I have used hydrophones for several years, mainly Aquarian H2a XLR and JFR piezo’s. But I have never been happy with the sound quality.
Comparison is hard to find on the internet and most hydrophone recordings there have been fixed in a post process so it doesn’t give me clear information how it actually sounds. Most hydrophone manufacturers don’t give a standard or important information so buying a quality hydrophone for thousands of dollars can be a risky task.
Last summer (2020) I spent several hours recording on Jökulsárlón Glacier lagoon. I noticed behind the sound of H2a was something very interesting soundscape which H2a couldn’t clearly capture.
So I decided to get a better hydrophone before I continue to record in this lagoon.
I contacted both B&K and Teledyne Reson to check the price and options. Both his manufacturers gave me useful information but the price was higher than I was ready to pay for a good car so I continued the search. Ambient has two interesting hydrophones but they do not show important information. When I ask for Frequency and S/N graphs, Ambient just sends me the user manual which is already on their website. Same with Cetacean Research and Dolphin Ears, which either give me strange answers or no answer So I continue to search through several manufactures.
Suddenly and surprisingly I found Benthowave, a company which was ready to custom made hydrophones for me, hydrophones where I got all necessary information for my needs. It was almost too good to be true, so it takes me several weeks to decide to let Benthowave build my dream hydrophone.
The base was built on BII-7122 but additional with balanced output w. BII-1082 ultra low noise amp and 15meter long cable.
No problem, I would get it after 6 to 8 weeks! But that was too late for me. My recording project actually started that time in late May. Before that project starts it is necessary to build a customized floating platform and power supply for the new hydrophone which can take two to three weeks.
So I ask Benthowave which low noise hydrophone they have in stock. It was only a single output BII-7121 with internal BII-1081 amp.
Two weeks later they were in my hands.
My first impression was a huge disappointment. I was almost sure I had thrown money out of the window. But after I build my second power supply, these hydrophones start to rock. It was two key figures that were important to know to let them work without problem.
1. While this hydrophone is so sensitive at low frequency, higher voltage means better performance to avoid internal amplifier overload/dropout.
2. These hydrophones are sensitive for EMF. From the power supply box (made out of metal) must be a wired ground connection to the water which will be recorded if any EMF pollution is around
Other things must be kept in mind because these hydrophones use an external battery power supply, but NOT Phantom power. Use a recorder which has „Combo input“, (XLR and jack input) or 3,5mm jack input. Use only Jack to connect the hydrophone to the recorder so you will never mistakenly get Phantom Power into the hydrophone. It might be possible to use Triton Big Amp for recorders that don’t have Combo input. But I haven’t tested it yet. I will write an update when it has been tested. I use non-standard XLR connectors in and out of the battery box so even blind men can not connect the hydrophones without correct cables.
Conclusion of the comparison
I am not sure if this comparison gives a correct picture of the Aquarian H2a. Mine have a LOM phantom power adapter which has a small amount of gain and probably makes tiny changes in the frequency curve.
Anyway, with this adapter it sounds similar to the original. I put this adapter to avoid extra noise which I got only in the SD744 recorder and with HPF off. This adapter also avoids other strange and unusual noises which I think is caused by the 48Volt phantom power which I think is too high. But with the adapter the H2a is driven by 5Volt.
This comparison was done in Sundahöfn port of Reykjavik. In a corner which is mainly used for depreciated ships so it was quiet, without loud ship engine noise.
The hydrophones were placed side by side in 1,5m depth with 2,5m separation, from a floating pier with two bonded boats. The gain on the MixPre6 was at 30dB for both hydrophones and HPF was off
There is pretty much difference between these two hydrophones, both in frequency range and sensitivity. Benthowave BII-7121 frequency range is from 0,5 hz to 60 Khz at +/-3dB V/μPa. On spectrograms I can see it can as well detect sound up to 80Khz or as high as I can record at 192Khz.
The hydrophone sensitivity is -158,7 +/- 0,2dB plus +26dB BII-1081 amplifier gain (-185dB) and self noise is 25dB μPa/1KHz.
Aquarian H2a frequency range is 20Hz-4.5KHz +/- 4dB and the self noise is „low noise“ whatever it means. Sensitivity is -180dB re: 1V/µPa. My Aquarian hydrophones are not new, so new model might have increased sensitivity. Compared to BII-7121 it seems be close to -165dB. With the LOM Phantom power adapter the sensitivity is closer to -170dB re: 1V/µPa and the frequency range seems to be on spectrogram close to 100-7Khz.
It was not easy to normalize the level of these hydrophones while they have so different frequency ranges. To do that I tried to normalize the level by listening to constant pump noise in the background in the recordings and make it as equal as possible in combination with the level meters.
At the moment I can’t say much about the self noise. It could be difficult to compare these two hydrophones while they are so different in frequency ranges. I somehow expected to hear lower self noise in BII-7121. But it might be as good as it gets. It is -24dB below sea state zero at 1Khz, which is very good, even for much more expensive hydrophones. But to know exactly what it means for me I need to test them in quiet lakes. I will put an update here as soon as I have done that.
I must say, the BII-7121 has very nice sound quality, Just as I expected. It is almost possible to hear the depth of the field, while H2a sounds flat and all high frequency is missing. But keeping in mind these two hydrophones have very different prices so in fact it is unfair to compare these two hydrophones. One piece of BII-7121 cost about USD 1,244.- (base price), while H2a XLR cost USD 194. I should rather compare Benthowave to Teledyne Reason like TC-4032 which cost EUR 3,640.- or Bruel & Kær like 8106 which cost DKK 86,470.- I think they will all sound similar.
Benthowave seems to be built on rather cheap plastic materials and glue, it looks very fragile and seems to be not as robust as Teledyne or B&K. But as long as Benthowave can offer me the same or similar sound quality at a lower price I am happy with that.
BII-7121 could have been heavier. It weighs only 95gr while H2a weighs 125gr which is even too light. The BII-7121 cable is Gepco MP1201 Quad Star.
In the recordings below you can hear in some headphones an unpleasant „low frequency noise“ due to the wind. This is because there was a lot of wind on the day when this was recorded. The cables to the hydrophones were mostly up on the pier where the wind got an awkward amount of play around them.
This „wind sound“ will usually disappear once I have built the „floating platform“ for the hydrophone.
I will post pictures when its done, plus other experiments too.
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Recording straight from the recorder, only add +12dB in post
Benthowave BII-7121
Recording straight from the recorder, only add +12dB in post
Aquarian H2a
Normalized recording BII-7121
Normalized Recording H2a
Recording with BII-7121 of a Tugboat propeller 250-300 meter away. This recording is straight from the recorder, no extra gain.
Notice at 3:55 the amplify goes up and down. I am not sure what it is. I think it could be a strange behavior of the hydrophone when they are in „strong“ current or water flow, in this case from the tug propeller, which was though not very strong. It could be as well the ground cable which lost contact to sea while waves pass the pier.
H2a normally makes a low frequency noise in water flow or current. But I am sure this current was not strong enough for H2a to make a noise.
I will write an update as soon as I figure out why BII-7121 acts like this.
All updates will be added here below by date.
More information about hydrophones & underwater sound:
Construction and testing of low-noise hydrophones (pdf)
Sound in the sea
Hydrophone Review: Ambient ASF-1, ASF-2, Aquarian Audio, JrF by Sach Poff
Quality open headphones are recommended while listening at mid level.
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Update 9th of May 2021 ——-
Hydrophones with frequency range down to 1Hz are very sensitive for all movement in water or water flow, which seems to cause an overload for the internal amplifier so the audio output turns on and off while it happens.
At the moment I would recommended for everyone who like to order Benthowave hydrophone to ask for custom HPF at 10Hz or 20Hz, otherwise the hydrophone will be only usable during calm days
If default HPF is in the hydrophone then is it necessary to make some kind of „flow noise reduction“ for the hydrophone, similar protection cages which CRT make for its own hydrophones.
Rycote BBG 25mm fits perfectly for this BII-7121. But that is not the final solution. Original BBG did not work well so it needs some changes which I am working on.
I will post an update later when I think I have found the final solution.
Update 30th of August 2021 ——-
The BIG-7121 tested with a 25 volt power supply (with six 18650 Li-Ion cells).
It does not completely fix the overload / dropout in the amplifier.
Compared to 16 volt and recording from SOT Kayak the hydrophone can now withstand much higher waves (up&down) and tide current (@8m depth)
Update 17th of October 2021 ——-
Underwater recording with 25 volt power supply @3m depth. Listen here
HLJÓÐMYND - SOUNDIMAGE 