Bodén H.

This paper presents the results of a study of non-linear acoustic properties of perforates. Signal analysis techniques have been used to study the acoustic properties of perforates in the non-linear regime. 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. In the linear case the perforate impedance is independent of the sound field but when the sound pressure level is high it 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. In this paper a study has been made of harmonic interaction effects by using multiple pure tone excitation. In a previous work by the author the possibility to use non-linear signal analysis techniques following the works of, e.g., Bendat for identifying the nonlinear perforate impedance and flow resistance was discussed. This would be an elegant technique since the measurements could be made over a wide frequency band using broadband noise excitation. With multi-frequency excitation this technique can not be used without reformulation. An alternative based on Volterra kernel formulations was presented by Keefe and Reisenthel. This formulation will be tested in the present paper. Tests will also be made to use a so called Microflown p-u transducer where the particle velocity and pressure are measured at approximately the same point in space. The advantages of using this technique for non-linear impedance studies will be explored in the paper.