A theoretical model, along with experimental verification, is developed to describe the generation, propagation and reception of a Rayleigh wave using angle beam wedge transducers. The Rayleigh wave generation process using an angle beam wedge transducer is analyzed, and the actual Rayleigh wave sound source distributions are evaluated numerically. Based on the reciprocity theorem and considering the actual sound source, the Rayleigh wave beams are modeled using an area integral method. The leaky Rayleigh wave theory is introduced to investigate the reception of the Rayleigh wave using the angle beam wedge transducers, and the effects of the wave spreading in the wedge and transducer size are considered in the reception process. The effects of attenuations of the Rayleigh wave and leaky Rayleigh wave are discussed, and the received wave results with different sizes of receivers are compared. The experiments are conducted using two angle beam wedge transducers to measure the Rayleigh wave, and the measurement results are compared with the predictions using different theoretical models. It is shown that the proposed model which considers the wave spreading in both the sample and wedges can be used to interpret the measurements reasonably.

In nonlinearity parameter ( β ) measurements of solids, the pulse-echo method is more desirable than the through-transmission method. Dual element transducers consisting of an outer ring transmitter and an inner disk receiver have been found to be an efficient alternative to single element transducers because such configuration provides an improved second harmonic generation and dependence on the total correction in the pulse-echo mode. For accurate and reliable measurements of β for relatively thin samples, further improvement is needed by optimizing the transmitter and receiver size of dual element transducers for a given sample thickness and frequency. The optimized transducer should provide the largest possible second harmonic reception and a total correction value close to one. In this paper, optimization of the dual element transducer is performed for 1 cm thick Al specimen at 5 MHz fundamental frequency, and the results are presented for the received second harmonic amplitude and the total correction. The optimized dual element transducer is fabricated and experimentally characterized to be applied to determine the absolute β of Al specimens whose microstructure changes due to precipitation heat treatment. The experimental results validate the transducer optimized for pulse-echo β measurements on 1 cm specimens, and provide a quantitative correlation between the β and the microstructural change.

This paper describes the acoustic nonlinearity parameter (β) determination for fluids using a pulse-echo method with the stress-free boundary. A newly derived β formula requires the measurement of the fundamental and second harmonic displacements with appropriate corrections for attenuation, diffraction, and boundary reflection. Measurements are composed of two steps: receiver calibration and harmonic generation. The β values calculated for water at several distances between the planar transducer and the water-air interface are in good agreement with literature, providing a validation for the method.

This paper presents analytical and experimental techniques for absolute determination of the acoustic nonlinearity parameter (β) in fluids using focused transducers. When focused transducers are used for β measurements, the geometrical and mechanical calibrations are generally required for accurate determination of the receiver transfer function from which the absolute pressure can be calculated. The fundamental and second harmonic wave amplitudes in harmonic generation measurements should be modified to account for beam diffraction and material absorption. All these issues are resolved in this study and the proposed technique is validated through the β measurement in water. An experimental method is developed to determine the effective radius and focal length of focused transducers. A simplified self-reciprocity calibration procedure for a broadband focused receiver is described. The diffraction and attenuation corrections for the fundamental and second harmonic waves are explicitly derived using the multi-Gaussian beam model, and the effects on the β determination are discussed. When the diffraction and attenuation corrections are all properly made, the measurement of β over a large range of propagation distances is possible with errors less than 8%.

This article presents a novel approach to determine the nonlinearity parameter using an optimized data fitting method. Based on the quasilinear theory of the Khokhlov–Zabolotskaya–Kuznetsov (KZK) equation, the fundamental and second harmonic pressure fields are expressed using multi-Gaussian beam (MGB) models which separate attenuation and diffraction effects from the plane wave solutions. The developed diffraction corrections are used with a curve-fitting method to extract the nonlinearity parameter together with other acoustic parameters including the attenuation coefficients at the fundamental and second harmonic frequencies. The source nonlinearity is also considered in the fitting process. The proposed scheme is validated through experiments in water and shows that a reliable nonlinearity parameter can be acquired within the range of suitable input power that satisfies the quasilinear approximation involved.