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Author Topic: 3 mic (LCR) distance  (Read 15106 times)

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Offline Organfreak

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Re: 3 mic (LCR) distance
« Reply #45 on: February 05, 2025, 04:13:17 AM »
A bit off topic because this is a 4-mic question:

My current set up is one straight home-made aluminum bar; two KM143 in NOS position in the center and two DPA4090 in AB position on the outside (distance 130 cm).
Would I benefit from putting the NOS pair a bit ahead of the AB pair to link the mics better?
I have the Schoeps tool, but that is limited to three mics maximum.
Mics: Rode NT55, DPA 4090, Neumann KM143
Recorder: Zoom F6

Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #46 on: February 05, 2025, 09:42:55 AM »
You might.  Worth a try.  Maybe push the NOS pair forward by some amount, or the omnis back by some amount, and compare that to having them all in a line just to determine if doing something like that seems to be moving in the right direction or not.

Michael Williams has specific suggestions for doing so in a way such that the imaging of each adjacent mic-pair segment perceptually links up with that of the next without overlaps or gaps.  Other than the Schoeps Visual Assistant which AFAIK is based on the same data but only covers 2 and 3 microphone positions, the easiest way to access those arrangements is through the interactive hyper-linked MMAD (multi-microphone array design) section of his website, however it currently only works for some 2 channel and 5 channel arrangements. The others are "In Preparation".  In addition to Image Assistant, I use that interactive MMAD site to inform my 3 position and 5 position arrangements (for 3 just use the 5 channel arrangements and just ignore the outermost or rear-facing pair).

However, even though the interactive website section for 4 mics is not operational, the data can be found in his AES papers.  Here's one which includes 4 channels across the front stage:

2004 -117th AES Convention, San Francisco, USA - AES Preprint 6230
« Multichannel Sound Recording using 3,4 and 5 channel arrays for front Sound Stage Coverage »
https://www.williamsmmad.com/Papers/6230%20San%20Francisco%202004%20(57%20pages).pdf

The arrangement shown below is close to your current spacings, which I pulled from page 46 of that paper.

Caveats-
1) Currently, all the MMAD arrays use the same microphone pattern for all microphone positions (using different pattern mics in various positions is one thing I intend to ask Mr Williams about). I choose hypocardioid as being somewhat close to the km143 pattern of your near-spaced pair in selecting the setup shown below.  Just substitute your 4090 omnis in the wide-spaced position in place of the hypocardioid pair.
2) These setups all assume playback using same number of speakers as microphones. In my past experimentation with multichannel playback speaker arrays these setups have worked really well for that.  However I also found that they "fold down" to 2-channel stereo very nicely.  That conclusion is my own finding and reflects how I use these most of the time these days (mixing them down to 2ch stereo), but it is not Michael William's intention as far as I know (this is another thing I intend to ask him about).  I just want to be completely clear about that.
« Last Edit: February 05, 2025, 09:46:47 AM by Gutbucket »
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Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #47 on: February 05, 2025, 10:21:56 AM »
As mentioned, the arrangement above is for four hypocardioids (somewhat approximating widecardioid).  Below are the closest ones to your current spacings for four omnis, and for four cardioids.

I've selected all of these simply because they are the closest 4ch MMAD arrangements to the mic-spacings you are currently using.  Notice that all of these indicate a forward spacing for the center pair that is pretty significant!  Although all MMAD configurations use some amount of forward spacing, there are others which do not use nearly as much forward spacing and instead have the mics closer to being arranged along a line, however they use lateral spacings between the mics that are different from your current spacings. Those other configurations will produce a different stereo recording angle for the center pair segment coverage and the lateral segments.  It's all a juggling of parameters required to get the linking to hand-off smoothly from one segment to the next.  In all of the examples I've attached here the SRA segment coverage remains the same despite the different pickup patterns: 90 degrees for the center pair segment and 40 degrees out to of either side of that for the lateral segments.

If you go and find these particular setups in the appendix of the paper linked above, scroll up and down a few pages and take a look at how the dimensions of the setup changes as the coverage angles are altered.  You might want to try a setup that keeps the mics more in a line but will require using a different spacing between mics.  Just keep in mind that doing so will alter the stereo recording angles.
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Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #48 on: February 05, 2025, 10:56:55 AM »
Here's an example of a 4 channel front-coverage MMAD arrangement of cardioids that uses much less forward spacing, but has a significantly wider center pair spacing.  The wider center pair spacing reduces the Stereo Recording Angle of the center segment down to 50 degrees (rather than 90 degrees in all previous examples above), while the lateral segments now extend out 80 degrees to either side of that center segment (rather than 40 degrees in the previous examples)
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Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #49 on: February 05, 2025, 11:29:31 AM »
To be clear, I'm not saying the above arrangements will be right or best for you.  Its just a presentation of one method for arranging things which is worth consideration as it's based on a lot of past work on stereo-image linking, which has successfully informed the arrangements I use.  But.. my setups that are based partly on this have all used minimal front/back spacings for a couple reasons: 1) The practical constraint of needing to support these things from a single mic-stand, along with the practical constraint of not overhanging the isles between seating rows in front or in back of the recording position. 2) I find I like to use spacing more along the L/R lateral axis and mic-pattern directionality more so along the front/back axis. That keeps transients that arrive front the stage in front aligned more tightly in all channels and minimizes front bleed into rear-facing mics.  And it may be that those things are what help these configurations mix down to 2-channel stereo so nicely for me.

I'll leave it here for now. Happy to discuss further if you like.
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<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline Organfreak

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Re: 3 mic (LCR) distance
« Reply #50 on: February 06, 2025, 01:45:10 PM »
Thanks Lee for this very useful and extended reply. Being both performer and taper I have the opportunity to record a piece played on the pipe organ, change mic positions, and then record the same piece again.
Compare at home by direct switching between the tracks using Audacity.
Because of your and all other good suggestions on this forum, my recordings improved a lot.

I am going to check out all this and try  :D.

One question: I use my Zoom F6 both for recording and playback. The device mixes the tracks real time and there is also an opportunity to delay one pair of the tracks. Each miilisecond of delay represents about 30cm distance.
I tried the delay effect in my current set-up and it gives an increased sense of depth and space, but also the sound becomes kind of "hollow".
Would this electronic delay gives same result as a physical move of the mics?

For example the 4 mic set-up as per figure 73 as you pasted; use same locations but instead of the 95.2 cm front-back distance keep all the 4 mics in line and apply ~3.2 millisec delay.
« Last Edit: February 06, 2025, 03:06:14 PM by Organfreak »
Mics: Rode NT55, DPA 4090, Neumann KM143
Recorder: Zoom F6

Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #51 on: February 06, 2025, 05:19:38 PM »
If you take a look at the lower right corner of each of the configuration diagrams I posted above, you'll see it states- "No Electronic Delay needed".

Some MMAD arrays do require electronic delay, others don't.  When they do, the electronic delay indicated isn't going to correspond directly to what would be needed to compensate for the offset distance between microphones if one were aligning the waveforms of all channels for a wavefront arriving from a certain direction (say directly from the front).  Electronic delay is instead used as another tool in addition to the pickup-pattern, spacing and angle between microphones to "steer" the edges of each SRA segment.

If you peruse the MMAD papers or the hyperlinked MMAD section of the site for the 5-microphone arrays you''ll come across some arrays that do require delay applied to some of the channels for the image linking to work properly.

First I'd try setting up the mics using the indicated physical locations.

Then after listening to that, I'd probably play around with delaying the front channels of those recordings on the computer by various amounts just to see what that does. 

Then you might try moving the mics into a different arrangement, say with the pairs spaced less far apart, and listen to that without delay and then with it to "sorta/kinda" emulate them being spaced farther apart.

BUT! Know that electronic delay is not the same as the use of physical spacing, especially in these arrays.  That's the "sorta/kinda".  Delay is one-dimensional.  These mic arrangements are two-dimensional. And the room is three-dimensional.  Application of delay can only line things up for wavefront arrival from a single direction, and with that achieved the arrival time difference for a wavefront arriving from any other direction is not going to be aligned.  What's more, the difference in alignment will increase as arrival shifts farther and farther off that axis until reaching a maximum for a wavefront arriving from the opposite direction, in which case the run-time delay from the physical spacing and the electronic delay will add together, doubling the timing offset instead of cancelling it out.

That geometric reality applies for any sound source bouncing sound around any non-echoic room, but I expect it to be especially relevant for pipe organ where the room is essentially an extension of the instrument, with wavefronts arriving from all directions and strong modal nodes distributed in 3-dimensions.

I do want to repeat that these arrays are actually intended for playback over quad, 5-channel etc, playback systems where the number of playback speakers equals the number of mic channels.  As mentioned, I've found the arrays that I've adapted to my own use work really well for 2-channel stereo mixdowns too, which is why I'm suggesting they might be worth a try.  But it also means the more significant front/back spacing between pairs in these particular arrays, which tend to be larger than what I'm using, throws things up in the air a bit too, in addition to the application of delay.

All I can say for sure is try it, play around with it, and please let us know how it works.


Oh yeah, almost forgot to discuss one important particular of the F6's channel delay function.. The F8 I use has the same capability.  I was initially excited to play around with that during playback when mixing on the recorder itself, but unfortunately the delay can only be applied during recording and not during playback.  That makes it far less ideal for experimentation when figuring all this stuff out in comparison to applying the delay on the computer.  You'd have to make lots of recordings with different delay settings instead of just changing the delay afterward as desired. It would be applicable if after playing around with delaying things on the computer you determine that you definitely want to use a certain delay all on certain channels of whatever array you settle upon using regularly - and it would only be applicable to that particular array. In that case applying the delay while recording would conveniently eliminate the need to do that afterward.
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<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline Organfreak

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Re: 3 mic (LCR) distance
« Reply #52 on: February 07, 2025, 02:36:35 AM »
I have tried numerous configurations using four mics. Looking around on Taperssection and Gearspace, lots of people did the same, apparently with no conclusive outcome what is best. This gives a very useful tool to get more grip on the situation.
I am really curious about the mathematics behind all these figures.

About delay during playback on the F6: I can do it via the remote function on the Zoom app on my IPhone. Press the button "pfl" at the track you want to delay during playback and then select "Input Delay" (0...30 msec could be selected).
« Last Edit: February 07, 2025, 02:49:03 AM by Organfreak »
Mics: Rode NT55, DPA 4090, Neumann KM143
Recorder: Zoom F6

Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #53 on: February 07, 2025, 11:38:59 AM »
Hmmm. Good to know you can do that on F6.  I'll have to try the Zoom app with the F8 and see if I can do the same.  Tried the app early on, but haven't used it since then, and never tried it for playback.

Not overly surprised there isn't a solid consensus on the best way to use four mics for recording pipe organ.  I think most play around with it until they find what works.  Probably varies a good bit with the particular organ and room.

[snip..] For example the 4 mic set-up as per figure 73 as you pasted; use same locations but instead of the 95.2 cm front-back distance keep all the 4 mics in line and apply ~3.2 millisec delay.
^
My posts above weren't meant to dissuade you from trying this, but rather to explain what's going on with the Williams arrays.  Yes sure, try that and play around with adjusting the delay during playback to best effect. Actually, what you suggest is more in line with what I do than the set-ups shown above which use significant front/back spacings. 

That's because a general guideline of mine for spacing the microphones in my multichannel arrays (please forgive me for repeating myself as this has been stated in the OMT threads probably several times) is to rely on spacing along the left/right axis more so than along the font/back axis.  I like using some amount of spacing along both axes, but rely more on microphone directionality along the front/back axis  - pointing directional mics forward and backward, without spacing them that far apart.  That's based on several things.  One is that I find my rear-facing mic channels much more useful when they exclude as much front content as possible.  I found this to be true regardless of whether I'm playing the recording back in multichannel surround or mixing it down to 2-channel stereo.  Loud sounds arriving from the front otherwise obscure the quieter stuff arriving from the back which the rear facing mics are intended to pick up. Even if I'm not using much of that rear-arriving content, its much more useful when its relatively isolated to the sound arriving primarily from the rear, and achieving that requires the use of microphone directivity rather than spacing.  Another is time alignment of transients, which is something that is arguably important for stereo mix-down and not multichannel playback. Using less front/back spacing keeps the transients from the front content that do leak into the rear-facing channels more closely aligned with the front facing channels, incurring less time domain smearing when mixed together.  A third is simply practical - its less of a challenge for me to deploy an array that is spaced widely in the L/R axis than one which has significant spacing along the front/back axis.  So I gravitate to the Williams arrays that have less front/back spacing to begin with, and end up modifying those to fit what I'm doing.

All that said, some of it isn't applicable to what you are doing!  Even though sound is arriving from all directions, especially so in a pipe organ hall, all four mic channels of your array are intended to be fully sensitive to sound arriving directly from the pipe registers in front, as well as to sound arriving from other directions in the room.  You aren't trying to exclude foreword arriving sound from rear-facing channels, but rather picking up sound from all directions in those wide-spaced omni channels.  The time-alignment aspect is probably more applicable, although you probably have more leeway there than other stereo mix-down applions just because of the nature of the instrument - its not super percussion rich with lots of sharp transients but rather more timbral, tonal, spacious and room filling.  Some time smear may actually be to your advantage.

Hope that helps more than confuses.  My apologies to others reading who find these wordy posts something of a bother.
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<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

 

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