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242
V. O. KNUDSEN
[J. S. M. p. E.
form absorptivity. By removing approximately two-thirds of the absorptive material from the ceiling and applying it to two adjacent walls, there was reasonably good agreement between calculated and measured times of reverberation. In general, it is desirable to distribute the absorptive material in a room, especially in a small room, so that the rate of decay will be approximately the same in all directions. This not only makes the measured reverberation approximate the calculated value, but also contributes to better acoustics.
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FIG. 6. Distribution of sound pressure in a six-foot cubical chamber excited by a tone of 162 cps. generated by a cone type loud speaker located in one corner of the chamber. The chamber is vibrating in the gravest mode involving all three dimensions. At top left is the distribution over a plane one foot from one end of the chamber; top center is the plane two feet from this end, etc.
The writer and two of his students, R. Neil and C. Hendrickson, have investigated the intensity distribution of sound in a six-foot cubical chamber at frequencies corresponding to several of the lowest eigentones. For the gravest mode of vibration involving all three dimensions, that is for p = q = r = I, in which case the frequency n is 162 cps. for the six-foot cubical chamber, the alternating pressure in the sound-field is a maximum at (or very near) the boundaries of the chamber and diminishes to zero (nearly) at the three planes that divide the chamber into eight equal cubes. Fig. 6 exhibits the pressure distribution over a series of six planes parallel to one wall of the