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Offline heathen

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Boundary mount angle questions
« on: May 31, 2018, 03:29:25 PM »
My caveman-level understanding of boundary mounting a mic is that (assuming the mic is properly mounted so it's close enough to the boundary) it reinforces the sound coming towards the boundary by reflecting it back to the mic.  So what I'm wondering about is how the angle between the boundary surface and the direction of the sound source comes into play.  It would seem to me that when the boundary surface is 90* perpendicular to the sound source, the sound arriving directly from the source is reinforced to the greatest degree possible, and when the boundary surface is parallel with the direction of the sound source there would the least amount of reinforcement.  If that's correct, then the amount of boundary surface effect would increase as the angle of the surface is turned back closer to 90* perpendicular, right?

With respect to the above I'm basically ignoring reflected sounds from other surfaces in the room.  But when those are taken into account, they would also be reinforced by the boundary, right?  So, if the boundary surface is parallel to the direct sound source, there would be minimal boundary effect with respect to the direct source, but maximum effect with respect to sound reflected off a side wall, right?

I realize my understanding of all this is "like chimps at their first fire."  The information I've been able to glean from Googling hasn't quite cleared up the things I ask about above though (obviously).
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Offline heathen

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Re: Boundary mount angle questions
« Reply #1 on: June 13, 2018, 02:34:59 PM »
Any thoughts about this?
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Offline kuba e

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Re: Boundary mount angle questions
« Reply #2 on: June 13, 2018, 04:35:54 PM »
I imagine it as following. If that's not right, please correct me.

The sound source causes a pressure waves in the air. The air molecules oscillate, approach each other and move away from each other. They have a certain velocity and create pressure between themselves. Near the surface, they have no place to move. So the energy of the velocity is transformed into pressure.

Edit:
Gutbucket explained it below.

This is why the acoustic pressure is higher in the boundary layer and the microphone has a stronger signal.
My guess is that the sound wave angle does not have a big impact because the molecules will not slip on the surface. But I can be wrong.

The main advantage of the boundary microphone is that you eliminate the reflection from the surface. E.g. when you have a microphone at some distance from the back wall, you first encounter the direct sound and then the reflected sound from the back wall. With the boundary microphone on back wall, you avoid this reflection and encounter only direct sound (and reflections from other surfaces).
« Last Edit: June 14, 2018, 03:27:09 PM by kuba e »

Offline Gutbucket

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Re: Boundary mount angle questions
« Reply #3 on: June 13, 2018, 08:06:34 PM »
The main thing is that sensitivity to direct arriving sound is increased by 6dB while sensitivity to indirect reverberant sound increases by 3dB.  Its that 3dB increase in direct/reverberant ratio which is the primary aspect of the boundary mounting "magic trick".

How much does boundary to source angle matter?  It probably does some near the far edges but I dunno how much or how to quantify it.  The occlusion effect of going past any boundary edge will be far bigger.  Probably best to try it in whatever situation you are thinking about to get a good empirical seat of the pants feel for what is happening.
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Offline ycoop

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Re: Boundary mount angle questions
« Reply #4 on: June 14, 2018, 04:55:29 AM »
Found this on a Schoeps page:

Quote
Boundary layer microphones have remarkably uniform frequency response for all angles of sound incidence. This provides an especially natural-sounding reproduction of room sound. Additionally, the boundary layer effect offers an increase in sensitivity.
https://schoeps.de/en/products/categories/boundary
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Offline heathen

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Re: Boundary mount angle questions
« Reply #5 on: June 14, 2018, 08:22:05 AM »
Thanks guys!
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Offline Gutbucket

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Re: Boundary mount angle questions
« Reply #6 on: June 14, 2018, 09:05:28 AM »
I think of boundary mounting in terms of 3d polar-pattern visualization as cutting whatever pattern you place against the boundary in half down the middle.  Omni becomes hemispheric.  Cardioid half an apple, etc.  Or maybe mirroring or folding in half, as that's somewhat more analogous to the sensitivity doubling across in the remaining half of the original polar pattern.
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Offline Gutbucket

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Re: Boundary mount angle questions
« Reply #7 on: June 14, 2018, 09:12:06 AM »
BTW, the +6dB direct / +3dB indirect increase in while sensitivity is "per boundary".  Imagine the microphone flush mounted in the center of a big flat 2-dimensional wall.  If you put the microphone in the corner between two boundaries the increase in sensitivity is doubled to +12 direct / +6dB indirect.  If you place in the apex of a 3-way corner (2-walls + ceiling or floor) sensitivity increase is +18/+9dB.
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Offline kuba e

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Re: Boundary mount angle questions
« Reply #8 on: June 14, 2018, 03:33:56 PM »
Thanks for clarification. Does it mean that only sound coming from perpendicular direction (for one boundary) has ratio +6dB direct/+3dB indirect in boundary layer? Have the sounds coming from other angles smaller ratio of direct/indirect?
« Last Edit: June 14, 2018, 03:35:43 PM by kuba e »

Offline Gutbucket

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Re: Boundary mount angle questions
« Reply #9 on: June 14, 2018, 03:49:21 PM »
It means that more-or-less regardless of the angle of incidence, sensitivity is increased twice as much for direct-arriving sound compared to the diffuse reverberant sound which arrives from all directions.

Quote
Does it mean that only sound coming from perpendicular direction (for one boundary) has ratio +6dB direct/+3dB indirect in boundary layer?
^Yes (but not only)
Quote
Have the sounds coming from other angles smaller ratio of direct/indirect?
^No (not significantly anyway)
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Offline kuba e

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Re: Boundary mount angle questions
« Reply #10 on: June 14, 2018, 04:31:13 PM »
Please, one more question. Why are direct sounds increased by +6dB in boundary layer while reverbant sounds only by +3dB? When it does not matter the angle of incidence, what causes this difference?

Offline Gutbucket

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Re: Boundary mount angle questions
« Reply #11 on: June 14, 2018, 05:36:12 PM »
Good question!

I have to think about it, and maybe research that a bit.. 

Lets see, the presence of the boundary creates a localized pressure build up which is what increases sensitivity.  Direct sound arrivals are coherent waveforms and reverberant diffuse sound is incoherent.

Incoherent means the reverberant sound arrives essentially as innumerable, indistinct waveforms with random phase relationships from all directions at once, and these get superimposed upon one another at the microphone diaphragm.  Since their phase-relationships are random, the the constructive and destructive interference of these innumerable arrivals are averaged, resulting in a 3dB sensitivity increase for non-coherent sound arrivals (reverberation). Whereas with phase-coherent direct arriving sound, the phase relationship between the direct arrival and its coincident reflection off the boundary are in phase with each other and constructively interfere, resulting in a +6dB gain without the destructive interference.

I think that's it.

This may address it (I read it years ago, and just found again with a quick search)-
THE OMNIDIRECTIONAL BOUNDARY MICROPHONE by Bruce Bartlett Copyright 2009
https://cdn.shopify.com/s/files/1/0247/3799/files/boundary_mics_a9abcfe4-9502-4ccf-a277-65df0b61c770.pdf?1690
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Offline kuba e

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Re: Boundary mount angle questions
« Reply #12 on: June 14, 2018, 06:14:31 PM »
Thank you very much for the answer and for the link. I will read it. It makes sense, I just need to think a little bit about it.

Offline heathen

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Re: Boundary mount angle questions
« Reply #13 on: June 14, 2018, 06:25:28 PM »
Maybe I'm thinking of this too much in terms of vectors like pool balls headed for the rail, but surely there must be some extreme angle where the direct sound just glances off the boundary and doesn't get the +6 dB boundary effect, right?  And if that's true (IF!), does the angle gradually increase the boost up to +6 as the angle moves from that extreme to being perpendicular to the direct sound source?
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Re: Boundary mount angle questions
« Reply #14 on: June 15, 2018, 07:23:24 AM »

GB thank you for the link and explanation!
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Offline kuba e

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Re: Boundary mount angle questions
« Reply #15 on: June 15, 2018, 09:15:03 AM »
Lets see, the presence of the boundary creates a localized pressure build up which is what increases sensitivity.  Direct sound arrivals are coherent waveforms and reverberant diffuse sound is incoherent.

Incoherent means the reverberant sound arrives essentially as innumerable, indistinct waveforms with random phase relationships from all directions at once, and these get superimposed upon one another at the microphone diaphragm.  Since their phase-relationships are random, the the constructive and destructive interference of these innumerable arrivals are averaged, resulting in a 3dB sensitivity increase for non-coherent sound arrivals (reverberation). Whereas with phase-coherent direct arriving sound, the phase relationship between the direct arrival and its coincident reflection off the boundary are in phase with each other and constructively interfere, resulting in a +6dB gain without the destructive interference.

It is right! I would not have done it myself.
For others like me, here is text with little graphics about combining coherent and incoherent waves.
https://physics.stackexchange.com/questions/389058/why-do-two-coherent-sounds-add-up-6db

Maybe I'm thinking of this too much in terms of vectors like pool balls headed for the rail, but surely there must be some extreme angle where the direct sound just glances off the boundary and doesn't get the +6 dB boundary effect, right?  And if that's true (IF!), does the angle gradually increase the boost up to +6 as the angle moves from that extreme to being perpendicular to the direct sound source?

If I understand correctly now ...

When sound wave hits the mic's diaphragm at any angle, it should have theoretically always gain +6dB. I imagine it like it is described in the document linked by Gutbucket. The mic is facing down to the surface and there is small gap between the mic and the surface. The direct wave and it's reflected wave from the mounting surface are combined together. The direct and reflected wave of "sound source" are in phase (because gap between mic and surface is small) and are combined in the sum by gain +6db. On the other hand, the direct and reflected waves of "reverberating sounds" have gain +6dB too, but are incoherent and are combined in the sum of gain only +3db.

When the mic is aligned with the surface and facing up, I imagine the gain +6dB as transformation of kinetic energy of air molecules in boundary layer to pressure energy (the mic's signal correspond to the pressure on the mic's diaphragm). And the combining of the incoherent waves of gain +6 dB is adding only gain +3dB in the sum.

« Last Edit: June 15, 2018, 09:22:39 AM by kuba e »

Offline Gutbucket

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Re: Boundary mount angle questions
« Reply #16 on: June 15, 2018, 10:04:30 AM »
Maybe I'm thinking of this too much in terms of vectors like pool balls headed for the rail, but surely there must be some extreme angle where the direct sound just glances off the boundary and doesn't get the +6 dB boundary effect, right?  And if that's true (IF!), does the angle gradually increase the boost up to +6 as the angle moves from that extreme to being perpendicular to the direct sound source?

Not sure the pool ball analogy is apt here, but pool balls still behave as vectors at even quite shallow angles to the rail.

I'll relate a situation from my own experience where the sound sources were predominantly positioned at very shallow angles to the boundary- In combination with a bunch of other mic techniques, I used to boundary-mount miniature omnis on a hard wooden stage surface for a series of non-PA amplified jazz trio gigs.  In that case the mic(s) were placed on the wooden floor (a hard boundary facing upwards) in front of the band, and the guitar amp, drum kit and small acoustic bass amp were sitting on the floor at various distances from the microphones.  As seen from the mic(s), the uppermost edges of the more distantly placed guitar and bass amps had to be no more than 5 degrees above parallel.  The drums in the kit ranged from 1 or 2 degrees (bottom edge of the kick drum low to the floor) up to 30 degrees or so above parallel with the cymbals up to maybe 45 degrees or so above parallel.  The acoustic bass was closer than its amp and the bass body probably ranged from 10 to 45 degrees or so above parallel.  Except for the occasional guest sax player and stage banter from nearly directly above the mics, all sound sources were at the very least 45 degrees away from perpendicular, with the majority of them much farther away from perpendicular.  The sound was very clean and clear for all sources.  I was especially astounding by how clearly the guitar amp came through, which was the furthest source 15 feet away or more, and I remember thinking that it sounded like the sound almost flowed more easily across the floor than through the open air.  That guitar amp was basically at a near right angle to the boundary surface.

I really liked placing the mics (not just boundary mounted, but more typical stereo setups) very close to the floor partly for this reason.

My seat of the pants take away from this, from other boundary mounting experiences, and from experimenting with close mounting mics to the surface of Jecklin disks and baffles that have soft fuzzy sound absorbing surfaces, is that the surface of the boundary matters and is likely to be the largest contributing factor for far off axis angles.

If the surface is hard, smooth and reflective (which it is supposed to be for proper boundary mounting effect), source angle away from the boundary plane doesn't seem to matter much if at all.  If the surface is soft, fuzzy, furry, and sound-absorbing, the behavior is different.  The highs are reduced, and the response seems to change more as the source angle gets close to parallel.  Not overly surprising I suppose.
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<<

 

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