assumed by such a uniform membrane in order to provide more subtle beam shaping effects one further advantage of the present invention is that the power required to achieve a displacement of the membrane 18 center of a few tens of microns is quite minimal because in effect it is the air 17 in the enclosure 20 which is being compressed rather than the liquid media 22 acoustic radiation losses into the liquid 22 are minimized both because of the slack provided by the proximity of the enclosure 20 and by the fact that the wavelength of sound in the liquid media 22 is much greater than the diameter d 39 of the pressure control plate 28 which typically will be comparable to the membrane 18 diameter d note that when d 39 is approximately equal to d the displacement amplitude a 39 of the pressure control plate 28 will also be approximately equal to the displacement amplitude of the center of membrane 18 most of the mechanical power dissipation in the system will come from the friction associated with the deformation of the plate 28 or of the deformable structure e g metal bellows by which the plate 28 is attached to the scanner wall 24 and is of the order of a tenth of a watt thus it can be seen from the above description that the present invention provides a simple and.