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.