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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.

__________________________________________________________________________________

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)

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I recently noticed the best tripod I can get to avoid high levels of „mechanical wind noise“ in recordings. It is simply a hummock. I have many times recorded nature sound by leaving omni mics on the ground. But it has not been as simple for cardioid mics because it changes the EQ on the frequency range, especially when the mics are in the windshield.
Here below is a recording I did in Flói bird reserve in south Iceland. The average wind was probably around 4-5m/sec with some gust up to 5-7m/sec. The high pass filter was at 40Hz so mechanical wind noise should be with a typical tripod clearly audible at 3m/sec.
Keep in mind this is a recording of silence. It was barely nothing audible while this was recorded
It was recorded with 48dB gain and in post the gain was increased again about 27dB.
Most of the background noise is the surf on the south coast which is very noisy. The wind is almost constantly wiping the ground and airplanes make a rumble noise for many minutes. You will also hear wind noise but far less than it would have been with a typical tripod. 
One of the reasons I think it is so effective to put the rig on the ground, is mainly because the LCT540s is a heavy mic. So while the windshield lay so heavily on the ground, the ground works like a damper for all vibration on the windshield which therefore make less „mechanical wind noise“
Most bird species are in distance so this is not a very attractive recording. But many things are going on in this wide open space in south Iceland and it is always interesting to listen to soundscape which is too quiet for most human ear.
Quality open headphones are recommended while listening at low to mid level, or in speakers at low level.

(mp3 256kbps / 60Mb)
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Recorder: Sound devices MixPre6
Mics: Lewitt LCT540s IRTcross setup
Pix: Canon EOS R
Location: 63.901024, -21.192173
Weather: cloudy, calm up to 7m/sec, around 12°C

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Wind is very often the recordist´s enemy. Especially if the rumbling noise of „hammering membrane“ is not acceptable.
Some recordists may say this rumbling noise is just normal. Everyone will hear similar rumbling noise when they stay in wind.
For me it sounds like „clipping distortion“.
Wind protection is one of the most important thing for nature recordings, especially in the country where wind below 3m/sec. is almost unique.
I have tried several wind protections. My own, Rode Blimp and Rycote Softie and Modular series. All of them sounds similar. So it was welcomed when Rycote introduced the Cyclone windshield. Cyclone have Floating Basket Suspension, which is very nice. Until now, it has been only be used in the overpriced Cinela products.
Rode NT1a is one of the best cardioid microphones available today for nature recording. But sadly it is not build for outdoor use, so it has some poor futures like handling noise.
So when I choose it on the field, it has been important to keep it in Rycote modular windshield, place it close to the ground (sadly very often too close) and pray for completely calm weather. Then pray again for nice outcome.
I per-order a pair of Cyclone mini windshield last summer and got it in mid September. My plan was to fix them with parallel MKH20/40. But I also gave my modified NT1a a try when I saw it was almost „plug and play“ to fix it .
Without fur Cyclone was not far from to be equal to Rycote modular series with fur. But when Cyclone was dressed in fur the rumbling noise almost disappeared in wind around 5-7 m/s. That was a huge success.
Following recording is a short part of overnight recording from Stafholtstungur, in the west of Iceland. The gust goes up to ca. 7m/sec. and the recorder HPF was set at 80Hz. The rig is about 1 meter above the ground.
Some rumble noise is audible in this recording, but some of it could as well be a vibration from the tripod. Keep it in mind this is NT1a which is particularly sensitive for handling noise.
I will spend more time to test this setup but it looks like I need to order another pair for my MKH20/40 rig.
Quality open headphones are recommended while listening at low to mid level.

(265kbps / 36Mb)

Recorder: Sound devices 744
Mics: Rode Nt1a NOS setup
Pic: Canon EOS-M

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When I use boom in recording I have used Audio Technica BP4025 stereo mic in Rycote windshield. It is not too heavy and it gives me stereo recordings. But very often I have been disappointed with the „musical“ sound quality, so I don´t often get quality ambiance recordings with this setup. Saying that, it does not mean BP4025 is a bad mic. BP4025 has a very low noise floor. It is the perfect mic for special circumstances like where size does matter and for very quiet environment/nature recordings.
DPA4060 is a pretty sounding miniature mic, sold in pairs and much lighter than any original stereo mic on the market. That means it is perfect to use for boom recordings. But because DPA4060 is an omni mic it is necessary to separate the capsules to get stereo. It is done in two ways. Separate the capsules with space (around 40cm) to get time difference, or place them in two sides of some sonic baffle materials.
The BP4025 was in a short Rycote WS2 windshield which has overall length (wide) 34cm. So it was important to place the capsules each side of some baffle material to get acceptable wide stereo.
I end up with a simple „binaural“ project I made out of wooden leftover (see pictures).
I decided not to use silicon artificial human ears because the ears will change the frequency curve at 2,5Khz and 5,5Khz, which means I needed to fix the EQ afterwards on all recordings made with this rig.
The result was stunning. The rig I made was lighter and better wight balanced than previous BP4025 setup. The overall wight is only 750gr (mics+wood baffle+WS2 basked+fur). It also gives me wider „stereo image“. But best of all, it withstands wind- and handling noise much better than previous BP4025 setup.
Following recordings are 4 and were made in three locations. The two first one are waves on seashore, then one from a cliff, 300 m above sea level with seabirds and the last one is from a football game in Reykjavik. All where recorded on Sound devices 744.
All recordings are straight from the recorder. Just cut and paste, fade in and out and then down grade from WAV to mp3.

(256kbps / 29Mb)

The two first recordings are big and then small waves. The mic have fur, HPF is off and gain at 45db. The mics are faced from the beach to the fjords. You will hear car pass by „behind“ the mic. Notice, you will hear a short „drop out“ when it pass. It is because it passes my car which was located on the road side. See location.
In the cliff recording the mic is without fur. It is possible to hear how strong gust sounds on the Rycote basket, but that is anyway much better (or different) than the my previous BP4025 setup. HPF at 40Hz and gain at 45dB. The boom with the mic reaches the cliff edge about one meter. See location.
The last recording is from a football game celebration in downtown Reykjavik. No fur, HPF at 40Hz and gain at 40dB. The mic stood on a boom about 1 meter above the crowds head. See location.

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Three stereo microphones noise and sensitivity comparison.
Shure VP88 – Rode NT4 – Audio Technica BP4025

This recordings include a spoken word from pocket radio at very low volume and ticking alarm clock in 1,6m distance. The volume settings on the radio was so low, the sound was hardly audible with bare ears. Noise from radiator pipeline is audible in the background. Miscellaneous bird life is outside and should be also clearly audible.
Keep in mind. This test is only noise and sensitivity comparison. High sensitivity and low noise is VERY important for nature recordings. This comparison does not give any information how this microphones sounds for music recording or how they withstand high pressure sound level.
See spectrogram and pictures
Quality headphones recommended while listen.

Shure VP88, Rode NT4 and Audio Technica BP4025 direct from recorder. All at same gain level at 55dB.

All three recordings are now independently level normalized up to 0dB.

Links to the products:
Shure VP88
Rode NT4
Audio Technica BP4025

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Some months ago my plan was to buy Rode NTG3. I was looking for a low noise and better sounding microphone than ME66 I already own. But after some research at home I found this microphone even worse than ME66 for nature recording.
The audible noise and sensitivity between these two mics was almost equal, but NTG3 have noticeably higher low frequency response.
But what surprised me most was all the noise in NTG3. If something, it was even more than in ME66. But spectrogram shows a shocking pictures for NTG3, a white noise in the whole frequency spectrum up to 50Khz. See spectrogram.
This NTG3 was sent back to the shop, but I was never sure if I did some mistakes in this measurements. So last week I vent to the shop and picked up some recording samples on the same NTG3.
The recorder was Sound devises 744 at 24bit/48Khz. NTG3 was in channel 1 and for comparison ME66 was in channel 2. They were laying side by side and the gain was in full position. Input filter for both channels was at 80Hz, 12dB/oct.
It is no doubt. There is some strange white noise in NTG3.
The sound sample below is about two minutes long. First minute is NTG3 and the second is ME66.
This particular NTG3 can be defective, so I am waiting for next supply of Rode NTG3 in the shop.
I will update this post as soon after I have got a chance to test some other NTG3 mics.
See more pictures and spectrogram in this last test.

Conclusion – Summary

The spectrogram looks very bad for NTG3 but the mic is not as bad as it looks. The ME66 datasheets display referred noise as 10dB(A) and the NTG3 datasheets display referred noise as 13dB(A), so  ME66 has lower referred noise than NTG3. This difference is probably what we see on the spectrogram.  This microphones does not have exactly same bandwidth, as the ME66 has a reduced audio bandwidth which in turn reduces the referred noise. The ME66 top end roll off starts around 14Khz (0dB) and is -6dB at 20KHz, NTG3 is +2dB at 14KHz and -2dB at 20KHz.
NTG3 is RF biased microphones designed to withstand high humidity and harsher environments than the ME66 condenser microphone. So while the NTG3 and ME66 are both shotgun microphones they are designed for different operating conditions.
NTG3 would be my choice for most “normal sound pressure” level. But for quiet nature sounds where most audible sounds are in low level and mid-high frequency, I would choose some other mic.
In my opinion Rode NT1a is one of the best.  Sadly NT1a is not a shotgun or small diaphragm condenser that can be fit nicely in a Blimp. But NT1a have only 5dB(A) noise and that makes the whole different for those who don’t like to use noise reduction software.    See solutions how to fit NT1a in Blimp in NOS and XY setup.
For those who are searching for sound quality in NTG3 I recommend this search: http://www.google.com/search?hl=en&q=NTG3+vs+ME66&meta=
Also this one:
http://www.google.co.uk/search?hl=en&q=NTG3+vs+MKH416&meta=

Download mp3 file (192kbps / 3Mb)

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Ekki er auðvelt að nálgast samanburð á hljóðnemum á netinu. Það er því ekkert grín þegar til stendur að fjárfesta í einhverjum slíkum. Sjálfur hef ég verið að leita að góðum og fyrirferðarlitlum MS hljóðnema. Flestir sem eiga að uppfylla þær kröfur eru ekki gefnir. Hvort sem MS uppsetningin muni samanstanda af tveimur hljóðnemum eða einum, þá er líklega Shure VP 88 nokkuð þekktur sem ódýr MS/steríó hljóðnemi. Þó hann sé mikill hlunkur þá freistar verðið til þess að honum sé gaumur gefinn. Í safni mínu er að finna Rode NT4 steríó hljóðnema sem er ögn ódýrari en Shure VP88. Hann er talinn nokkuð góður þó ég telji þrönga steríómyndina takmarka notkun hans.
Á dögunum fékk ég að prófa VP88. Við fyrstu kynni varð mér ljóst að ég var ekki að kynnast tímamótahljóðnema. Ég stillti hann á víða steríómynd og prófaði hann samhliða NT4. Stillti ég þeim báðum á sama stað á borði þar sem þeir lágu á púða. Í herberginu var lágt stillt útvarp í gangi í um þriggja metra fjarlægð. Veggklukka tifaði á vegg í tveggja metra fjarlægð og kæliskápur var í gangi í þriggja metra fjarlægð.
Hljóðnemarnir voru báðir tengdir við Sound device 305 formagnara þar sem slökt var á hljóðsíum og styrkur hafður í botni til að fá fram grunnsuðið. Tekið var upp á Korg MR1000 upptökutæki.
Sjálfur kynni ég svo hljóðnemana þar sem ég sit einn metra fyrir aftan þá.
Hljóðdæmið gefur ekki fullkomna mynd af þessum tveimur hljóðnemum en segir þó til um suð og næmni.

English summation:

Rode NT4 and Shure VP88 was placed in the same place. Connected to Sound device 305 preamp. All filters at zero and gain and faders at 100%.
You shold hear the radio at low level (3 meters away), clock on a wall (2 meters) and a refrigerator (3 meters)
This is not a perfect test, but will give some information about noise and sensitivity between this two mics.

Sækja mp3 skrá.   (192kbps / 1,83Mb)

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