Determining the sensitivity of air-coupled piezoelectric transducers using a comparative method - theory and experiments

The sensitivity of an ultrasonic transducer is an important parameter for evaluating its effective frequency band, electroacoustic conversion efficiency, and the measurement capability of the system. Determining the sensitivity of a traditional immersion or contact piezoelectric transducer has been well investigated. However, due to the high attenuation of wave propagation in air and the large acoustic impedance mismatch between the active piezoceramic material and the load medium, there are few reports on the calibration of an air-coupled piezoelectric transducer. In this work, we present a comparative method to measure the sensitivity of an air-coupled transducer: the air-coupled transducer is used to receive a broadband pulse signal for evaluating its frequency spectrum; and a toneburst signal with the known vibration displacement is measured by the air-coupled transducer to calibrate its amplitude of the sensitivity. The effects of transmitter sensitivity, input pulse, attenuation and diffraction are taken into account to improve the precision of the sensitivity determination. In addition, measuring the sensitivity amplitude by comparing the measured displacements avoids the complicated task of characterizing the effects of the electrical equipment. The determined sensitivity is checked by comparing the measured displacements at different frequencies and nonlinearity parameters of the material using different kinds of transducers. Agreements of the results obtained using different methods demonstrate that the calibrated sensitivity of the air-coupled transducer is valid, and that the presented method is effective.