Calibration method using a narrowband signal for measurement of the acoustic nonlinearity parameter

Abstract

The measurement of absolute acoustic nonlinearity parameters using piezoelectric transducers consists of two steps; calibration and harmonic detection. In calibration, a frequency domain transfer function that converts the current output signal amplitude into the acoustic displacement amplitude is measured. Then, in harmonic detection, the amplitudes of the harmonics generated in the ultrasonic waves propagating through a medium are detected and converted to displacement amplitudes using the transfer function obtained in the calibration step for calculation of the acoustic nonlinearity parameters. However, in the conventional method, a broadband pulse signal is used for the calibration and a narrowband tone-burst signal is used for the harmonic detection; therefore, this method inconveniently uses different driving devices for calibration and harmonic detection. In this study, we propose a more convenient technique that can perform calibration using a narrowband signal so that a single driver of a narrowband signal can be used for both calibration and harmonic detection. To verify the validity of the proposed method, transfer functions were measured at typical frequencies for two types of material (aluminum alloy and fused silica) against two types of transducers (commercial PZT and LiNbO3 crystal), and the results were compared with those obtained using conventional calibration method. The transfer functions obtained by the conventional method and the proposed method showed good concordance, independent of material and transducer type.