Experimental determination of acoustic properties using a two-microphone random-excitation technique *

An experimental technique is presented for the determination of normal acoustic properties in a tube, including the effect of mean flow. An acoustic source is driven by Gaussian white noise to produce a randomly fluctuating 'sound field in a tube terminated by the system under investigation. Two stationary. wail-mounted microphones measure the sound pressure at arbitrary but known positions in the tube. Theory is developed, including the effect of mean flow, showing that the incidentand reflected-wave spectra, and the phase angle between the incident and reflected waves, can be determined from measurement of the autoand cross-spectra of the two microphone signals. Expressions for the normal specific acoustic impedance and the reflection coefficient of the tube termination are developed for a random sound field in the tube. Three no-flow test cases are evaluated using the two-microphone randomexcitation technique: a closed tube of specified length, an open, unbaffled tube of specified length, and a prototype automotive muffler. Comparison is made between results using the present method and approximate theory and results from the traditional standing-wave method. In all cases agreement between the two-microphone random-excitation method and comparison' data is excellent. The two-microphone random-excitation technique can be used to evaluate aenustie properties very rapidly since no traversing is necessary and since random excitation is used (in each of three test eases only 7 sec of continuous data was needed). !n addition, the bandwidth may be made arbitrarily small, within limits, so that the computed properties will have a high degree of frequency resolution.