Acoustic-Lining Concepts and Materials for Engine Ducts

Abstract

A summary is given of a research program to develop acoustic-lining technology for engine ducts. The environment to which linings are exposed in the inlet and duct of a typical turbofan engine installation and the noise spectrum to be attenuated are described. Concepts of acoustic linings and their mechanisms for attenuating the propagation of sound in ducts are discussed, with particular application to turbomachinery. The procedure used for screening and selecting acoustical facing materials for linings is reviewed. A summary is given of the experimental methods employed in this procedure—measurement of the specific flow resistance, the specific acoustic impedance, and of the acoustic absorption coefficient—and a synopsis of the results is made. Suitable materials were evaluated as part of duct linings in a laboratory flow-duct apparatus, simulating an engine-duct environment in which linings were subjected to sound levels up to 155 dB over-all sound-pressure level (SPL) and airflows up to Mach number 0.4. The most important acoustic and geometric characteristics that influence the acoustic attenuation and the results obtained are summarized. The attenuation of linings in a laboratory flow duct and in a full-scale engine are compared. Finally, a description and some results of a theoretical study developed for predicting the attenuation of linings is given and compared with the experimental measurements.