Measurement of the Radiation Impedance of a Transducer by a Single Sensor

Abstract

A technique is described for measuring with any uncalibrated linear sensor the radiation resistance and reactance in a medium of any transducer whose motion can be approximated to be a single-degree-of-freedom damped harmonic oscillator. The measurement requires only a single velocity, acceleration, or displacement monitor on the transducer and its advantage over other measurement methods is that it does not require monitoring the pressure at the surface of the transducer, nor does it require an absolute measurement of velocity amplitude and phase at the transducer surface. The magnitude and phase of the output of the sensor on the transducer are measured in air and in the medium for the same frequency and driving force, and from the ratio of the results the radiation resistance and reactance of the transducer can be calculated. The mechanical impedance of the transducer, which is easily obtained by a variety of standard techniques, must also be known to perform the calculation. By using this procedure to first determine the self-radiation impedance of the transducer in a free field, the change in the radiation impedance of the transducer due to its position in an array or in a baffle can also be found by repeating the procedure.