I’ve been building my own hydrophones for awhile, using them alongside inexpensive options from Aquarian Audio and JrF, but I always wondered how much of a difference I would hear with expensive professional hydrophones. I recently had the opportunity to do multichannel comparisons of the Ambient ASF-1, Ambient ASF-2, Aquarian Audio H1a, and JrF D-Series. Here is my review and comparison files. (Also see Choosing a Hydrophone For Field Recording for general advice on underwater field recording and a big comparison table of commercially available hydrophones.)

Ambient ASF-1 ($1600)

A totally pro hydrophone. It feels like a serious research tool. It’s flat well into the ultrasonic range and the sensitivity is well matched to pro audio equipment. I have no complaints about the tonality, but the noise performance is worse than I would expect for the high price. (This will only be an issue for very quiet sounds, though.) It’s also very heavy and the cable is stiff and hard to handle. This feels like a device I’d rent for special occasions, not something I would make part of my field recording kit, regardless of price.

Gotham Sound in NYC offers it for sale or rental.

Pros

Linear frequency response throughout the range of 192kHz sampling
No resonances or sonic coloration
Phantom Powered XLR is very convenient
High output (built-in 12dB preamplifier)

Cons

Very expensive
Noise floor may be audible when recording very quiet signals
Heavy, paired with a stiff cable, makes handling and storage difficult

Ambient ASF-2 ($500)

This is the more practical of Ambient’s offerings. It’s much more affordable, the cable is thinner, and the hydrophone itself is compact. Unfortunately it sounds worse than the ASF-1. NOTE: The hydrophone I tested below was partially broken.The high frequency response was attenuated above 10kHz. In this later review I tested another unit with full frequency response. I’m still not sure I’d want to spend $500 on something with such low sensitivity.

Gotham Sound in NYC offers it for sale or rental.

Pros

Cheaper, smaller and more practical than ASF-1
Relatively balanced frequency response
Phantom Powered XLR is very convenient

Cons

Still quite expensive compared to other options
I had problems with high frequency response, but it was a faulty hydrophone!
Low output leads to noticeable noise in quiet sources (even with best-in-class Sound Devices recorder)

Aquarian Audio H1a ($170)

This is a tried and true affordable hydrophone. (I have the passive H1a version because I have an external piezo preamp, but I recommend the preamplified H2a version for most people.) For the money it’s a good choice, but it’s a bit resonant in the midrange and there is very little information above 5kHz so you’ll miss the sparkle and detail of more expensive options. If you’re primarily interested in mid-range sounds, the resonance leads to higher output which makes this hydrophone less noisy than its expensive competitors for certain quiet sources.

(Read more about my philosophy on resonant v. flat response in Choosing a Hydrophone For Field Recording.)

Pros

Inexpensive
Compact, rugged, easy to handle
Phantom Powered XLR is very convenient (for preamplified H2a-XLR version)
Slight resonant coloration in mid-range increases sensitivity, lowers noise floor

Cons

HF response above 5kHz is limited, so it sounds muffled
Slight resonant coloration in mid-range

JrF D-series (approx $80 + ship)

This has been a popular option for years, handmade by UK sound artist Jez Riley French. It’s an optimized version of the typical DIY method of gluing a piezo disc to an air cavity like a bottle cap and sealing the assembly with liquid rubber. I have made many similar ones for my own use, and I’m glad there’s an “off-the-shelf” option for folks who are less inclined to experiment. It’s highly resonant, so there is significant coloration to the sound, but it also has a very high output if you run it through a low noise piezo preamplifier. (I used my own preamp for these tests.) This type of hydrophone will last for years of occasional use, but the liquid rubber will eventually peel from the cable if submerged for days/weeks.

(Read more about my philosophy on resonant v. flat response in Choosing a Hydrophone For Field Recording.)

Pros

Inexpensive
Compact, easy to handle
Strong mid-range resonance increases sensitivity, lowers noise floor

Cons

Not preamplified, so you need to add a piezo preamp or at least an impedance matching transformer
HF response above 3kHz is limited, so it sounds muffled
Strong mid-range resonance adds coloration to everything
Long term durability of liquid rubber coating is questionable

Comparison Tests

Below you’ll find comparison tests of the same signal recorded with multiple hydrophones, several inches apart. (Recorded with a Sound Devices MixPre-6 recorder.) The sensitivity and tonality of each sensor are very different, so each clip is normalized subjectively to make the comparisons as relevant as possible . Not every hydrophone is compared in every test, but I think you’ll get a clear idea. NOTE: The uneven noise floor in the JrF tracks on some examples are an RF oscillation problem with my preamp, not the hydrophone!

Location 1: Urban river

This location is full of broadband noise from infra to ultrasound. It’s a great test of frequency response, resonance, and character, but says little about noise floor.

Broadband River Noise #1 (ASF-1, ASF-2, Aquarian, JrF)

Broadband River Noise #2 (ASF-1, ASF-2, JrF) @ 192k SR

Normal speed:

Slowed 4x:

River Musical Engine (ASF-1, ASF-2, JrF)

River Shuddering and Creaking (ASF-1, ASF-2, JrF) @ 192k SR

Normal Speed:

Slowed 4x:

River Splashes and Passing Boat (ASF-1, ASF-2, Aquarian)

Location 2: Quiet Pond

This is a quiet freshwater environment that requires lots of gain. You can also hear how the different tonalities of each sensor color the voices of the insects and fish.