Pressure zone and boundary microphones 1

[otaio]

Dear all,

I would like to understand the physics of why sound pressure doubles at a surface and how the thickness of the this boundary can be calculated.

B

 

[MikeyP]

It is analogous to transmission line theory in electronics, where the instantaneous voltage at the end of an unterminated wire is the double the voltage travelling along the wire.

In the acoustic case, the instantaneous pressure at the interface is equal to the sum of the pressure due to the incident and reflected waves. If the interface is perfectly reflecting, then the normally incident pressure wave and the reflected wave will be idientical (except the direction will be reversed of course) and the instantaneous pressure will be double that of the incident wave.

If the interface is not perfectly reflecting, then there will be some loss at the boundary and possibly a change of phase and the sum of the two waves will be less than doubled.

M