HTI 92-WB ($1350 +)

This hydrophone isn’t marketed for field recording. It’s more of a custom product for underwater research (and maybe anti-submarine warfare?). It requires a low voltage power supply (battery box) and is typically supplied with no connector on the cable. It can be difficult to work with because the sensor is huge and the cable is quite stiff. This is not something you can toss like a fishing lure! If you can overcome these obstacles, though, it sounds better than anything else I’ve tried.

Pros

• Extremely low noise and high sensitivity
• Balanced frequency response up to the ultrasound limits of 192kHz recording

Cons

• Expensive and only available directly from the manufacturer in U.S.
• Bulky (large sensor, stiff cable)
• Requires battery box (no phantom power)
• U.S. export restrictions prohibit sale to certain countries

Ambient ASF-2 ($500)

I wrote about the ASF-2 in an earlier post, but I think the unit I rented was faulty. (It had very little high frequency output above 10kHz.) In the tests below I used a brand new hydrophone that seemed much better. Unfortunately I still don’t like it much: The sensitivity is very low. In my comparison tests it needed at least 40dB more gain to match the HTI-92-WB, and the results were extremely noisy. I also had a lot of problems with water currents creating loud fluttering bass frequencies (not shown in the tests below). I think the angular edges of the casing create turbulence which interacts with the tension of the cable to produce oscillations. I reduced it by adjusting cable length and mounting methods, but it seems like a smoother casing would avoid the problem. (Note that almost all commercial hydrophones are smooth except this one.)

Pros

• Phantom Powered XLR is very convenient
• Small and easy to deploy
• Easy to buy from major vendors.

Cons

• Low output leads to noticeable noise in quiet sources, and it can’t resolve small details like underwater reflections.
• Ultrasound is present, but noise increases with frequency.
• It sounds slightly pinched and strident compared to the HTI. I see a mid-range resonance around 4k.
• Physical design encourages bass oscillations (flow noise) in water currents. (Not a problem in still water.)

DIY Resonant Hydrophone

This is just a pair of 35mm piezo discs epoxied to a metal ring to create an air gap, then dipped in plasti-dip liquid rubber to waterproof the whole thing. I have built many variations of this over the years, including the sensors for my Pond Station installation.

It is extremely resonant, so don’t expect hi-fi results, even after corrective EQ! Similar hydrophones are available from JrF and others, but you should really pair them with a preamp. (I used my PIP piezo preamp for these tests.) Read my notes about resonance, preamps, etc on Choosing a Hydrophone For Field Recording.

Pros

• Cheap and accessible, so you can feel more free to take risks
• Very high output (due to natural resonance and preamp design) makes it possible to record very quiet sounds with low noise

Cons

• Extremely resonant
• Passive design requires piezo preamp (and most commercial preamps are quite noisy)