Vibro-acoustography of small spheres

Abstract

Vibro-acoustography is a method for imaging the acoustic energy emitted from objects in response to an oscillatory radiation force produced by two interfering focused beams of ultrasound [M. Fatemi and J. F. Greenleaf, Science 280, 82–85 (1998)]. To facilitate quantitative study, a model is presented that describes the behavior of an elastic sphere in two plane ultrasound waves of slightly different frequencies. The vibrating velocity and the resulting acoustic emission of the sphere at the difference frequency are derived from the radiation force and the impedance of the sphere. For small steel spheres in water, the velocity and emission are predicted to have a simple relationship with the difference frequency and the size of the sphere. Experiments on small stainless steel spheres of different size are carried out for a number of difference frequencies, with a laser interferometer to measure the velocity of the sphere and a calibrated hydrophone to measure the acoustic emission from the sphere. The experimental results fit the model description well. This means that vibro-acoustography may be useful in quantitative measurement.