Acoustic impedance measurement with grazing flow

Abstract

Lasting increasing of air traffic will only be feasible if noise pollution near airports, due to aircraft flyover, is significantly reduced. Among acoustic emissions from the aircraft, engine noise, and particularly fan noise, is a preponderant source. In order to reduce this noise, the nacelle is lined with absorbing materials made of a thin layer (such as perforated plate) bounded to partitioned air cavities. These treatments are submitted to high sound pressure levels (up to 160 dB) and high flow Mach numbers (up to 0.7). The key design parameter is the acoustic impedance of treatment panels, which depends on sound frequency, sound pressure level and flow Mach number. Acoustic impedance properties are mainly quantified by semi or totally empirical formulas, which depend on the experimental set-up used and are specific for each type of tested layer. Therefore, EADS acoustic laboratory has got an experimental set-up measuring acoustic impedance of liners used on Airbus aircrafts. This paper presents some of existing models for perforated plates impedance, describes the experimental set-up used and shows some results that are compared with empirical models from literature. It is shown that models valid up to Mach 0.2 seem to be valid up to Mach 0.6.