting to fig.8 in the desired spatial direction. And when we add the omni signal in a certain ratio to the resulting fig.8, we get the resulting desired directional pattern rotated in the desired direction.

The principle of higher order ambisonic microphones (eg OctoMic) is different. There are multiple microphone capsules that are distributed on a spherical surface (imaginary or rigid) and a mathematical model is used to evaluate their signals. The mathematical model is based on the fact that the values ​​of acoustic pressure on a spherical surface can be expressed using the sum of the infinite number of spherical harmonics. And these spherical harmonics correspond to our polar patterns.
The input to the mathematical model are signals from individual capsules. A matrix operations are performed with these signals and the output are spherical harmonics. 1st order spherical harmonics and their combinations correspond to the generic microphones patterns. Higher orders and their combinations are ambisonic specialties. The more microphone capsules we have, the higher spherical harmonics we can derive.

The principle of the higher order microphone (capsules on a spherical surface/space + mathematical model) is different from the principle of the 1st order microphone (combination of 4 coincident cardiodes). It would be interesting to hear their comparisons, eg TetraMic vs OctoMic with 1st order patterns.
« Last Edit: August 19, 2020, 01:52:19 PM by kuba e »

Offline kuba e

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Re: The World's Finest Blumlein Array
« Reply #62 on: August 19, 2020, 12:35:55 PM »
Thank you Len, you are right. I read that omni capsules are also used in higher order microphones. The mathematical model is very similar. But the physical design of the microphone is different. It is necessary to separate the capsules from each other to obtain signal differences from the omni capsules at low frequencies. But this in turn brings limitations for high frequencies. It is probably only used in a microphone, which consists of a large number of capsules.
« Last Edit: August 19, 2020, 12:44:29 PM by kuba e »

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Re: The World's Finest Blumlein Array
« Reply #63 on: August 19, 2020, 12:38:18 PM »
Ambisonic microphones have their advantages and disadvantages. Unfortunately, I don't understand why the OctoMic should have a better polar pattern than generic microphones. The input to the OctoMic mathematical model are signals from cardioid capsules. These signals are transformed into spherical harmonics (polar patterns). And the quality of these spherical harmonics corresponds to the quality of signals of cardioid capsules - generic microphones. How can the OctoMic be better than a generic microphone?
(I don't consider equalizing low frequencies as an improvement. DSatz has already explained to us that we can equalize any microphone, this is not an OctoMic advantage.)

It is also not clear to me how can be the proximity effect removed from the B-format?

I am left with these ambiguities. But those are the details.
« Last Edit: August 19, 2020, 01:33:19 PM by kuba e »

Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #64 on: August 19, 2020, 02:01:22 PM »
Unfortunately, I don't understand why the OctoMic should have a better polar pattern than generic microphones.

It's truly so. Please have a look at OctoMic's measured patterns, and compare them to the best of the world's mono mics of all directivities.

It has to do with the resolution of OctoMic's spatial sampling and the precision of its calibration. That's all I'll say for now.

Quote
It is also not clear to me how can be the proximity effect removed from the B-format?

It's not removed. It's never there from the start.

---

Also, please note that OctoMic is not the kind of spherical microphone you described.
« Last Edit: August 19, 2020, 04:05:45 PM by Len Moskowitz (Core Sound) »
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Re: The World's Finest Blumlein Array
« Reply #65 on: August 19, 2020, 05:31:46 PM »
Thank you Len for your patience.

I have seen charts of fig 8.

Yes,  I probably understand now. The question is what do you mean by better polar pattern.
I understand this that pattern is not changing with frequency. Quality generic microphones meet this. OctoMic also meets this up to 10 kHz.
But you probably mean that OctoMic is better because it has a little different shape of polar patterns than generic microphones. Is this difference in shape really beneficial? This should be judged by listening.
(Haha, I got idea how to do it. From the OctoMic, we can take the direct signal of two cardiods that have a larger angle with each other. This gives us a recording with generic microphones in xy. And also we can create the second recording by ambisonic virtual patterns identical as the two cardioids in xy. Then it is easy to compare it.)

The proximity effect is difficult. I believe you. I only asked because I wanted to understand how ambisonic works. It is not easy -  the B-format, which is based on cardiod signals, has no proximity effect. And when we compose a virtual pattern from the B-format, the proximity effect is created there. Not everything may be clear. I will need time to understand.

Thank you again for all your help.
« Last Edit: August 19, 2020, 06:24:23 PM by kuba e »

Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #66 on: August 19, 2020, 11:51:48 PM »
The question is what do you mean by better polar pattern.
I understand this that pattern is not changing with frequency. Quality generic microphones meet this. OctoMic also meets this up to 10 kHz.

Have a look at all of the directivity patterns. You'll see that OctoMic has more stable patterns for all directivities.

Quote
But you probably mean that OctoMic is better because it has a little different shape of polar patterns than generic microphones.

No, that's wrong. The ideal pattern shapes are identical across both ambisonic and mono mics. You may be mistaking the linear scale plots for log plots.

---

We've strayed very far from the subject of the original post. I'm going to lock the thread.
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Re: The World's Finest Blumlein Array
« Reply #67 on: August 20, 2020, 11:22:51 AM »
We've strayed very far from the subject of the original post. I'm going to lock the thread.

Is this not a discussion forum?
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Re: The World's Finest Blumlein Array
« Reply #68 on: August 20, 2020, 03:22:01 PM »
Just started a new thread dedicated to discussion of ambisonics in general-
Ambisonics (general discussion, as related to recording)
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

Offline Len Moskowitz (Core Sound)

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Re: The World's Finest Blumlein Array
« Reply #69 on: August 20, 2020, 03:32:44 PM »
Quote
Just started a new thread dedicated to discussion of ambisonics in general-
Ambisonics (general discussion, as related to recording)

Thank you!

Quote
Is this not a discussion forum?

This post was transferred here by the moderator from Retail Space.
« Last Edit: August 20, 2020, 03:34:52 PM by Len Moskowitz (Core Sound) »
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