Ellipsoidal hydrophone with improved characteristics [diagnostic US application]

A new style of hydrophone is being developed to meet or exceed the new recommendations set forth in the AIUM Acoustic Output Measurement Standard as they apply to ultrasound measurements for submission to the U.S. FDA. The device can be calibrated and used over a frequency range extending from below 200 kHz to above 40 MHz. The new hydrophone is basically a membrane style hydrophone; however, the membrane is bonded to and backed by the small radius end of an ellipsoid of acoustically lossy epoxy. The active element in the device is defined by the end of an axial wire that comes away perpendicular to the membrane, which is a small disk of 4 or 12-μm thick copolymer film. This configuration has the advantage of minimal shunt capacitance, excellent shielding, and the reduction of spurious artifacts in the frequency response due to surface waves. Surface waves are inevitably generated by ultrasound incident upon a hydrophone, but with this design they carry on around the elliptical cross section with little or no reflection. The large size of the device (in wavelengths) also avoids a 6 dB step in the frequency response due to the transition from a free field pressure sensor at low frequency to a mirror at high frequency. This is an inherent problem with needle type hydrophones as discussed by Fay et. al. (1994). The authors are also addressing this mode transition issue (and other problems) by choosing materials for casting the ellipsoid that have acoustic impedances close to water. A major advantage of this design is the fact that it can be well shielded electrically without loss of sensitivity, as is often the case with membrane hydrophones. The shielding currently in use is an evaporated gold film over the front end of the device. This gives them the appearance of being a “golden lipstick” which has become the nickname used to refer to these things in the shop. Active element sizes as small as 25 μm have been tested, typical active element sizes are 85-400 μm, and larger apertures are also possible. The hydrophones can be made with high input impedance preamplifiers cast in the backing directly behind the active element in order to achieve very high sensitivities relative to other types of hydrophones. The hydrophone´s freedom from artifacts yields frequency responses that are flat to ±1.5 dB over the entire calibration range (200 kHz to 40 MHz). Some theory of operation and experimental results obtained with these new devices is presented