I have been a happy user of Aquarian Audio H2a hydrophones for some years but on a recent relocation I lost track of them. Hopefully they will resurface (pun intended), but in the meantime work knocked on the door and I had to get a new pair. I found that what was the H2a is now called the H2d and has an upgraded signal conditioning circuit to improve sensitivity and signal-to-noise ratio.

This was gold to my ears. However, speaking to Aquarian Audio’s (AA) customer support about delivery times, they mentioned a new model in the works, which they had started selling to enquiring customers: the Aquarian Audio A5 hydrophone. Five days later and two A5s with custom 30 meter long cables arrived through the post.

Aquarian Audio A5 hydrophone coiled on top of its cardboard box

On first impressions the A5 feels well built and sounds much more pleasing to my ears. Aquarian’s engineers confirmed it is intended to replace the H series in due course and it’s clear they have tried to improve on it, as you would hope from a new model. They also sent me an unpublished web page, showing the temporary (pre mass production) specifications for the A5. This, together with the actual units and my correspondence with AA, is what I’m basing this article on. Please note I have avoided giving specifications that AA considers temporary, as per their request.

The exterior design has changed considerably compared to the H series.

Aquarian A5 capsule and minijack connector

Aquarian Audio A5 hydrophone with dummy weight

Smaller capsule

Smaller in diameter than an XLR connector — the H2x capsules are larger. This offers the obvious advantage that you can fit the hydrophone through smaller gaps. But it doesn’t stop here as there is a way, through the other end, to access even smaller gaps. More on this below.

Detachable dumb weight

The stainless steel weight (115 g vs 105 g on Hx’s) is now removable. It has the same width as the capsule (keeping it to XLR diameter) and is designed to screw directly onto it, resulting in a seamless coupling. It is sold separately however, but in my opinion is essential if you want you want your hydrophone to stay submerged and relatively still, particularly in waters with currents.

New cable

The cable has been improved as well. My H2a’s came with the CA-51 ‘low-noise hydrophone cable’. The A5 uses their new CA-52 ‘coaxial hydrophone cable that offers a tighter shield with steeper helix angle, which substantially improves triboelectric noise while slightly improving flexibility’. In other words, it’s a better shielded and more flexible cable. (Shielding is the protection against incoming and outgoing electrical interference, which can translate in unwanted signals in your audio. Made of braided tinned copper in this case). It definitely feels less stiff and I would describe the cable memory as OK; it coils relatively easily considering it’s new and that I have 30 meters of it. AA’s hydrophones usually ship with a choice of 3, 6, 9 or 15 meter length cables. You can also choose a custom length and 30 meters is their maximum recommended length before potential signal attenuation problems. Handling cable noise seems the same or a little improved too, but I have no equal conditions tests to base this on. Note: judging from AAs website the upgraded H2d comes with the newer cable as well.

Smaller connector, with a twist

Instead of soldering the cable directly to an XLR connector (or other connector of your choice, as per H series) the A5 terminates in a low-profile 3.5 mm TRS (tip ring sleeve) output plug; your classic mini jack. This looks well sealed and the low profile design allows it to easily pass through small orifices (their words not mine), essentially through anything wider than 3.5 mm. The mini jack is designed to accept plug-in power (PIP) — aka phantom power — from digital recorders, which all AA hydrophones require to operate. But AA recommend using their PIP to XLR adapter to connect to an XLR input, and for good reason.

Aquarian A5 PIP to XLR adapter PCB

Aquarian A5 minijack clicked onto XLR adapter

PIP (Plug-in-power) to XLR adapter

The adapter converts the PIP mini jack to a balanced XLR output. It is sold separately and requires phantom power to work. Again the design is kept low-profile with all the circuitry tucked into a standard XLR connector. Using their adapter as opposed to other commercially available ones will lower the noise, and give it a true balanced output that will work with any recorder. It is installed by simply clicking in the cable’s male mini jack to a female socket inside the XLR adapter.

Personally, I find the adapter a double-edged sword. If I have to point out a weak link this would be it. Connection points in any audio setup involving liquids usually are, offering an opening to all sorts of problems. But also in this case the connection can come loose and un-click, given enough pull. This happened to me and it took a while to figure it out. Suffice to say I was delighted to know there was no problem with the hydrophone proper. It’s a shame I lost a whole day’s worth of recordings, but in future I know to where to check first if I loose signal again — and you know too.

This considered, AA have done a good job at minimising the “weakest link” by compartmentalising it. In the H series the connector was soldered directly onto the cable so if it caught any dampness, it travelled down the cable. I also got a hint that this modularity will make it compatible with planned cable extensions.

But the most significant changes are internal.

The sound

The new circuitry, is what gives the A5 a different sound. As I said, I cannot give specific figures but I can confirm that the sensitivity is higher than that of the H series. It also has a more sophisticated signal-conditioning board that will ‘give consistent gain, regardless of what it is plugged into’. The sales team told me the useful frequency range is also broadened. This confused me initially because the H2x’s boast a frequency response from <10 Hz to >100KHz and the bandwidth I had seen for the A5 was much narrower, certainly not in the ultrasonic spectrum. So I asked them what they meant by “useful”, useful to who?

This is what I got:

‘The "useful range" specification is not intended to be misleading, but it does confuse people. We have many users that will use the H2 for monitoring the presence or timing of high-amplitude ultrasound signals, like location pingers or anti-fouling equipment. So we do consider them useful at these ultrasound frequencies. But output is very low. Sensitivity is down 40 dB at 100KHz. The linear range is what you are concerned with. The A5 is actually filtered above a certain frequency, so it is not a good ultrasound receiver. It does sound much more linear throughout our auditory range though. We don't really want anybody to consider these measurement devices. They are designed for general audio recording and ecotour support, so neither will have data sheets similar to what you would see with our scientific grade AS-1’.

To my ears, the linear frequency response up to 15KHz results in a more pleasant sound than the somewhat mid-frequency dominant sound of the H2x’s. In fact, they remind me of the considerably more expensive Ambient ASF-1 hydrophones, which I rented on one occasion.

Price-wise the A5 is little more expensive than the H series hydrophones, but not by much. Getting the A5’s instead of the H2d’s, cost me an additional USD 50 per unit. This is mainly down to the additional parts, such as the PIP-XLR adapter and the dumb weight, being sold separately. Ultimately, I think this modularity is an advantage. Whilst there is a small amount of assembly required on unboxing, the instructions provided are clear and simple. There is one part I’m missing which was not yet ready to ship with the A5, and that is the contact mic converter. It consists of a moulded rubber cup that fits on the hydrophone capsule giving it a flatter profile, essentially converting it into a contact microphone. I found it really useful with the H2a - albeit not the easiest contact microphone to attach to different surfaces — and I’ll be getting those if they become available. Bear in mind that there is import duty if you are buying a custom length hydrophone direct from Aquarian’s US headquarters, or if you ever need a repair. As an indication, for two A5’s the duties, taxes and fees were GBP 97.66 (summer 2023).

What’s next?

First, it would be great to see the final data sheets for the A5 and how they stack up to other hydrophones, including AA’s H series. Personally, within the Aquarian family I would be interested in using their scientific grade AS-1 model. Beyond Aquarian I would be really curious to try and build one from scratch following DJJules’ World Class Hydrophone Instructables, which I believe some WSRS members are doing or have done already. (If so, an article on that experience would be a great read.)

A word about AA in general

I cannot speak highly enough of Aquarian’s customer service. They respond quickly, even considering the Europe-US time difference, and mindfully, always going the extra mile to solve any queries. In their website it reads ‘every product design is a balance of strengths and compromises. We strive to provide tools that are relevant to our customers without needless expense.’ It is fair to say i’m pleased with the A5s and in my opinion they honour their mission statement.

I cannot provide a pound for pound comparison until (if!) I find the H2a’s but in this link you can find two recordings made in a similar location, a sea loch in the Outer Hebrides - with a pair of H2a in 2018 and with a pair of A5s in 2023. The conditions and time of year were similar, in August on a calm sea. Both were recorded from a dinghy, close to shore, and the recorder is also the same, a Sound Devices 744T. They were not made on the same loch though or at the same depth, hence one (H2a, 15 m deep) sounds busier than the other (A5, 3 m deep). I also increased the gain on the H2a recording by 5 dB.

Underwater Sea Loch - A5’s

Underwater Sea Loch - H2a’s

(feel free to use these sounds under the Sound Ark License Agreement).

Here you can see the location where the A5 recording was made. The H2a recording was made on the next loch.

Also see these videos on David de la Haye’s Youtube channel, comparing the H2a to the A5:

Comparing hydrophones part 1 - David de la Haye

Comparing hydrophones part 2 - David de la Haye

Aquarian said they can still make H2a hydrophones for those wanting to match a previous purchase, whilst supplies last. These are also sold at discounted prices by several distributors I checked.

Important note: I’m not affiliated to AA or the distributors.

That’s all thank you for reading!