COMPARISON OF METHODS FOR DETERMINING SPECIFIC ACOUSTIC-IMPEDANCE

An investigation was conducted to explore potential improvements provi ded by alternate implementations of two impedance-measurement methods, known as the two-microphone and multipoint methods. This investigatio n is part of a continuing technology development to find more efficien t and convenient methods for impedance measurements in harsh environme nts at high frequencies. The two alternate methods are compared with o ther methods to determine relative merits. As expected, the ''best met hod'' depends upon trade-offs between convenience and accuracy. The si ngle-microphone method eliminates one of the two microphones typically used in the two-microphone method. The single-microphone method shoul d be useful where mechanical constraints allow only one microphone to be placed into the test environment. It is found to be quite accurate with a single discrete frequency source, but the most inaccurate with a random noise source. An alternate implementation of the multipoint m ethod, achieved by replacing the single-tone source with a pseudo-rand om noise source, requires significantly less time with minimal loss of accuracy. Typical agreement between this implementation and the accep ted standard is within 0.1 rho c units. However, slightly larger devia tions at frequencies above 2.5 kHz indicate challenges in the proposed extension to high-frequency (up to 25-kHz) applications. (C) 1997 Aco ustical Society of America.