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Author Topic: Relationships between spaced omnis and center mic  (Read 12583 times)

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Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #45 on: November 02, 2021, 10:42:34 AM »
DSatz raises a good point about potential phase interaction complexities as multiple microphone channels are mixed together.  I've found using more than two microphones in an array to be beneficial for many taper situations and provides a lot of flexibility, but it does require thought and attention regarding how those microphones are arranged and used in order to minimize potential problems and maximize the potential benefits. In light of that, limiting a multichannel microphone arrangement to three or four channels is likely to represent the sweet spot for most tapers who want to play around with what a multiple microphone array can do, as it strikes something of a reasonable balance between simplicity and complexity in providing additional degrees of freedom without the complexity getting out of hand.

Phase interactions arise from the output of multiple microphones placed in close proximity but slightly different points in space being mixed together.  A pair of spaced omnis plus a center microphone represents three separate points in space, two of which are summed to each playback channel (Left omni and center microphone summed to the Left channel; Right omni and center microphone summed to the Right channel).  One reason I recommend a coincident X/Y or M/S pair in the middle for a four microphone array rather than a near-spaced pair arrangement such as ORTF, NOS, "DIN" or whatever near-spaced configuration a taper might prefer over a coincident-pair when when recording only two channels, is that the substitution of a coincident pair for a single microphone does not increase the total number of different microphone positions in space.  It's four channels, but still only three separate positions.   

Another way of thinking about it is that a coincident pair (ideally) produces no phase difference between it's two channels, so the introduction of a fourth microphone placed coincidentally with one already present in a spaced three microphone array will not increase phase interaction complexities over those that are already present with just a single microphone in that position.

Keep your ear open for phasiness with any spaced configuration.  Especially those using more than two physically spaced apart microphone positions that will be summed together.  But don't let the potential for that keep you from trying multi-microphone spaced array techniques if they are interesting to you.
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 dyneq

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Re: Relationships between spaced omnis and center mic
« Reply #46 on: November 02, 2021, 11:13:06 AM »
I stumbled upon a recent example: http://phish.in/2021-10-23
Quote
Source: DPA 4006a w/30mm APE spheres(split 7.5'@9')->Portico 5012 + DPA 4011c(xy - centered between 4006s/-9.5db)->SD788t(24bit/96kHz)->MBit(16bit/44.1kHz)->FLAC

Recorded and transferred by Scott Schneider
Even with the spheres in use, there is an element of distance to this recording and I'm still perceiving a hole in the middle (on headphones).

Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #47 on: November 02, 2021, 11:52:54 AM »
Some more in-depth dirt on phase interactions between channels-

Fundamentally, there are two basic strategies for reducing potentially problematic phase interactions between microphone channels, and they are essentially opposite approaches to each other.  One seeks to eliminate the phase differences between channels by placing both microphones in a single "coincident" point in space, making the phase relationship between those channels identical.  The other seeks to make the phase relationship between channels different enough that they will not interact problematically when summed, achieved by spacing the microphones far enough apart from each other that the phase relationship between them is essentially randomized. That's the basis of the 3-to-1 spacing rule for close microphones that will be summed together into a single channel (Note- the 3-to-1 rule is not intended to apply to stereo microphone configurations where the channels are not summed. Many sources erroneously state that it does apply to stereo mic'ing, but a stereo pair is not summed, and 3-to-1 is geometrically impossible to apply anyway).  A complication of the spacing  approach is that the phase relationship between spaced positions varies in a complex way.  It varies with spacing distance, by frequency, and with the angle of arrival of the wavefront, and all three aspects interact with each other.  For wavefronts that arrive from any angle other than very close to perpendicular / directly on-center, the phase relationships at the high end of the frequency range is essentially randomized down to an increasingly lower frequency as spacing is increased.  The greater the off-axis angle from which the wavefront arrives, the greater that difference becomes.  The difference varies with frequency as well - there is increasingly less phase shift as the frequency decreases. 

When two similar signals with different phase relationships are summed, at each multiple of one full frequency cycle or 360 degree of phase difference the signals will constructively interfere to the greatest degree, and at each half cycle or 180 degree difference the signals will destructively interfere and cancel to the greatest degree.  Below some frequency where the difference in path length to each microphone is close enough to the wavelength of the frequency in question, the phase difference between channels will be less than 360 degrees and no longer effectively random in a perceptual sense.  At low enough frequencies with significantly long wavelengths in comparison to the spacing between microphones, the phase difference will be less than a 1/4 wavelength and the two signals will essentially remain in phase, acting effectively like coincident placement below that frequency.  That may or may not help explain how spacing a pair of omnis more widely can create more low frequency difference between channels.

How does all this relate to why a pair of wide spaced omnis with a coincident pair in the middle might be a highly effective four microphone arrangement for taping from an audience position?  And how might it be applied to deciding how to arrange it?  Consider the different qualities of direct-arriving verses reverberant sound. Direct-arrival sound requires a more highly correlated, or identical phase relationship between channels to present a well defined stereo image that features directional clarity and well-defined imaging, while reverberant sound has a more highly-randomized phase relationship and requires the preservation of that between channels to sound open, airy and natural rather than closed-in, flat and monophonic.  A recording will translate better if we can arrange things so as to preserve or even enhance both of these relationships, however the two are at odds with each other.  When using a near-spaced pair we are finding something of a best middle-way compromise solution that may not be optimal for either but works well enough for both. 

When using a coincident pair in combination with a wide-spaced pair we can set things up so as to have each pair better optimized for what it can best contribute.  A coincident pair produces no phase difference between channels and does a great job in translating clear and distinct directional imaging of direct arrival sound. A wide omni pair randomizes phase across a significant portion of the frequency range, which does a great job of translating a big lush open reverberant sense of space, and at lower frequencies can provide useful non-random phase differences that makes for more enveloping, non-monophonic bass.  There is still a compromise to be made however, although now it is a different one - managing the phase interaction complexity between both pairs.  Using a coincident pair in the middle helps with that, as does spacing the omnis far enough apart that they won't produce objectionable phase interactions with the coincident center pair (yet not too far), and a somewhat wider spacing that can be used when a center microphone or pair is included also helps with portrayal of the reververant aspects of the recording.
« Last Edit: November 02, 2021, 03:56:54 PM by Gutbucket »
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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: Relationships between spaced omnis and center mic
« Reply #48 on: November 02, 2021, 01:05:11 PM »
As mentioned above, although a wavefront arriving from somewhat off axis will produce phase differences across two spaced microphone positions, particularly so at higher frequencies, a wavefront arriving absolutely directly on-axis, perpendicular to the spacing axis will not produce a significant phase difference between channels.  This is an interesting phenomena.

This aspect can sometimes be apparent when listening to a pair of wide spaced omnis where a hole in the middle is evident.  Listening closely, one may perceive a narrow "island" of sound centered in the middle of the hole.  It's also why using a particularly wide spacing can sometimes work amazingly well for recording a PA amplified concert from a more distant section position without producing a gaping perceived hole, where a hole would otherwise seem assured.  In that case there actually is a hole in the middle with regards to the reactions of specific audience members that aren't directly centered, which are likely to be heard as being clumped well over toward one speaker or the other (even though diffuse non-specific general audience din, roar and applause should be portrayed in a more even way). But because much of the PA sound is monophonic, if each widely spaced omni is essentially equidistant to the PA stack on its side, pickup of that PA content with such a wide arrangement can produce relatively minor phase differences so the direct sound from the PA sounds well centered without a hole.  The cross-path delay from the PA stack on one side to the spaced microphone position on the other likely serves to create some short-delay source-widening effect that also helps the centered PA material fill the soundstage.

One interesting implication of this may be it's potential application to the positioning of the center microphone or pair. We've talked here or in other threads about either placing the three microphone positions directly inline with each other, or shifting the center position forward somewhat, and if so by how much?   It might be useful to shift the center position forward just enough so that the path from the left PA stack arrives perpendicular to the axis between the left omni and center pair, and vice versa with the PA stack and omni/center-pair axis on the other side.  That seeks to minimize phase difference between the wide omni and center position for pickup of the PA on each side.  The farther back the recording position, and the narrower the distance between PA stacks, the less far forward the center microphone position would be placed in front of the omnis.
« Last Edit: November 02, 2021, 03:59:16 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<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline rocksuitcase

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Re: Relationships between spaced omnis and center mic
« Reply #49 on: November 02, 2021, 01:19:50 PM »

One interesting implication of this may be it's potential application to the positioning of the center microphone or pair. We've talked here or in other threads about either placing the three microphone positions directly inline with each other, or shifting the center position forward somewhat, and if so by how much?   It might be useful to shift the center position forward just enough so that the path from the left PA stack arrives perpendicular to the axis between the left omni and center pair, and vice versa with the PA stack and omni/center-pair axis on the other side.  That seeks to minimize phase difference between the wide omni and center position for pickup of the PA on each side.  The farther back the recording position, and the narrower the distance between PA stacks, the less far forward the center microphone position would be placed in front of the omnis.
I've tried this once without a scientific measurement between the three sides of the triangle.
This paragraph would suggest you are getting toward a Decca Tree type arrangement for the center (fwd) channel(s). Am I headed in the correct direction as to your intent?
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Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #50 on: November 02, 2021, 02:21:43 PM »
Sort of.. in a way, yes.  It would imply a more triangular arrangement when recording from closer in (as the apparent angle to the PA stack on each side becomes wider) as the opposite extreme to a very distant position where the wavefront arrival is flat and all three microphone positions would be arranged in essentially a straight line.

A Decca tree is traditionally placed above the conductors head, almost inside the the orchestra with sections wrapping around to each side.  I guess it might be thought of in that sense as having three axes normal to its sides where that kind of thing occurs: one facing forward (and rearward into the hall), one facing toward the left rear corner and one facing toward the right rear corner of the orchestra.  But I don't think that particular aspect or this analogy really works or extends to the widely distributed sources of an orchestra.  This is more specifically about the oddity of left/right spaced PA stack amplification and why super wide omni spacings which shouldn't work actually do work in that situation, and how we might make the best of that when adding a center pair.

So yeah, it turns into something of a triangular arrangement similar to a Decca tree, but perhaps for different reasons than how a Decca tree is traditionally thought of and used.

What was your experience in making those recordings?  May be hard to judge without a good baseline comparison with a straight line arrangement from the same position.

I'll say this- In moving to a directional center mic or pair I tend to not want the center coincident pair position out too far forward because I feel like I want the impulses from a front arriving wavefront well aligned across all forward facing mic channels, and I don't want to have to deal with the hassle of delaying the center channels to achieve that.  Yet I used to frequently use four omnis in a spaced diamond configuration from an FOB position in the center of the audience relatively close to the stage, using anywhere from a meter or two of horizontal spacing and a half-meter to a meter or so of front/back spacing.. and I never noticed a problem with smeared transients doing that.  In that case the axis between the center omni placed forward of the L/R omnis and the omni over on one side was probably close to being perpendicular to on-axis to the PA on that side much of the time.  I didn't think of it this way back then, though.  I thought of it more in terms of just achieving more spacing between the center and side omnis than I could otherwise, by pushing the center position forward, and also as a modification of the traditional Decca tree arrangement with a fourth microphone spaced out to the rear forming a diamond arrangement.
« Last Edit: November 02, 2021, 04:05:23 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<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline heathen

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Re: Relationships between spaced omnis and center mic
« Reply #51 on: November 24, 2021, 09:53:05 AM »
Here's an example I made:  https://archive.org/details/garajmahal2018-05-11.AT4031.CA14

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Garaj Mahal
Be On Key Psychedelic Ripple
Denver, CO
May 11, 2018

Source 1: Audio-Technica AT4031s (XY at ~100*) > Zoom F8 (24/48 wav)

Source 2: Church Audio CA14 omnis (spread ~5') > Church Audio CA9200 (+10 gain) > Zoom F8 (24/48 wav)

Location: First column closest to stage (~10' from stage), ~7' high

Post: Mixed sources 1&2 (source 2 at -7 dB compared to source 1), amplify, fades, track splits, and conversion to flac using Audacity; File tagging using Mp3tag; ffp created using Trader's Little Helper
Mics: AT4050ST | AT4031 | AT853 (C/SC) | Line Audio CM3 | Sennheiser e614 | Sennheiser MKE2 | DPA 4061 Pre: CA9200 Decks: Zoom F8 | Roland R-05

Offline dyneq

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Re: Relationships between spaced omnis and center mic
« Reply #52 on: November 24, 2021, 02:46:35 PM »
Here's an example I made:  https://archive.org/details/garajmahal2018-05-11.AT4031.CA14

Thanks for the nice example; sounds fantastic! 10' from stage in a small, relatively quiet club helps ;^)

Did you choose 100* in order to PAS?

Offline heathen

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Re: Relationships between spaced omnis and center mic
« Reply #53 on: November 24, 2021, 08:57:55 PM »
Yeah, I was aiming for PAS and 100* was my estimate of what that was.  The driving force was the PAS part not the numerical part, if that makes sense.

Funny you should call that place relatively quiet...it often was anything but.  I think attendance was somewhat sparse that night though IIRC (or at least sparse compared to what I thought the band deserves...if there was any justice in the world a band of that caliber wouldn't even be able to fit the audience they'd draw into that place, but I digress).

Also note there are other sources available, like Scott's on stage DPAs.  That could make for interesting comparison, even if it's not necessarily like-for-like.
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Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #54 on: December 29, 2022, 12:34:50 PM »
Reworking my recording rig to install Movos in place of BAS windscreens in preparation for a recording at an amphitheater earlier this month, I spent some time checking the current spacing relationships between microphone positions, modeling it's various 3-position relationships in the online Schoeps Image Assistant simulations and checking those against what Image Assistant suggests as optimal for the taping positions I commonly encounter.

It's really interesting to play around with 3-position arrays in Image Assistant enough to get a better feel for what changes actually effect which aspects, by how much, and how changing multiple variables tends to cause different aspects to interact or not.  Of the aspects it models, IA implies that center microphone spacing forward of the L/R pair primarily effects imaging, and that center microphone pattern doesn't matter much.

Now imaging isn't everything, and is not even most important IMO (even with regards to IA where I'm careful to check diffuse-filed correlation in addition to image linking), but in general Image Assistant suggests a more forward spacing of the center mic position than most of us are using.  I found myself juggling multiple variables and looking at the resulting plots, and realized I may be able to derive a table for 3-microphone-position PAS similar to the one for improved 2-microphone PAS.  Similar to Improved 2-ch PAS, it would include choice of pattern for the L/R pair, and because IA suggests that choice of center mic pattern is not strongly correlated with the other variables, I suspect the resulting table may be equally applicable to a coincident pair in the center in place of a single microphone. 

I considered starting a new thread to discuss this moving forward, but this one seems the most appropriate place to do so.  I will likely post in the current OMT thread additionally, specifically on how this may apply to arrays of 4 or more channels.  If it works out, I'll update the Improved PAS thread to include tables for both two and three position PAS. Stay tuned..
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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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^ I've been playing around with this a bit more, working toward a PAS table for 3-position microphone configurations, consisting of 3 or 4 channels: 3 if using a single center mic, 4 if using a coincident center pair.  So far I find it generally advantageous to position the center microphone or coincident pair about 20cm (8") forward of the baseline between spaced omnis, which works for a relatively wide range of omni spacings.  Overly narrow spacing benefit from more forward center spacing.  More details to follow..
« Last Edit: January 11, 2023, 12:30:16 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<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline vantheman

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Re: Relationships between spaced omnis and center mic
« Reply #56 on: January 16, 2023, 11:47:53 AM »
Forgive me if this is off topic or too specific for the thread, but I’m testing things out for a show this coming week, various combinations of omnis and wide cards (all Line Audio). Single or coincident cards in the middle and omnis about 28” apart as a starting point. I’m loving what I’m hearing with the coincident pair but I’m wary that I might think that simply because it’s a bit louder. I’m recording a rock trio so a really strong, straightforward center could work really well but it could be interesting to have them all more spaced.

For my question, are there rooms where I don’t want to deploy omnis? It’s a very small, 50 capacity room. The band will be very close together. I’ll be at stage lip and close miking. What is concerning me is that I could run wide NOS with the cards and know I’ll have a nice spread with space in the middle to mix vocals later, and I could run omnis as a second option and just take the better of the two. But if omnis are inherently no good in such a small space, and I go all in with LCR (preferable, all things being equal), then I’ll be in trouble if it fails.
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Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #57 on: January 16, 2023, 02:15:55 PM »
If you are liking what you are hearing from the 28" spaced omnis + center coincident pair configuration, but are concerned about using omnis in a small, reverberant room that has challenging acoustics, and have an additional pair of directional mics available, substitute the extra pair of directional mics angled +/-45 degrees in place of the omnis while otherwise keeping the configuration the same.  With that 28" spacing you can probably angle them as tightly as +/-30 degrees or so when necessary, say if taping from the audience and that gets them pointed more PAS.  But depending on the stage setup, when recording at stagelip +/-45 degrees is probably about right, as it is likely to be from a recording position that is otherwise not overly distant.

Your 28" spacing is close to the spacing I use for a +/-45 degree directional L/R pair positioned between the center coincident pair and wider-spaced omnis, which I record as channels 5&6.  I basically keep the center and near-spaced directional portion of the array unchanged and include the omnis if/when appropriate.

In your case, think of this as substituting a directional pair for the omnis in situations where omnis don't work well.  Combination with the coincident pair in the center allows for more flexibility of spacing and angle in the "near-spaced" directional pair than would otherwise be possible with a 2-channel near-spaced pair used on its own in isolation (such as NOS, DIN, ORTF or whatever).  A bit more or less spacing and a bit more or less angle can work fine.  Still, a good starting point for spacing a near-spaced pair intended to work in combination with a center coincident pair is twice as much spacing as would typically be used for 2-channel near-spaced pair on its own.  The 28" you specify is right in that ballpark. 

NOS's 12" spacing is optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair, but isn't spaced enough to work especially well in combination with a center mic position. EDIT- (that statement is too strong) NOS's 12" spacing was originally intended and optimized for use as a single near-spaced pair without the inclusion of a center mic or coincident pair. It isn't spaced enough to work really optimally in combination with a center mic position, even though the inclusion of a center mic may very well serve to improve the recording over 2ch NOS alone.  Your call on whether 2-ch NOS or 3 or 4chs consisting of 28" wide L/C/R using a coincident center pair is the way to go. If you are able and its not a hassle to do so, record six channels including the omnis as well.

Also as mentioned, when including a center coincident pair, best to place it somewhat forward of the other pair or pairs if you can.
« Last Edit: January 19, 2023, 11:06:38 AM 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<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline vantheman

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Re: Relationships between spaced omnis and center mic
« Reply #58 on: January 16, 2023, 08:30:20 PM »
Sadly I don’t have an extra pair for this purpose but that’s a great tip for the future. I actually misspoke and it’s a “wide ORTF” mount from SRS that I mentioned worked well with mixing in vocals, etc, to fill in the center, but I think your advice still applies. How does one determine that a room is too small for omnis? Would that depend heavily on SPL?
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Offline Gutbucket

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Re: Relationships between spaced omnis and center mic
« Reply #59 on: January 17, 2023, 11:41:06 AM »
^ Generally a room that sounds overly reflective and boomy.  Sometimes hard to tell without making a recording and assessing it that way, essentially getting to know the room and the usable recording locations. In my experience its easier for me to tell which rooms are likely to be good, only because some that seem like they'd be terrible aren't so bad after all.  A lot depends on recording location, the PA, the sound guy..  It can depend somewhat on SPL, I've frequently recorded in rooms that worked decently for acoustic and lightly amplified stuff but turned into a mess with more amplification.  You can sort of hear the room "load up" and get "boggy and messy", particularly in the lower frequencies.

But I often get away with using omnis from a closer position even in a not so good room.  The less good the room, the closer they need to be.  At one extreme, on stage is usually okay almost anywhere.  At the other, from a distance, I'd want to be outside for omnis work really well.  Put another way, the imperative of a good recording location being the most important thing becomes even more important.  Omnis will often work nicely from the down-front audience sweet-spot where we'd prefer to record from but may not be able or willing to do so.  Positions where the level of clean direct sound is proportionately much greater than the indirect reverberant sound.
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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