Experimental constant beamwidth transducer

Abstract

The theory of a broadband constant beamwidth transducer (CBT) which is based on Legendre function shading of a spherical cap was described in a previous report [P. H. Rogers and A. L. Van Buren, J. Acoust. Soc. Am. 64, 38–43 (1978)]. Theoretical calculations showed the CBT to have uniform acoustic loading, extremely low side lobes, virtually no nearfield, an essentially constant beam pattern for all frequencies above a certain cutoff frequency, and a flat transmitting current response over a broadband for piezoelectric drive. In this paper we present experimental results obtained for a prototype CBT based on fifth-order Legendre function shading. The transducer is a spherical cap with a diameter of 30.5 cm and a total cap angle of 50°. Measurements on the CBT showed it to possess very low side lobes and a nearly constant beamwidth over the frequency range from 20 to 140 kHz for ambient pressures up to 3.4 MPa. We also present formulas useful in the design of a CBT which is to be used as a transmitter and/or receiver. The formulas involve simple algebraic and trigonometric expressions which can be readily evaluated using a pocket calculator.