Characterization of airborne transducers by optical tomography

This paper describes the application of an acousto-optic method to the meas urement of airborne ultrasound. The method consists of a heterodyne interfe rometric probing of the pressure emitted by the transducer combined with a tomographic algorithm. The heterodyne interferometer measures the optical p hase shift of the probe laser beam, proportional to the acoustic pressure i ntegrated along the light path. A number of projections of the sound field, e.g. a set of ray integrals obtained along parallel paths, are made in mov ing the transducer to be tested. The main advantage of the method is its ve ry high sensitivity in air (2 x 10(-4) Pa Hz(-1/2)), combined with a large bandwidth. Using the same principle as X-ray tomography the ultrasonic pres sure in a plane perpendicular to the transducer axis can be reconstructed. Several ultrasonic fields emitted by wide-band home made electrostatic tran sducers, with operating frequencies between 200 and 700 kHz, have been meas ured. The sensitivities compared favorably with those of commercial airborn e transducers. (C) 2000 Elsevier Science B.V. All rights reserved.