Page 264 - Master Handbook of Acoustics

maximum at the drape and the greatest frictional losses occur. The same effect occurs at odd multiples
of λ/4, such as 3λ/4, 5λ/4, 7λ/4, and so on. At a half-wavelength distance, particle velocity is at a
minimum; hence absorption is minimum. In practice, because drapes usually have some degree of
fold, the quarter-wavelength point occurs at different distances from the wall. Thus, the frequency

peaks and nulls tend to be broadened, with less specific effect on overall response.
In Fig. 12-17B, the spacing of the drape from the wall is held constant at 12 in as the absorption is
measured at different frequencies. The same variation of absorption is observed: maximum when
spacing from the wall is at odd quarter wavelengths and minimum at even quarter wavelengths. At
this particular spacing of 12 in (1 ft), a wavelength of spacing occurs at 1,130/1 = 1,130 Hz, a quarter
wavelength at 276 Hz, a half wavelength at 565 Hz, and so on. When referring to quarter

wavelengths, sine waves are inferred. Absorption measurements are invariably made with bands of
random noise. Hence we must expect the variations of Fig. 12-17B to be averaged by the use of such
bands.
2
Figure 12-18 shows reverberation chamber measurements of the absorption of 19 oz/yd velour.
The solid plot is presumably for a drape well removed from any walls. The other plots, very close
together, are for the same material spaced about 4 and 8 in from the wall. The 4-in distance is one
wavelength at 3,444 Hz; the 8-in distance is at 1,722 Hz. The odd multiples of both the 4- and 8-in
quarter wavelengths are seen on the upper part of Fig. 12-18.