Arrays versus Lenses as Directional Acoustic Sensors\

Abstract

Abstract : There are two major techniques today for creating directional acoustic sensors. These are: the use of an array of point receivers, coupled into an electrical beam forming network on the one hand and the use of a refractive lens to focus on the sound rays to a localized region on the other hand. The essential difference between these two methods lies in the way in which the transformation of a plane wave acoustic field into a diffraction limited region takes place. In the case of the lens, the actual acoustic field is modified locally by refraction and the transformation of the plane wave field into the diffraction limited region takes place within the acoustic field itself whereas the array samples the acoustic field without perturbing it and then electrically combines the samples to generate the transform of the plane wave acoustic samples into a diffraction limited signal. It is interesting to look at the implications of these differences in terms of the performance of these two techniques as directional sensors. In general there is no inherent difference in resolution or in directional response pattern between the two. In both cases the transformation which is made is aperture limited and a lens of a given, physical aperture, can have the same performance as an array of the same aperture and conversely, an array of that aperture can have the performance of a lens of a comparable aperture. Thus the differences between the two techniques must lie in other areas.