Tomographic Reconstruction of 3D Ultrasound Fields Measured Using Laser Vibrometry

Abstract

A method for obtaining the amplitude and phase distribution of an ultrasound field in air, using laser vibrometry and computed tomography, is described. Radiating ultrasound transducers causes pressure variations in the air, which lead to refractive index changes, which in turn can be recorded by the use of interferometric instruments like the laser vibrometer. This point measuring method for the measurement and visualization of sound fields was proposed by Zipser et al. [ 1 , 2 ]. In order to measure the whole sound field, the laser beam is scanned over the area of interest. With this technique, the absolute phase of the field is obtained directly from the measurements while the amplitude needs some calculations using the Gladstone Dale equation [ 3 ]. Moreover, the obtained result using laser vibrometry for sound measurements is an integral of the sound field along the laser light path. The projection effects due to this integral were studied by Olsson and Tatar [ 4 ]. In this paper however, the projection property of the measurement method is taken into advantage. Tomographic 3D reconstruction of the sound field is obtained using several projections taken at different angles. The phase and the amplitude distributions of an ultrasound field due to several transducers, emitting at a frequency of 40 kHz, positioned both symmetrically and non-symmetrically are reconstructed.