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, wall-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 incident- and reflected-wave spectra, and the phase angle between the incident and reflected waves, can be determined from measurement of the auto- and 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 random-excitation 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 acoustic properties very rapidly since no traversing is necessary and since random excitation is used (in each of three test cases only 7 sec of continuous data was needed). In addition, the bandwidth may be made arbitrarily small, within limits, so that the computed properties will have a high degree of frequency resolution.
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Seybert et al. (1977) studied this question.