Ellipsoidal hydrophone with improved characteristics

A new style of hydrophone is being developed to meet or exceed the new recommendations set forth in the NUM Acoustic Output Measurement Standard (l) 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-pm 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 cany 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. 121. We 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 pm have been test4 typical active element sizes are 85 - 400 pm, and larger apertures