Nonlinear effects on tonal sound in lined ducts

Abstract

Porous sound absorbers are a familiar and supposedly well-understood feature of the noise control palette. As in many other aspects of engineering, however, aerospace applications require a degree of mastery well beyond the common place: sound absorbers must be designed to be effective at high noise levels within strict mass and volume constraints. In the process of fine-tuning sound absorbers for jet aircraft, it was discovered that high incident sound pressure levels cause the performance of lined ducts to diverge from small-signal predictions. The underlying physics was not fully understood prior to the early 1980s, at which point NASA-sponsored research disclosed and quantified the primary mechanism. Further research by others continues building on that foundation down to the present. This paper gives a brief overview of the topic, summarizes one of the research efforts, and adapts its findings for lined duct applications.