Experimental Investigation of Harmonic Interaction Effects for Perforates

Abstract

This paper presents the results of a small experimental study of acoustic non-linear harmonic interaction effects for perforates. Impedance measurements using multiple pure tone excitation has been used. The results are potentially of interest for perforates and other facing sheets used in aircraft engine liners as well as perforate pipes used in automotive mufflers. I. Introduction n Ref. 1 experimental techniques for determining acoustic impedance and flow resistance for perforates under non-linear conditions were discussed. Experiments were made using both pure tone and random excitation and the relevant parameters controlling the non-linearity were discussed. In the linear case the impedance is independent of the sound field but when the sound pressure level is high the perforate impedance will be dependent on the acoustic particle velocity in the holes. For pure tone excitation it is obvious that the impedance will be controlled by the acoustic particle velocity at that frequency. If the acoustic excitation is random or periodic with multiple harmonics the impedance at a certain frequency may depend on the particle velocity at other frequencies. The total rms-value or the Reynolds number based on the particle velocity in the hole are for instance candidates for being the relevant parameters. In this paper the experimental techniques discussed in Ref. 1 are further developed for studying this problem. A study is made of the harmonic interaction effects by using multiple pure tone excitation. By controlling the amplitude level of one pure tone component the effect on impedance at other frequencies can be studied. Many investigations of nonlinear effects occurring when high amplitude sound waves are incident on perforated plates or orifice plates have been published, see e.g., Ref. 2-11. In many of the early works a standing wave tube with single frequency excitation was used. Ingard and Ising 4 in their classical study on nonlinearity of orifices used a differential pressure measurements and a hotwire probe in the hole, still with pure tone excitation. In later papers 8-11 acoustic nonlinearities and power losses of orifices and perforated plates were studied using impulse excitation. In Ref 12 a dimensional analysis of single orifice impedance was made and correlations with previously published single frequency experimental results were made. It is generally agreed that the non-linear losses are associated with vortex shedding at the outlet side of the orifice or perforate openings