Variation of acoustic impedance caused by a cut in ultrasonic transducers

The authors describe the measurement of the acoustic impedance of the backing of an ultrasonic array transducer consisting of piezoelectric ceramic and backing and matching layers. The transducer is made by sawing the piezoelectric ceramic into array elements. The backing is also cut slightly so that each element can be driven independently, without acoustic cross-coupling between elements. The authors devised a way to measure the acoustic impedance of the sawn part of the backing, which presented difficulties because of its small size. The method uses the reflection coefficient spectrum at the surface of the sawn backing. The measured impedance of the sawed part was 5.33×10⁶ kg/m² s, and that of the solid part was 6.35×10⁶ kg/m² s. It is shown that the impedance of the sawn part of the backing is less than that of the solid backing, even though the material is the same. From this, the array transducer´s electroacoustic conversion characteristics were obtained by computer simulation, accounting for the effect of the sawn part of the backing, and were in excellent agreement with experiment