Acoustic modal analysis experiment

Abstract

This paper proposes an experimental modal technique for acoustic ducts, mufflers, and resonators over the plane-wave frequency regime. Global modal properties, such as natural frequencies and modes of gas oscillation, are extracted from the coincident-quadrature response curves of measured cross-point acoustic impedances at a number of observation locations. The acoustic system is excited by a vibrating piston which is driven by an electromagnetic shaker with bandlimited binary random noise signal. The acoustic impedance is determined using the following two transducers: (i) an accelerometer attached to the piston—its signal is processed to yield volume velocity information, and (ii) a microphone traverse. Digital data acquisition and processing techniques are used to generate the necessary impedance data at a number of locations for modal analysis. In order to demonstrate the validity of our experimental technique, we have applied it to the following example cases, and obtained excellent correlation between theory and experiment: closed–closed tube, closed–open tube, and symmetrical and unsymmetrical lumped parameter systems. In this paper, we discuss the conceptual, analytical, physical, and measurement considerations of the acoustic modal analysis. We also point out some areas of further research.