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Author Topic: The World's Finest Blumlein Array  (Read 2241 times)

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Offline Len Moskowitz (Core Sound)

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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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Offline kuba e

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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 »

Offline Len Moskowitz (Core Sound)

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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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Offline DSatz

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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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Offline kuba e

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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 »

Offline Gutbucket

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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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Offline DSatz

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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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Offline EmRR

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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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Offline Gutbucket

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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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Offline Gutbucket

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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 »
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 #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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Offline 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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Offline 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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Offline 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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Offline jerryfreak

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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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