Page 219 - The Master Handbook Of Acoustics
P. 219

194   CHAPTER NINE









































                    FIGURE 9-11
                   Sonex contoured acoustical foam simulating anechoic wedges. This is an open-cell type of foam.


                                      The distance a drape is hung from a reflecting surface can have a
                                   great effect on its absorption efficiency. This is best explained by Fig.
                                   9-15. In Fig. 9-15A a drape or other porous material is hung parallel to
                                   a solid wall, and the distance d between the two is varied. The fre-
                                   quency of the sound impinging on the porous material is held constant
                                   at 1,000 Hz. 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. Looking at the situation closely reveals that the wave-
                                   length of the sound is related to maxima and minima of absorption.
                                   The wavelength of sound is the speed of sound divided by frequency,
                                   which in the case of 1,000 Hz, is 1,130/1,000   1.13 feet or about 13.6
                                   inches. A quarter wavelength is 3.4 inches, and a half wavelength is
                                                                                        1
                                   6.8 inches. We note that there are absorption peaks at   4 wavelength,
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