4aEA3. Modification of two methods for determining acoustic impedance.

Session: Thursday Morning, May 16

Time: 8:30

Author: Michael G. Jones
Location: Lockheed Martin Eng. and Sciences Co., 144 Res. Dr., Hampton, VA 23666
Author: Patricia E. Stiede
Location: NASA Langley Res. Ctr., Hampton, VA 23681


Results of an investigation to modify two impedance measurement methods (two-microphone method, TMM; and multipoint method, MPM) to improve efficiency and convenience are presented. The TMM is modified by eliminating the need for one of the microphones in situations where only one microphone can be placed into the test environment. The MPM modification uses pseudorandom noise to significantly reduce the time necessary to complete acoustic impedance measurements. These modifications are the results of on-going technology development at NASA Langley Research Center to simplify and increase accuracy of measurements in harsh environments. Evaluations of efficiency and accuracy requirements of each of the normal incidence acoustic impedance measurement techniques in use at NASA Langley Research Center are provided to determine which method should be used for a given problem. The single-tone source implementation of the MPM is found to be the most accurate, but the most time-consuming technique currently in use. A pseudorandom noise source implementation of the MPM requires significantly less time, with minimal loss of accuracy. The single-microphone method (SMM) is shown to be accurate with a single discrete frequency source, but degrades in accuracy with a random noise source.

from ASA 131st Meeting, Indianapolis, May 1996