ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

5aPAa6. On the assumption of normal reaction in acoustic characterization of air-filled rigid porous materials.

James M. Sabatier

Craig J. Hickey

Natl. Ctr. for Physical Acoust., Univ. of Mississippi, University, MS 38677

Carl K. Frederickson

Univ. of Central Arkansas, Conway, AR 72035

The ground has been modeled as a rigid air-filled porous media for the purpose of determining the characteristic impedance important to outdoor sound propagation. It is possible to use these models to acoustically measure the porous parameters of the ground important to agriculture. To completely determine the porous parameters, both reflection and transmission measurements must be made. For both measurements, the ground has been considered to be locally reacting giving transmitted signals that propagate in a direction normal to the surface. This is not nessessarily the case. Using the local reaction assumption, transmission measurements only determine the normal component of the propagation constant. This error lowers the caclulated value of the tortuosity. The error in the tortuosity depends on the angle of incidence of the incident signal. There is no effect on the calculated effective flow resistivity. A theoretical determination of the tortuosity correction will be presented along with data that demonstrate the effect. [Work supported by USDA.]