DETERMINATION OF TRANSMISSION LOSS, ACOUSTIC VELOCITY, SURFACE VELOCITY AND RADIATION EFFICIENCY BY USE OF TWO MICROPHONE TECHNIQUES

BJORN HENRY FORSSEN, Purdue University

Abstract

This thesis is concerned with the development of novel measurement approaches to estimate surface velocity, radiation efficiency and transmission loss of panel structures. These approaches are based on the estimates of the auto and cross spectral densities of the sound pressures measured by two closely spaced microphones. Some of these approaches involve the use of the recently developed two microphone acoustic intensity technique, which is reviewed extensively. A two microphone velocity technique was developed and used to measure the acoustic velocity of a point source. Excellent agreement is obtained between velocities estimated by use of this technique and that measured by a single microphone (yielding the exact velocity), up to a frequency above which the finite difference error becomes significant. This error is well predicted, for a point source. This technique is also used to estimate the surface velocity of an aluminum panel. Fair agreement is obtained between the measured and predicted difference in surface velocities measured by use of accelerometer and a two microphone probe. Near the panel coincidence region the two microphone velocity technique is quite sensitive to the probe orientation and a small misalignment can result in a significant overestimate in the surface velocity. The radiation efficiency of the aluminum panel is estimated in the case of acoustic and mechanical excitations. The radiation efficiency is estimated accurately for subpanel critical frequencies from the aforementioned surface velocities and the radiated intensity measured by use of the two microphone acoustic intensity technique. A statistical energy analysis approach is developed to determine the dissipation loss factor of a panel structure. Good agreement is obtained between this approach and a conventional approach. A new approach based on the two microphone acoustic intensity technique and the use of only one reverberation room is developed to determine panel transmission loss. Good agreement is found with both theory and the conventional method for several different panel structures. This approach is also used successfully to identify areas of different levels of intensity transmitted through a composite panel.

Degree

Ph.D.

Subject Area

Mechanical engineering

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