A novel calibration technique with narrowband signals for measuring the absolute acoustic nonlinearity parameter

The absolute acoustic nonlinearity parameter, which is defined by displacement amplitudes of the fundamental and second-order harmonic waves transmitted through a medium, has been known to be positive for the diagnosis of structural degradation. It can be measured using piezo-electric transducers with calibration to obtain transfer functions that convert the electrical amplitude into the acoustic displacement amplitude. Because the amplitude of the harmonic component is small, a narrowband signal is typically used to increase the reliability of harmonic detection. However, for calibration, a broadband signal in the form of a pulse is preferred because the transfer function at both the fundamental and harmonic frequencies should be obtained simultaneously. Here, we propose a calibration technique that uses a narrowband instead of a broadband signal to measure acoustic nonlinearity parameters. Firstly, we explain the theoretical reason for selecting an identical time width for all received signals in the conventional technique and then introduce the advantages of the proposed technique. Next, to validate the proposed technique, transfer functions obtained using the proposed and conventional techniques were compared for a fused silica sample with a lithium niobate crystal transducer, confirming the close agreement. To demonstrate the versatility of applications in terms of the transducer and material type, four combinations were tested by adding a lead zirconate titanate transducer and an aluminum alloy, showing good concordance regardless of the material or the transducer type. Finally, to validate the effectiveness of the proposed technique in measuring the absolute acoustic nonlinearity parameters, fused silica was measured using both techniques. The result from the conventional technique was 10.87, while the result from the proposed technique was 11.49, a difference of only 5.70%.