Beam profile measurements and simulations for air-coupled ultrasonic transducers

This paper describes a technique for predicting and measuring acoustic radiation from ultrasonic transducers operating into air in continuous wave mode over the frequency range 50kHz to 2MHz. Using surface displacement data for the radiating aperture, the transmitted pressure field is computed. The field calculation includes simulation of propagation channel attenuation and, where required for comparison with experimentally measured fields, the directional response of the hydrophone. Surface displacement data may be obtained directly from laser interferometric measurement of the excited aperture, or indirectly from finite element analysis of the transmitter. To provide validation by experimental measurement of transducer beam profiles, an environmentally controlled, draught-proof scanning system was utilised. Comparison of experimental and simulated results for different transducer technologies proposed for use in air has been performed. Results indicate good agreement between experiment and theory and a selection of these results is presented in this paper. Where appropriate, comparisons are also made with fields predicted from classical aperture diffraction theory and significant differences are highlighted and explained.