Finite amplitude method for measuring the nonlinearity parameter BA in small-volume samples using focused ultrasound.

On the basis of finite amplitude and comparative methods, the acoustic nonlinearity parameter BA of a liquid sample of as small as 0.1 ml is measured using an 18.6-MHz focused Gaussian beam. The sample fills the space between a polystyrene plate and a tungsten reflector set about 1 mm apart from each other within the focal region. The sound speed c and attenuation coefficient alpha are determined using the time of flight and the insertion loss of the sound passing through the sample, respectively. The density rho is estimated from the reflection coefficient at the interface between the polystyrene plate and the sample, where the transformation from longitudinal to transverse waves is considered. To compensate for the effect of velocity dispersion on the second harmonic generation, the relative phase of the second harmonic sound is also measured using dual-frequency sound. By summarizing all the linear properties and amplitude data of the second harmonic component in the sound transmitted through the sample, the BA value is finally determined. The measurement is validated through the experiments on nondispersive liquids and weakly dispersive biological samples with known BA values.