The use of antiphased zones in an acoustic Fresnel lens for a scanning sonar transmitter

A simple acoustic Fresnel lens is described, which, when used with a plane array of sonar transducer elements, produces a wide beam in azimuth. Test results are compared to computed beam plots and good agreement between theory and measurement was found when all the array elements were in phase. When alternate zones are 180° out of phase, the lens can be reduced in thickness as the step height is halved; however, more careful placing of the transducer elements is required. Such a reduction in lens thickness could be desirable at low operating frequencies, or if a very wide sound beam is desired.