Comparison of methods for determining specific acoustic impedance

An investigation was conducted to explore potential improvements provided by alternate implementations of two impedance-measurement methods, known as the two-microphone and multipoint methods. This investigation is part of a continuing technology development to find more efficient and convenient methods for impedance measurements in harsh environments at high frequencies. The two alternate methods are compared with other methods to determine relative merits. As expected, the ''best method'' depends upon trade-offs between convenience and accuracy. The single-microphone method eliminates one of the two microphones typically used in the two-microphone method. The single-microphone method should 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 method, achieved by replacing the single-tone source with a pseudo-random noise source, requires significantly less time with minimal loss of accuracy. Typical agreement between this implementation and the accepted standard is within 0.1 rho c units. However, slightly larger deviations at frequencies above 2.5 kHz indicate challenges in the proposed extension to high-frequency (up to 25-kHz) applications. (C) 1997 Acoustical Society of America.