I've done a few sessions using H2a hydrophones as contact mics, and they really do a great job of capturing clean, clear signals. My primary problem with using them in that context is that, even with the rubber cup mount, they don't really stabilize unless your surface is completely flat and gravity is working in your favor. Even with mild angles or curves you lose that direct coupling between the mic and the surface, thus reducing the amount of signal you can pick up. A couple of solutions that have worked for me:
- Duct or electricians tape!
- Velcro wraps (tricky to keep them from slipping off the cylindrical mic but doesn't leave a sticky residue like tape)
Sometimes I forgo the rubber cup altogether and press the mic directly onto the desired surface with my hand. I'm pretty confident that this yields a better recording since there's nothing coming between my noise source and the mic capsule, but you've got to be very, very still since the H2a is so sensitive.