Page 262 - Master Handbook of Acoustics

that folds are introduced until the resulting drape area is half that of the straight fabric. (Mankovsky.)

The distance a drape is hung from a reflecting surface can have a great effect on its absorption
efficiency. In Fig. 12-17A, a drape or other porous material is hung parallel to a solid wall, and the
distance d between the two is varied. The frequency of the sound impinging on the porous material is
held constant at 1 kHz. If the sound absorption provided by the porous material is measured, we find

that it varies greatly as the distance d from the wall is changed. Examination reveals that the
wavelength of the sound is related to maxima and minima of absorption. The wavelength of sound λ is
the speed of sound divided by frequency, which in the case of 1,000 Hz, is 1,130/1,000 = 1.13 ft or
about 13.6 in. A quarter wavelength is 3.4 in, and a half wavelength is 6.8 in. There are absorption
peaks at one-quarter wavelength, and if we carry it further than indicated in Fig. 12-17A, peaks are
present at each odd multiple of quarter wavelengths. Absorption minima occur at even multiples of
quarter wavelengths.