The Absorption of Sound by Rigid Walls

Absorption assumed to be due to porosity, the absorbing walls being so massive that flexural yielding is presumably negligible. Upon this assumption, Rayleigh's theory indicates that the absorption by such a wall is very nearly proportional to the square root of the frequency of the sound.

Absorption is approximately a linear function of the square root of the frequency, as shown by experiments for a range of frequencies between 64 and 4096 periods per second, but the absorption frequency curve does not pass through the origin, and at higher frequencies the absorption is greater than can be accounted for by porosity alone.

Selective transmission of a frequency whose wave-length in the wall is twice the thickness gives a possible explanation. Absorption experiments at still higher frequencies are desirable.