An exposimetry system for characterization of acoustic fields above 20 MHz

A brief discussion of the methods and guidelines associated with high frequency (>20 MHz) acoustic field characterization is given. Using these guidelines as a basis, a computer-controlled exposimetry system for measurement of acoustic fields above 15 MHz was built. The system incorporates five degrees-of-freedom for positioning both the ultrasound device and hydrophone. Overall system bandwidth can range over 100 MHz, and is limited by the type of hydrophone used. A calibrated needle-type polyvinylidene fluoride (PVDF) hydrophone with a 0.04 mm spot size was used for most measurements. A 50 MHz spherically focused LiNbO₃ transducer was tested and intensity results are reported. A comparison with theoretical field simulations yielded some variance between the field patterns of pulse-average intensity, however, there is good agreement between empirical (8.6 mm) and designed (9.0 mm) focal points. The measured -3 dB beamwidth was 208 μm, slightly larger than the calculated value of 128 μm