HARPs and arrays use hydrophones to convert sound pressure into voltage which is amplified and filtered, and then digitized for recording onto disk drives. Our group designs and uses broad-band (10 Hz – 100 kHz) hydrophones for recording sounds from low frequency baleen whales up to high frequency dolphin clicks and all the sounds in between such as other marine mammals, fish, anthropogenic sources (ships, sonar, seismic exploration) and ambient ocean noise.
The internal components of a HARP hydrophone: two transducers, signal conditioning electronics circuit board, and connector. These components are packaged in a thin-walled, pliable, polyurethane tube filled with oil to provide good acoustic coupling of the transducers with the seawater while protecting the circuit board from the environment.
       Because the background ambient ocean noise decreases approximately 60 dB from 10 Hz to 100 kHz, our hydrophones provide signal whitening to flatten the frequency response of the ambient ocean noise.  This approach allows for measuring large amplitude sounds across a wide frequency band without reaching the clip level of the analog-to-digital converter. To accomplish this and to minimize electronic self-noise, we produced a two-stage design for our HARP hydrophone and various array configurations.
Stage 1 frequency band is 10 Hz to about 3 kHz and utilizes six AQ-1s
(www.benthos.com) transducers in series.  Stage 2 frequency band is from about
2 kHz to 100 kHz and uses a single ITC-1042 (www.itc-transducers.com).

       In addition to their high sensitivity, our hydrophones are design for low power consumption (50 mW), a necessity for long-term deployments with autonomous (battery powered) instruments such as the HARP.

Credit: Sean Wiggins