Abstract
A thermoacoustic measurement technique based on the transformation of theincident ultrasonic energy into heat inside a cylindrical absorber is investigated. To enablequantitative acoustic intensity measurements, a thermoacoustic sensor with an absorber 3 mmin diameter is calibrated in the far field of a planar source transducer. For ultrasoundfrequencies in the range from 1 to 9 MHz, the time-averaged intensities are derived frompressure field measurements using a membrane hydrophone. In a second run, measurementsare performed with the thermoacoustic sensor at the same position in the acoustic field andunder the same excitation conditions. The equilibrium temperature enhancement at the rearside of the absorber is determined at each frequency, and the transfer function of the sensor isgiven by the temperature enhancement per ultrasound intensity averaged over the sensor crosssection. The calibrated thermoacoustic sensor is then applied to acoustic output measurementson a diagnostic ultrasound machine at various parameter settings. The results for M-mode andpulse Doppler mode, i.e. for non-scanning beams, are compared with the intensities derivedfrom additional hydrophone measurements. If the spatial averaging effect of thethermoacoustic sensor is taken into consideration, agreement can be observed between theresults of both methods.
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