I've tested the frequency response of two hydrophones for use with an AudioMothDev. The Aquarian H2a uses plug-in power and doesn't require any modifications to be used with the AudioMothDev board. However, I found that its flat frequency response only extends to 3 kHz. Signals levels are down 20 dB by 10 kHz and were undetectable at 30 kHz. The HighTech HTI-96-min requires the addition of a resistor (10-12 kOhm) from the audio input to the ground. It also requires external power (3 v is enough from one version, but I could not use the external voltage provided by the board without adding noise). The HTI-96-min has a flat frequency response to 30 kHz and a useable frequency response up to 140 kHz.