Wedge Design for National Bureau of Standards Anechoic Chamber

Abstract

Design techniques for anechoic room wedges must always be used with caution because of the possibility that a manufacturer may change the properties of the material. This paper describes efforts at the National Bureau of Standards to adapt to relatively new glass wool manufactured in the United States. A “hybrid” wedge designed for a large anechoic chamber at NBS consists of glass wool of two densities: 3 lb/ft3 for the 55-in.-long tapered portion and 1.1 lb/ft3 for the 11-in.-long base. Experimental evidence indicates that the light-weight material and the 4-in air space behind the wedge, in conjunction with the heavier material, serve to produce a useful resonance absorption. The “cutoff frequency” attained for the 70-in.-long structure was about 45 Hz as measured in a 31-ft plane-wave tube. Normal acoustic impedance measurements looking into the wedge from the tip were made for the hydrid wedge and a half-scale model.