An experimental procedure is suggested to determine the directional scattering coefficient (DISC) or directivity balloon which fully represents the backscattered energy from any acoustical surface as a function of the incident frequency and direction. For absorbing materials, this directional information complements the random-incident absorption coefficients, determined using the ASTM reverberation chamber method, and upon appropriate integration should also yield the random-incidence value. In the case of reflecting, diffusing, and diffractingsurfaces these data will begin to fill a void in the acoustics literature. The DISC is necessary in the design of critical listening and performance environments, where strategic rather than statistical application of acoustical materials is desired, and where both magnitude and phase information are necessary. Also a directional, rather than a random-incidence, scattering parameter is of more significance in the various image and ray tracing computer room modeling programs to improve the accuracy of impulse response predictions and auralizations, derived from convolutions of anechoic program material with these impulses. A new automated polar mapping technique, which is capable of yielding the full three-dimensional directivity balloon, is described. It is hoped that the suggestions made will eventually lead to an ASTM standard and that these data will improve the accuracy of predictive as well as actual acoustical designs.
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