Surface waves above thin porous layers and periodic structures

Abstract

Airborne surface waves above thin air-saturated porous layers and periodic structures have been studied in the low-ultrasonic frequency domain. These waves were observed performing two different kinds of experiments. Because surface waves are related to a pole in the reflection coefficient, we adapted a setup based on near-field acoustical holography to the frequency range of interest. In this way, it was possible to measure the reflection coefficient of the porous layer as a function of the angle of incidence. The existence of a surface wave was observed for different layer thickness. Second, the velocity of the inhomogeneous waves has been determined directly by time-of-flight measurements of ultrasonic bursts, by phase velocity measurements with a sine signal, and from interference patterns of standing waves. The results are compared to predictions using several models. The reflection coefficient above the reticulated polyurethane foams was predicted by an equivalent fluid model, the frame being motionless at the frequencies considered here. For the calculations of the velocity above rectangular- and triangular-groove gratings, we adapted models developed for electromagnetic waves above these structures to acoustics.