Calibration of miniature medical ultrasonic hydrophones for frequencies in the range 100 to 500 kHz using an ultrasonically absorbing waveguide

Enhancements to the existing primary standard optical interferometer and narrowband tone-burst comparison calibration methods for miniature medical ultrasonic hydrophones of the membrane type over the frequency range 100 to 500 kHz are described. Improvements were realized through application of an ultrasonically absorbing waveguide made of a low-frequency-absorbing tile used in sonar applications which narrows the spatial extent of the broad acoustic field. The waveguide was employed in conjunction with a sonar multilayered polyvinylidene difluoride (PVDF) hydrophone used as a transmitting transducer covering a frequency range of 100 kHz to 1 MHz. The acoustic field emanating from the ultrasonically absorbing waveguide reduced the significance of diffracted acoustic waves from the membrane hydrophone ring and the consequent interference of this wave with the direct acoustic wave received by the active element of the hydrophone during calibration. Four membrane hydrophone make/ models with ring sizes (defined as the inner diameter of the annular mounting ring of the hydrophone) in the range 50 to 100 mm were employed along with a needle hydrophone. A reference membrane hydrophone, calibrated using the NPL primary standard optical interferometer in combination with the ultrasonically absorbing waveguide, was subsequently used to calibrate the other four hydrophones by comparison, again using the ultrasonically absorbing waveguide. In comparison to existing methods, the use of the ultrasonically absorbing waveguide enabled the low-frequency calibration limit of a membrane hydrophone with a ring diameter of 50 mm to be reduced from 400 kHz to 200 kHz.