EFFICIENCY OF A MONOPOLE SOUND SOURCE IN THE VICINITY OF A WATER-LOADED PLATE

Abstract

This paper presents numerical results for the power output of a sound source in the vicinity of an elastic structure. The source is a monopole characterized by a constant rate of volume injection. The structure is a thin elastic plate of infinite extent. The coupling of the plate's flexural motion and the acoustic field is taken into account. The acoustic energy emitted by the source is carried away to infinity by acoustic radiation as well as by flexural waves that travel along the plate, accompanied by a coupled, subsonic surface wave in the fluid. For the case of steel plates in water, numerical results have been obtained for the total power delivered by the source as well as for the various power flows involved. The dependence of these results upon two parameters is investigated by extensive numerical calculations. These parameters are: (i) the diffraction or Helmholtz numberk0d, wherek0is the fluid wavenumber anddis the distance from the source to the plate, and (ii) the ratio of the plate thicknesshandd. The power output of the source depends strongly upon the region of interest in this two-parameter space. Fork0d≥1 the total power output approaches the free field value, but also shows the typical modulation due to the finite distance to the plate. Forh/d≥0·1 this modulation effect is comparable to that for an infinite rigid wall, while forh/d≤0·01 it resembles the effect for a free surface. Fork0d<1 andh/d≤k0d/20 the results do not vary withh/d, but the effect of the plate is very close to that for a perfectly compliant surface. In a third regime, i.e., for 0·1≥h/d≥k0d/10 the efficiency of the source is independent ofk0h, but increases strongly with decreasingk0d2/h.