Abstract
An indirect experimental check is now obtained for the calculated “shape factor” for an absorbing strip of material in a reflecting wall, reported previously The effect of the strip on each of the component “partial waves” in the incident plane wave is studied by placing the strip in a special enclosure, the shape of which allows the partial waves to be studied individually as isolated normal modes of vibration. A semicylindrical room is employed; its concave wall tends to “focus” sound upon the center of the opposite, flat wall on which the absorbing panel is centered. If the wave equation is set up in elliptic cylinder coordinates with the edges of the panel as foci, the boundary conditions are readily satisfied. The theory is strictly applicable to a semielliptical room only, but holds well in the semicircular for strip widths up to one-third the diameter. The experimental enclosure has a 14-inch radius, a “thickness” of 4 inches between the parallel walls, and is constructed with highly reflective metal surfaces. The selective damping of various normal modes is investigated for strip widths from about one to ten inches, for several values of normal acoustic impedance. The results substantiate the calculated values of “shape factor” which were given in the previous paper.
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