New approach to a constant beamwidth transducer

Abstract

The theory of a broadband constant beamwidth transducer which is to be used primarily as a projector is presented. The transducer is a spherical cap of arbitrary half-angle α shaded so that the normal velocity is given by ur = u0Pv(cosθ), where Pv is the Legendre function whose root of smallest angle occurs at θ = α. The required value for v can be obtained to within 1% for α ⩽ 1 rad from the approximation v ≃ 0.5(4.81/α − 1). The transducer is shown to have uniform acoustic loading, extremely low side lobes and an essentially constant beam pattern for all frequencies above a certain cutoff frequency. Under piezoelectric drive the transducer is shown to have a flat transmitting current response over a broad band.