ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

1pEA15. Application of cardioid beamforming in both impedance tube and free-field measurements of ground impedance.

David C. Swanson

Edward Maniet

Appl. Res. Lab., Penn State Univ., P. O. Box 30, State College, PA 16804

Measurements of the specific acoustic impedance of ground covered by institutional grass were performed using both a cardioid beamforming method and the established transfer function method [J. Y. Chung and D. A. Blaser, 907--921 (1980)]. The cardioid method steers a beampattern to separate the incident and reflected waves. Both methods were applied to impedance tube and free-field measurements. With proper calibration, the cardioid and transfer function methods are mathematically identical and produce similar experimental results. Comparison of the impedance tube and free-field measurements suggest that the latter approach produces more reliable results. However, due to practical limitations of the source/receiver geometry free-field measurements at low frequencies are more complicated in that the plane wave assumption is not valid.