TABLE OF CONTENTS While Tonpilz transducers are usually designed for the range from 1 kHz to 50 kHz, transmission line sandwich type transducers are used mostly in the range from 10 kHz to 500 kHz and piezoelectric ceramic bars or plates in the range from 50 kHz to beyond 2 MHz. In the frequency range above 10 kHz, lumped elements become very small and in this range of small wavelengths the wave nature of the transducer becomes more influential. Also, the area ratio between the piston head and the drive stack must be smaller because of more pronounced effects of head flexure with extensive overhang. In this section, we consider sandwich and plate designs as well as composite material designs for transducers operating in the frequency range above 10 kHz.
The sandwich transmission line transducer was first introduced by Langevin [35] as a layered symmetrical structure of metal, quartz, and metal (see Fig. 1.4) that lowered the resonance frequency of a thin quartz thickness mode plate and provided a practical underwater sonar projector. A number of other sandwich transducer designs using piezoelectric ceramics have been given by Liddiard [36]. The front section of the sandwich is approximately matched to the medium by use of a matching material with intermediate impedance, such as aluminum, magnesium, or glass-reinforced plastic (GRP), (e.g., G10), rather than through a change in area, as in the case of the Tonpilz transducer. The material of the rear section of the sandwich, often steel...
