Effective Absorption Coefficient of Rectanguar Areas of Absorbent Material

Abstract

Extensive measurements of the variation of effective absorption coefficient with sample area have been made by Chrisler [J. Research Natl. Bur. Standards 13 (August, 1934)]. There is some indication that the measured variation follows a simple law based upon a very elementary consideration of diffraction effects. An attempt is made to deduce such a law from a more detailed consideration of diffraction effects at (a) the single edge of a semi-infinite, absorbent sheet and (b) the two edges of an absorbent strip. An explicit result is obtained in case (a) which leads to a calculated “area effect” of the type observed by Chrisler. The result is, however, of limited validity and cannot be applied to sound striking the absorbent material near grazing incidence. It is shown that case (b) would lead to a result valid for all angles of incidence.