Page 220 - The Master Handbook Of Acoustics
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195
                                                                                ABSORPTION OF SOUND


                      and if we carry it further than indicated in Fig. 9-15A at each odd mul-
                      tiple of quarter wavelengths. Absorption minima occur at even multi-
                      ples of quarter wavelengths.
                         These effects are explained by reflections of the sound from the
                      solid wall. At the wall surface, pressure
                      will be highest, but air particle velocity
                                                                       1.0
                      will be zero because the sound waves can-
                      not supply enough energy to shake the            0.8    4"
                      wall. At a quarter wavelength from the
                      wall, however, pressure is zero, and air         0.6     3"
                      particle velocity is maximum. By placing                   2"
                      the porous material, such as a drape, a        Absorption coefficient  0.4
                      quarter wavelength from the wall, it will
                      have maximum absorbing effect because            0.2
                      the particle velocity is maximum. At half
                      wavelength, particle velocity is at a mini-       0
                                                                         125  250   500  1 kHz 2 kHz 4 kHz
                      mum, hence absorption is minimum.
                                                                                 Frequency - Hz
                         In Fig. 9-15B, the spacing of the drape
                      from the wall is held constant at 12 inches                            FIGURE 9-12
                      as the absorption is measured at different  Absorption coefficients of Sonex contoured foam of
                                                                                4
                      frequencies. The same variation of absorp-  various thicknesses.
                      tion is observed, maximum when the spac-
                      ing is at odd quarter wavelengths and
                                                                       1.0
                      minimum at even quarter wavelengths
                      from the wall. At this particular spacing of
                      12 inches, a wavelength of spacing occurs        0.8
                      at 1,130/1    1,130 Hz, a quarter wave-
                      length at 276 Hz, and a half wavelength at       0.6
                      565 Hz, etc.                                   Absorption coefficient  18 oz/sq yd
                         Figure 9-16 shows actual reverberation-       0.4            14 oz/sq yd
                      chamber measurements of the absorption                            10 oz/sq yd
                      of 19 oz/sq yd velour. The solid graph is        0.2
                      presumably for a drape well removed from          0
                      all walls. The other graphs, very close            125  250   500  1 kHz 2 kHz 4 kHz
                      together, are for the same material spaced                    Frequency - Hz
                      10 cm (about 4″) and 20 cm (about 8″) from                             FIGURE 9-13
                      the wall. The 10-cm distance is one wave-  The sound absorption of velour hung straight for
                                                                                                       18
                      length at 3,444 Hz, the 20-cm distance is at  three different weights of fabric. (After Beranek. )
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