Acoustic Lining Concepts and Materials for Engine Ducts

Abstract

The requirement for attenuating engine fan noise is summarized. The inlet and duct environment to which linings are exposed in a typical turbofan engine and the noise spectrum to be attenuated are reviewed. Concepts of acoustic linings and their mechanisms for attenuating the propagation of sound in ducts are discussed with particular application to turbo-machinery. The procedure used for screening and selecting suitable acoustical facing materials for linings is discussed. This is followed by a summary of the experimental methods used for this procedure to obtain the specific flow resistance, specific acoustic impedance, and the acoustic absorption coefficient. Suitable materials were evaluated as part of duct linings using a laboratory flow-duct apparatus in which linings were subjected to sound levels up to 155 dB over-all SPL and airflows up to Mach number 0.4. Parametric studies are then discussed showing the important acoustic and geometeric characteristics that influence the acoustic attenuation of linings. A comparison is made of the attenuation of linings obtained in the flow duct and in a full-scale engine. The acoustic materials evaluated in the study are reviewed. Finally, a description is given of a theoretical method developed for predicting the attenuation of linings, and compared with experimental measurements.