FACTORS WHICH DETERMINE THE TUNABLE FREQUENCY-RANGE OF TUNABLE TRANSDUCERS

Tuning or varying the resonance frequency of a transducer by adding an additional layer of ceramic, called the control ceramic, is a method for designing a wide bandwidth transducer. By loading the control cera mic with an inductive electrical load the resonance frequency of the w hole structure is variable or tunable over a wide range of frequency. Two designs of tunable transducer are discussed in this paper; a two-p late thickness-driven structure with a fundamental frequency of 240 kH z and a sandwich structure with a fundamental frequency of 30 kHz. Tun able ranges of 1.5 and 1.8 oct are obtained, respectively. It is estab lished that the limit of the tunable range for the two-plate transduce r is between the fundamental harmonic and the second overtone of the w hole structure, which are defined with the control ceramic short circu ited. For the sandwich transducer the tunable range is between the fun damental harmonic and the first overtone of the whole structure, which are defined when the control ceramic short circuited. An analysis bas ed on the Mason transmission-line model shows that the fundamental lim it of the tunable range is determined by the fundamental resonance fre quency and the dynamic coupling coefficient of the drive ceramic and i s affected by the presence of any bonds and shims between the ceramics . To obtain a continuously tunable frequency range the position of the drive ceramic must be between the radiating front-end of the structur e and the control ceramic. (C) 1996 Acoustical Society of America.