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Offline Massive Dynamic

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New mic config
« on: February 19, 2022, 08:21:38 PM »
Years ago, I had a phone conversation with a speaker designer. When I mentioned that I did concert recording, we talked a bit about microphone configurations. My memory is a bit hazy, but I think he suggested putting microphones in the position I tried to draw in the attachment (triangles indicate capsule position and direction).

If you would typically use a DIN config, consider keeping the microphones @ 90º to each other, but swap positions left to right. I can't recall what advantage this was supposed to provide, but I have never seen any taper use this approach. Has anyone experimented with this type of configuration?
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Offline morst

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Re: New mic config
« Reply #1 on: February 19, 2022, 10:37:53 PM »
Years ago, I had a phone conversation with a speaker designer. When I mentioned that I did concert recording, we talked a bit about microphone configurations. My memory is a bit hazy, but I think he suggested putting microphones in the position I tried to draw in the attachment (triangles indicate capsule position and direction).

If you would typically use a DIN config, consider keeping the microphones @ 90º to each other, but swap positions left to right. I can't recall what advantage this was supposed to provide, but I have never seen any taper use this approach. Has anyone experimented with this type of configuration?
Yeah Ever since I lost my ORTF bar, then lost my X/Y swivel I do this all the time with a stereo bar.
It makes the sign off confusing but sounds great.
"THIS IS THE LEFT-POINTING MICROPHONE" etc
I try for 90 degrees

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Re: New mic config
« Reply #2 on: February 20, 2022, 09:30:55 AM »
so its like a spaced XY on the same horizontal plane ?

Offline aaronji

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Re: New mic config
« Reply #3 on: February 20, 2022, 11:54:59 AM »
Intuitively, this doesn't seem like a great idea. For near coincident configurations, shouldn't the left mic should pick up sound from the left side both earlier (time-of-arrival) and louder (sound pressure level) than the right mic and vice versa?

Offline datbrad

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Re: New mic config
« Reply #4 on: February 20, 2022, 01:43:55 PM »
I have seen PA speakers setup turned inward to minimize reflections in steel barns that sounded awful as a rule. I've run mics like that on-stage with the channels set to match the side they are placed when facing the stage.

Farther out in a concert setting I think this setup would sound just like a NOS with the channels swapped by accident, and poorly aimed to boot.

The farther away sound source that is on axis will be masked over by the louder sound pressure closer to the mic, since cardioid mic sensitivity typically stays the same out to 45 degrees off axis.

If the channels aren't swapped, then that still leaves the stereo recording angle. There would be a heavily centered image that is out of phase, with confusing intensity differences.

Someone here should still try do to it. I've put up all kinds of weird setups in clubs and odd shaped rooms that looked crazy, but somehow achieved what I was trying to isolate or reject.

(Edit to add: I tried to model the setup with Neumann's mic config app and it wouldn't allow the mics to have a 90 degree included angle spaced at all. Fyi)

 
« Last Edit: February 20, 2022, 01:51:45 PM by datbrad »
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Offline morst

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Re: New mic config
« Reply #5 on: February 20, 2022, 01:47:48 PM »
Someone here should still try do to it.


I do this all the time with a stereo bar.

Mixed with a board feed it can come out like this:
https://archive.org/details/ChadGalacticSuperfood2020-01-18

Offline Gutbucket

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Re: New mic config
« Reply #6 on: February 21, 2022, 01:41:57 PM »
That arrangement will not be generally conducive to good imaging.  It can create a situation where the time-of-arrival cues and the level difference cues produced across the two channels contradict rather than complement each other.


The tldr-
How a listener uses those cues to determine direction of the sound source is described by the duplex theory of hearing - which is a model for how our brains determine direction by using the interaural time differences (phase differences) between the two signals arriving at the ears at lower frequencies and interaural intensity differences (level differences) at higher frequencies, and how we switch between those two modes of direction finding in a shared frequency region in between.

In frequency regions that loosely correspond with duplex theory, typical near-spaced stereo microphone configurations that use a pair of directional microphones angled outward (ORTF, NOS, DIN, etc.) generate time-of arrival (phase) differences between channels based on the spacing between microphones and generate level-differences between channels based on the angle between the microphones and their directional sensitivity.  Both of the direction-finding hearing mechanisms of duplex theory are simultaneously in play in a shared frequency region in the middle.  Angling a stereo pair of near-spaced directional microphones inward rater than outward produces opposing directional cues at high and low frequencies and contradictory directional cues in the shared region.  Time-of-arrival phase differences will tend to indicate a source is placed to one side while level differences will tend to indicate the source is on the opposite side.

How much of a mismatch such a near-spaced directional microphone configuration is going to produce will be determined by the spacing between the microphones.  It will be less detrimental at quite small microphone spacings that are significantly smaller than the distance between our ears, because the time-of-arrival / ITD / phase-differences produced by such a close spacing will occur in a frequency region higher than the one in which we are sensitive to direction finding based on phase difference.  This is why small handheld recorders with built in microphones sometimes "get away" with arranging the microphones this way.  It's not an ideal arrangement, yet doesn't produce conflicting directional cues in the shared psycho-acoustic region. Arranging the microphones on the recorder in that way is simply a way of solving the packaging problem of how to compactly arrange an X/Y pair of directional mics on a small device, effectively avoiding the audible imaging problems that it would incur if the spacing between microphones was larger.. such as the spacing used in a typical near-spaced ORTF, NOS, DIN, or whatever stereo pair.

It comes down to a good or not so good fit between the cues produced by the stereo microphone pair and the cues our brain's expect to hear.

I can't think of any good reason for a taper to do this with a near-spaced stereo pair.  I might see someone doing this with an A-B spaced omni pair as a way of tuning high-frequency tonality by keeping the mics pointed at the source to get more high frequency extension on-axis.  But in that case any contradictory level cues will be above the range of directional sensing, and are likely to be swamped by level differences caused by the source being closer to one than the other anyway.


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Offline Gutbucket

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Re: New mic config
« Reply #7 on: February 21, 2022, 02:12:20 PM »
Hi Brad,

A few comments on your post above..

I have seen PA speakers setup turned inward to minimize reflections in steel barns that sounded awful as a rule. I've run mics like that on-stage with the channels set to match the side they are placed when facing the stage.
This can work for wide spaced mic arrangements on stage.  But in that case the mic configuration is wide-spaced rather than near-coincident.  If the mics are near-spaced, the distance to the sidewall reflection points from the opposite side isn't going to be significantly different enough that this would improve the situation.

Quote
Farther out in a concert setting I think this setup would sound just like a NOS with the channels swapped by accident, and poorly aimed to boot.
A channel swapped NOS (or other near-spaced pair arrangement) will just be mirror-image reversed.  You can fix it by swapping L<>R afterward,  It will not have the same contradictory imaging problems as this kind of arrangement which cannot be fixed later by swapping channels.

Quote
There would be a heavily centered image that is out of phase, with confusing intensity differences.
Actually the dead center of the image will have no imaging contradiction problems, as a centered source produces no phase or level differences between channels either way.  The problems will manifest with sound sources that are not arriving at the microphone array along its dead center axis.  If by phase you mean polarity, that's a separate issue and it won't matter which way the microphones are angled (and is correctable afterward either way by swapping the polarity of one channel).

Quote
Someone here should still try do to it. I've put up all kinds of weird setups in clubs and odd shaped rooms that looked crazy, but somehow achieved what I was trying to isolate or reject.
Imaging isn't the most important thing by a long shot.  The resulting recording may not be that bad, but I'd still not recommend it except perhaps in the special cases of omnis and wide-spaced directional pairs on stage.

Cheers!
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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 rocksuitcase

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Re: New mic config
« Reply #8 on: February 21, 2022, 03:12:13 PM »
I'll chime in with a different perspective. If you took the image in the OP and took the mic capsules, and placed the back R/L in the same front/back plane as the front L/R pair, and did this with figure eights, then you would have a variation on "Blumlein" simulating using coincidentally paired crossed fig 8's.

The original idea presented has the microphone pairs too far apart. However, for playback using four speakers, aligning the speakers as if he told you to align the mic capsules might result in a handy dandy "quad simulation" effect.

I tried to crop & edit the OP snap to illustrate, it is poor but maybe can show what I mean:
« Last Edit: February 21, 2022, 03:21:16 PM by rocksuitcase »
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Offline morst

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Re: New mic config
« Reply #9 on: February 21, 2022, 04:36:08 PM »
That arrangement will not be generally conducive to good imaging.

(Snip)
I can't think of any good reason for a taper to do this with a near-spaced stereo pair.
Because that's how the bar is shaped!
Thanks for the breakdown.
In the future, I'll just throw arrival time to the wind, and go for X/Y on the other screw threads.


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Re: New mic config
« Reply #10 on: February 21, 2022, 05:44:08 PM »
^ If those were the only setup options doable with that mic-bar, I can imagine one might value the "sound of a near-spaced non-coincidence mic setup" (even with the compromised imaging it may produce), over "the sound of coincident X/Y" which provides more technically correct directional imaging.  Imaging is not the most important aspect of a recording.

However, if it were me and I didn't want coincident X/Y, I'd just flip the bar around the other way before attaching the mics.  From the looks of it that will provide a minimum spacing of about 11" between capsules in the top photo and about 8" in the bottom photo, with the mics facing outwards.
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Offline Gutbucket

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Re: New mic config
« Reply #11 on: February 21, 2022, 06:29:28 PM »
Hi Kyle,  I'm having trouble following you. 

Just to make sure we are all on the same page here let me try and clarify the diagrams above, both for myself and everyone else (Massive D, please correct me if I'm reading it wrong).  As far as I can tell we are viewing a representation of two stereo pairs as viewed from above.  The triangles represent individual cardioid microphones and the right angle point of each triangle indicates the direction in which each microphone is facing.  The stage or sound source of interest is toward the top of the page. All good so far?

If we move the back pair forward until they are coincident with the front pair, we end up with two forward-facing coincident 90° X/Y positions spaced about 20cm apart (DIN spacing). If we swap all four cardioids for figure-8's we get.. well, it depends on the channel summing..  we get either 1) two Blumlein pairs near-spaced 20cm apart from each other.  That's easy enough to imagine, except I don't expect a near spaced pair of them summed together would be useful. We'd have comb-filtering in both channels, from the sum of both right-facing fig-8's spaced 20cm apart into the right channel, and the sum of both left facing fig-8's into the left channel.  ..or 2) We'd get a single pair of "virtual" figure-8's facing directly forward in a parallel 20cm spaced A-B arrangement.  That's because if we sum the two figure-8's of a Blumlein pair the result is a single virtual fig-8 facing directly forward (the Mid channel of X/Y Blumlein converted to Mid/Side). 

Okay enough of that brain-tease..

If you wanted to emulate a Blumlein pair using four cardioids instead of figure-8s, you'd place them all coincident and at 90° to each other, then swap the L/R routing and invert the polarity of the rear-facing two.

If instead of placing them coincident you were to place them outward facing in a near-spaced square ~20cm on each side, you form an IRT-Cross arrangement, which is a common recording format for 4-channel sound location ambiance beds.  That works better for quad playback as its more open and spacious feeling than if you were to take the coincident four cardioid arrangement and fed each to its own speaker placed in opposing corners.

The inverse of that is fun to think about, and brings us back full circle to situations where a wide-spaced inward-angled arrangement may be applicable- imagine four cardioids placed at the vertices of a big square facing inward toward the center, with the sound sources located inside the square.  Think festival camp jam under a square pop-up tent canopy with a small circle of performers playing under it.  I always wanted to try hanging an ambisonic mic from the apex of such a canopy but never got around to it.  This would be the inverse of that.  In fact, the two techniques might actually work well in conjunction but would require 8 channels.  Craziness!
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 morst

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Re: New mic config
« Reply #12 on: February 21, 2022, 08:12:59 PM »
^ If those were the only setup options doable with that mic-bar, I can imagine one might value the "sound of a near-spaced non-coincidence mic setup" (even with the compromised imaging it may produce), over "the sound of coincident X/Y" which provides more technically correct directional imaging.  Imaging is not the most important aspect of a recording.

However, if it were me and I didn't want coincident X/Y, I'd just flip the bar around the other way before attaching the mics.  From the looks of it that will provide a minimum spacing of about 11" between capsules in the top photo and about 8" in the bottom photo, with the mics facing outwards.
yeah that's a top view. I could just face them outwards from the ends of the bar, sure.

Offline rocksuitcase

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Re: New mic config
« Reply #13 on: February 21, 2022, 10:48:13 PM »
lee, I meant one pair of figure 8's, but spaced like corners of a box which is how I took the OP configuration.

sort of like this
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Re: New mic config
« Reply #14 on: February 22, 2022, 09:41:01 AM »
Reminds me of the Hamasaki Square technique for reveberant mic'ing. Except that's four eights, one at each corner of a square.  I've tried two eights for that, half the HS arrangement with less spacing, but found that pointing the null side of a fig-8 at the stage still picked up too much much sound from the front, so switched to rear-facing cardioids (provided better front rejection), then supercards (better still), realizing the thing I was searching for was reduced sensitivity on average across the entire front hemisphere, rather than a deep but overly narrow null.



Here's an example of IRT Cross, using cardioids or supercards-

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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