Measurement of the Nonlinearity Parameter in Dispersive Liquid using Focused Ultrasound

Taking into account the velocity dispersion in a sample inserted into the focal region of the focused field, the second harmonic component of the acoustic pressure observed beyond the sample is investigated. An amplitude increase due to the degradation of the cancellation effect of the second harmonic components generated at the pre- and post-focal regions is predicted. When focused ultrasound is applied to the finite-amplitude method to estimate the nonlinearity parameter B/A in dispersive liquid, the B/A value can, therefore, be overestimated. This prediction is confirmed by the B/A measurement for water-oil mixtures with various oil fractions employed as dispersive samples. The methods used to measure the sound-speed difference between the fundamental and the second harmonic frequencies using the same focusing system and to measure the B/A accurately are presented in this paper.