Properties of transducer arrays: Directivity index (Urick, 1967)

Abstract

The chapter, ‘‘Properties of Transducer Arrays: Directivity Index,’’ in Urick’s text includes not only the directivity index (DI) of an array, but also many of the important metrics of spatial performance which remain important in assessing the performance of a sonar system. Beampatterns, tapers and sidelobes, signal gain, noise gain and even superdirectivity are all included. The DI metric in the sonar equation indicates gain against 3-D isotropic noise; alternatively, it is used to convert a beam noise level to power per steradian. While easy to compute and appropriate when a field is diffuse, the DI is not a good metric for spatially discrete noise such as shipping which can often dominate a noise field. Recently, the signal and noise gain metrics are used more often. Signal gain is an issue as sonar systems use very large apertures; noise gain is also an issue when the directionality of the noise fields is exploited. Certainly, the most significant evolution in spatial processing not included in Urick’s chapter was the use of adaptive processing which has been enabled by the availability of high-speed DSP for real time processing. The presentation will review Urick’s important contributions to describing the performance of arrays and how they have impacted sonars.