The Design of Sound Absorptive Materials to Meet Special Requirements

Abstract

Bulk sound-absorbing materials are not suitable for many applications involving liquids, air flow, elevated temperature, vibration, and severe space limitations. In aircraft all of these design constraints can occur together. The simplest alternative to the bulk absorber is the laminar absorber, comprising a permeable facing sheet over a closed compartmented air space. Laminar absorbers meet some of the above design requirements but their impedance-versus-frequency characteristic is inherently inferior to that of a good bulk absorber. A wide variety of improved absorption characteristics may be attained by utilizing more acoustical elements and parallel as well as series connections among the elements. The use of resistance paralleled by inductive resistance provides a facing sheet whose resistance increases with frequency while the positive reactance decreases. Dissimilar air cavities arranged in parallel provide air space reactance that remains near zero over a broad range of frequency. The cutoff characteristics of miniature acoustic horn arrays may be exploited to provide a dual-frequency range of absorption. The upper frequency range resembles that of a laminar absorber, whereas the low-frequency absorption occurs in a controllable region of considerable bandwidth. These novel sound-absorbing structures may provide practical solutions to such diverse problems as fan jet engine buzzsaw noise, combustion noise, and noise due to cooling towers and car wash installations.