Eng. Mech. Discipline, Open Univ., Milton Keynes MK7 6AA, England
Experiments carried out by Donato [J. Acoust. Soc. Am. 63, 700--703 (1978)] demonstrated a continuous wave field behavior consistent with the formation of an air-coupled surface wave above an impedance boundary consisting of a hard-backed rectangular rigid lattice. He found it necessary to adjust the impedance of the lattice, predicted according to a thin-air-layer model, to fit the measured impedance and attributed the required adjustment to clamping of the sample in the impedance tube. A repetition of Donato's continuous wave experiment is reported. A rectangular pore model for the impedance of the lattice is found to give better agreement with measured impedance data than a thin-layer model. Pulse experiments are reported that show clear evidence of the air-coupled surface wave as a separate arrival and the observed dependence on the measurement geometry is in good accord with predictions.