Estimating radiation conductance of a thickness-drive transducer from electrical driving-point admittance measurements

For many thickness-drive transducer applications, it is desirable to have a good estimate of the radiation conductance (or other property based on power output) of the transducer. Generally, however, it is difficult to measure directly transducer power output. It is therefore desirable to have a procedure for estimating radiation conductance from easily measured quantities. The measured quantities used in this paper are electrical admittances at the terminals of the transducer when it is in air, and when in water. The transducer model used to interpret the measured admittance is a lumped-constant equivalent circuit; it has been refined to represent an electrically excited thickness-drive transducer over the frequency range 0.5–40 MHz. Two methods are used to estimate radiation conductance Grad (f), and efficiency. One, the general-case method, gives values of Grad over a wide range of frequencies, the other gives values of Grad only for the special case in which the transducer is at a half-wave resonance. For the general case, a novel procedure, similar to iterative optimization, is used to estimate parameters for the equivalent circuit. Provision is made for estimating a tuning-coil resistance that is frequency dependent. The validity of the two methods is tested by comparing resultant Grad (f) values with those measured by the National Institute of Standards and Technology (formerly National Bureau of Standards). The maximum discrepancy at the lowest transducer resonance is approximately 15% for the small number of transducers on which measurements have at present been made. The transducers studied herein have active elements with characteristic impedance much higher than that of water. Polymer-film transducers are not considered.