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Boundary mount angle questions

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heathen:
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).

heathen:
Any thoughts about this?

kuba e:
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).

Gutbucket:
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.

ycoop:
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.

--- End quote ---
https://schoeps.de/en/products/categories/boundary

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