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Room Modes

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DonH56

DonH56

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Loose fibers = pink fluffy insulation
Compacted fibers = Corning OC-70x panels (commonly used to make absorbers), Rockwool, etc.
Solid fiberglass is used for car bodies, boats, musical instrument cases, etc.

Note an air gap can significantly increase absorption. At a boundary (e.g. wall, ceiling) velocity is ideally zero (and pressure is maximum). Since common absorbers work by converting sound "velocity" (kinetic energy) into heat (not enough to warm them up) they work better placed a little off the wall where the sound waves exhibit higher velocity. I have 2" soft foam blocks in the corners of mine to hold them away from the wall a bit.

That said, at the low fundamental mode in most rooms, absorbers won't do much unless you have a lot of (very thick) ones in the room.

FWIWFM - Don
 

incoherent

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Don, I'm new to the ASR forums and am really enjoying the tutorials you have written. It is obvious you carefully developed them, and they are very helpful.

Finally, when the sound wave hits a reflecting surface, it inverts and takes off at an angle equal to the incoming angle but on the other side: it makes a triangle.

I wanted to bring up the point you made about reflections from a surface. I do not believe there will be a phase inversion for the reflection of an in-air plane wave incident on a nearly rigid surface. The normal incidence pressure reflection coefficient is R = (z2-z1)/(z2+z1), where z1 and z2 are the specific acoustic impedance of the air and the wall, respectively. In the case of a rigid wall, z2 >> z1, so the pressure reflection coefficient will be very close to +1. If the media were swapped so that the plane wave was propagating in the wall and incident on the air boundary, then there would be a phase inversion for the wave reflected back into the wall with a reflection coefficient very close to -1.
 
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DonH56

DonH56

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Don, I'm new to the ASR forums and am really enjoying the tutorials you have written. It is obvious you carefully developed them, and they are very helpful.
Thank you!

I wanted to bring up the point you made about reflections from a surface. I do not believe there will be a phase inversion for the reflection of an in-air plane wave incident on a nearly rigid surface. The normal incidence pressure reflection coefficient is R = (z2-z1)/(z2+z1), where z1 and z2 are the specific acoustic impedance of the air and the wall, respectively. In the case of a rigid wall, z2 >> z1, so the pressure reflection coefficient will be very close to +1. If the media were swapped so that the plane wave was propagating in the wall and incident on the air boundary, then there would be a phase inversion for the wave reflected back into the wall with a reflection coefficient very close to -1.

You are correct; I need to go fix that. :facepalm: My background is mostly RF/mW stuff so I tend to think of wave transmission differently than acoustic waves. Rookie mistake, will fix, thanks for catching that.

Edit: Done. Back to shoveling snow...
 

Andysu

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i need do this again not that really want do it again have other idea that sort of eluded me the sine wave on the scope
 
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