ANALYSIS OF LOW FREQUENCY ACOUSTIC RESPONSE IN A DAMPED RECTANGULAR ENCLOSURE

This paper is concerned with the prediction of acoustic response in a well damped rectangular enclosure. The acoustic characteristics of the enclosure are modelled using the modal expansion approach, with suitable modification of the damping and frequency shift introduced into the rigid wall acoustic modes of the enclosure. The prediction of acoustical transfer impedance in the enclosure has demonstrated that a frequency dependent modal parameter may be used to describe the acoustic response. This modal parameter is defined as the specific acoustical modal admittance, which describes the contribution of all the boundaries (locally and modally reactive and air leakage) to the modal damping and the shift of nature frequencies. The sound absorption properties of each boundary type and its effect on the response of the enclosure are analysed. The predicted and measured space-average acoustic transfer impedances in a damped rectangular enclosure are compared and reasonable agreement is found. The Helmholtz resonance effect of the cockpit in a helicopter is also included in the model. The measured dip at about 23 Hz in the space-average acoustic impedance inside the passenger cabin of the helicopter is accurately predicted.