Let's Build Some World Class Hydrophones

Updated September 2022: SEE STEP FIVE FOR ADDITIONAL INFO

New Audio Demo Too!

Thomas Rex Beverly, a professional sound recordist, took a pair of these to Greenland in July. He released a commercial sound library using them. I put a link to the demo of the library at the end of this Instructable.

I spent 20 years in the Navy as a Submariner. Although I wasn't a Sonar Tech, I spent a lot of time in the Sonar Shack listening. You could hear all sorts of things; whales, crustaceans, pile drivers building north atlantic oil rigs, and of course, the occasional Russian Submarine. Being the Audio Guy I am, I found this fascinating and always wanted my own set of hydrophones to continue the journey. Which leads us to today's Instructable: Build your own professional grade hydrophones. These may not match the durability of what the Navy and commercial ones do, but they certainly meet or exceed the acoustic properties they do. Best of all, they are inexpensive to build.

We need three critical things for the acoustics:

1. The right transducer
2. A high impedance, low noise buffer amplifier PCB Here: https://www.jlielectronics.com/diy-accessories/p4...
3. Resin to embed items one and two in

Other items for the build:

1. Male XLR jack
2. Microphone cable (that can be submerged)
3. A mold to put the parts in - I have a couple ways to do this.

Supplies

Fully populated PCB available from JLI Electronics: https://www.jlielectronics.com/diy-accessories/p48...

Male XLR: https://www.redco.com/Neutrik-NC3MX-B.html

Microphone cable: https://www.redco.com/Mogami-W2549.html

SIngle Edge Razor Blade

Soldering Iron

Hand Tools

Step 1: Background

What is a Piezoelectric material anyway? The phenomenon was discovered by two french physicists (Yea, Go Physics!) Pierre and Jacques Curie in 1880. In essence, applying a force to some crystal structures, causes a voltage to be generated. The converse is also true, if you apply a voltage to the crystal it will deform. Like most discoveries back then, it was hard to turn the concept into useful things. Then in World War I, the need to find submarines became important, leading to the invention of Sonar in 1917. Of course we kept improving that over the years, leading to the modern submarine sonar systems on my three boats. Interestingly, the basic transducer designs didn't really change. The most common material today for the transducers is PZT or Lead Zirconium Titanate. It is used for ultrasound, sonars, fish finders and all sorts of other fun things.

It was so important to the Navy they wrote a standard for it: https://quicksearch.dla.mil/qsDocDetails.aspx?ide...

The cool thing for us is that you can buy single quantities of already prepared PZT material in various shapes, already silver plated on two sides, just waiting to be turned into a hydrophone. All we have to do is solder on some leads! The shape that ended up using and sounded the best to me was the cylinder, same as the NOAA article. There is far more to this and for a lot more theory read this for background.

https://www.americanpiezo.com/knowledge-center/pie...

Then there is this amazing paper from the Naval Postgraduate school

https://apps.dtic.mil/sti/pdfs/AD1068326.pdf

Before we talk about the embedding of the transducer into urethane, a review of underwater acoustics is in order. Sound behaves differently in water than in air. Water is a liquid and non compressible. We tend to think of sound being 20-20,000Hz in air. That is because that is what we can hear. Bats and other nocturnal critters can extend that into the 100Khz plus range, which they mostly use for echolocation. In the water, sound waves go from about 10Hz to 1Mhz. Yup, one megahertz. That is mainly for imaging and advanced sonar applications. Above that, sound gets absorbed quickly. It also travels much further in water. And when you are in the open ocean, lower frequency sounds can travel really far. The other thing about bodies of water is reflections at the surface and the bottom. Lakes and the ocean are like gigantic caverns with lots of echo and reverberant properties.

The other difference between air and water is “speed of sound”. In water, it is totally dependent on density. Which for the Navy and sonar, makes salinity, depth, and temperature really important. For example it is 1450 Meters per second in freshwater and 1500 Meters per second in salt water. In air it is about 345 Meters per second. Pay attention -- there is a quiz… For us we can ball park it 4.25 times faster. The reason I point this out is well, if we are going to record underwater sounds, we might as well do it in Stereo. So we should multiply the standard spacings for microphones by that amount to get a decent sound. Our ears are about 20cm apart or 7-8 inches, we should space the hydrophones at least 3 feet apart. For those of you with the calculators out… I am winging it. :-)

The last thing to take into account is how sound transfers from water to other things. This one can get complex. Sound is traveling in water as a wave. Changes in density cause a change in the speed and reflect or refract it. So, if we keep the resin we are molding the piezo element in as close to the density of water as we can, we will get maximum sound transfer without worrying about how the shape of the mold modifies the sound. That eases mold design.

Step 2: The Transducer

The most critical component to these is the actual piezo element we are using. I found one that works really well and is less than $20 each. What I found doing my research was that there are two types of transducers used, which also matches my Navy experience. The first is the Active transmitter. Think underwater speaker. The second kind, and what we are doing, is called Passive. Think underwater microphone. In my search I found a great article from NOAA This one showed the basics. I could not find the specific transducer they were using, but I found a company called Steminc, which has a plethora of them! I tried several which all worked, but this one, which is cylindrical, is the one I settled on.

Step 3: The Electronics for the Buffer:

After building many microphones with an Op Amp as the impedance converter, I smiled when I saw what they were doing in the NOAA article. They used a great one, but it was at EOL or “End Of Life”. It only made sense for me to adapt my circuit for use in a hydrophone. Piezo elements need a high impedance circuit to pick up the voltage they produce. This is similar to a condenser microphone. One of the problems with piezo elements, and this applies to pickups on musical instruments, is that they need a high impedance preamp on them. We need about a 1Meg input impedance vs the 1Gig that a condenser needs. This has to do with the inherent capacitance of the transducer. The piezo element is 6600pf. So with a 1 meg resistor we have a low cutoff frequency of 24hz. This works really well. If it goes into a preamp with an input impedance of say 10K Ohm, (typical of line level inputs) it has a cutoff of about 2.2Khz. Which will sound really tiny and bad. Thus piezo pickups have a bad reputation. For those thinking ahead already, this circuit makes a fantastic pick up interface that runs off of phantom power.

This uses the same Opamp as in my Condenser Microphone instructables.

https://www.instructables.com/OPA-Based-Alice-Mic...

https://www.instructables.com/True-Condenser-OPA-M...

In fact we are using almost the exact same circuit. The board is smaller 26mmX29MM to allow embedding into resin. And a couple components (the capacitors) are smaller in value but provide excellent results.

Here is how it works:

The Blue section has our two incoming 47Ohm resistors coupled into two 22uF capacitors. This passes the signal out to the XLR connector and the mic preamp. Tapping off of this are two 2.2K resistors. These feed the incoming phantom power to the Green section. The Green section has a 12V zener and associated resistors and capacitors which generate filtered 12VDC. Then in the Pink section, we have a resistor divider with two 47K resistors and a filter capacitor. This generates a stable voltage that is roughly half the 12 volts. We are using this as a “virtual ground” for the operational amplifier in the Red section. There are two op amp stages. One is a non-inverting buffer connected directly to the Piezo element with a 1M resistor connected to the virtual ground from the Pink section. The Yellow section are filter caps for the op amp. The .1uF should be as close to the supply pins as possible.

The op amp is the heart of the circuit. The OPA1642 has really low noise, low distortion, and high bandwidth. That gives us a flat buffer amplifier, probably good to a megahertz. Then we have an inverting buffer from the second op amp stage and its two 2.2K resistors. This lets us drive a differential signal into the mic preamp. All in a little 26mm X 29mm PCB.

You can order the board from PCBWay if you want to assemble it using Surface Mount Components, or you can order one premade and populated from JLI Electronics. I recommend that.

PCBWay: https://www.pcbway.com/project/shareproject/OPAAli...

JLI Electronics: https://www.jlielectronics.com/diy-accessories/p48...

Step 4: The Resin and the Mold

I tried to find the exact urethane resin that the NOAA paper used but couldn't. What I did find was perfect as it has a specific gravity, (which is related to water density pure water = 1) of 1.05 which is really close. And, the specific gravity of seawater is 1.03 so it is even closer to the seawater.

We are using this: https://www.amazon.com/gp/product/B00E3ZJ9XW/

It is easy to work with and most importantly, it has a Specific Gravity of 1.05 g/cc or almost identical of water. This is really important for sound transfer. Just like glass in water. The closer to the index of refraction for water that the glass is, the harder it is to see when it is submerged. For comparison, Water is 1.33 and glass about 1.5 so that is about a 10-12% difference. Acoustically this resin has a 3-4% difference making it quite transparent to underwater sound.

Step 5: Connecting It All Up

NOTE: For 60hz Hum I am adding a Ground Wire to be in contact with the water!

See the photos with the solid copper wire connected to the shield. This was added September 2022 due to feedback from several builders who experienced this. Doing some more research there are commercial ones that have a metal portion that is grounded and I found another NOAA guide from 2013 that recommends this as well.

The ground wire does not have to be long. Just that is is contact with the water the hydrophone is used with.

Wiring the Piezo Elements

These are made from ceramic piezo material. It has silver plating on the inside and outside surfaces which can be soldered with some caution. PZT material has something called a “Curie Temperature” Normally that is associated with magnets. As in if you heat it above the curie temperature, it is no longer a magnet… For piezoelectric material, it means that above that temperature, it affects the piezoelectric properties. Which we want to avoid. Also, it is ceramic so heating it unevenly will cause stress and potential breakage. With that said, just be careful and watch the video. It is easy but be careful.

To maintain the polarity the same for the two transducers, we need to wire them the same. I chose to wire the Signal lead to the outside and shield or ground to the middle.

I chose this particular microphone cable for its durability, strength and ease of working with it. Note, there is “Waterproof” microphone cabling available. That is really for permanent installations where the cable is submerged for years. And it is priced accordingly. I have tested the fully built hydrophones for 24hours in my swimming pool with no issues.

Wire the PCB end mic cable per the picture and the video

The XLR connector end is standard Pin one shield, Pin two (hot) to blue and Pin three (cold) to clear.

IMPORTANT! After you solder the cable to the PCB and XLR connector, TEST IT BEFORE MOLDING!!!!

Step 6: Embedding the Assembly

My initial prototype was with two 50Ml measuring cups. I cut the bottom off of one and then glued it with hot glue upside down to the other one. Use enough hot glue to seal them together. A small bead the whole way around is recommended. There is no acoustic difference in the final build. We are using a two part resin. You mix equal parts “A” and “B” then stir it up. Couple things to note. The resin gets just a bit thinner (less viscous) as it mixes and blends. It gets a little clearer at that point too, That is about a minute of mixing. This is when it is ready to pour. Over the next few minutes it starts to set and gets thicker. About five minutes later, it turns white as it sets and hardens. In another 15-20 minutes, it is set hard enough to break the mold away. I used a single edge razor blade to score the mold on one side and then break it free. I did not use a release agent on the mold. This resin is pretty forgiving that way. One final note on the resin. You will see bubbles as it mixes and you pour. They do not affect the sound quality at all.

Before pouring the resin in, make sure the PCB and piezo element are not touching the wall or bottom. That ensures everything is sealed.

Note:

The resin I am using worked for me without a Release Agent. You SHOULD use a release agent with any other kind of resin.

Step 7: Testing and Use

These are quite amazing! The circuit I am using gives about 100dB of dynamic range and between the cylinder and the circuit the frequency response extends to several hundred kilohertz. Along with capturing biologics, which I plan on when I can travel again, you can use them to record sounds that can be slowed down significantly to produce really amazing surreal soundscapes. These are a sound designer's dream. To do so, record with them at a high sample rate like 192Khz. I'm using a Zoom F6. Bring the file into any editor (Audacity is a great one for this) then slow it down 4-8 times. The above graph shows the sonic profile of the hot metal into water. Recorded at 192Khz into the Zoom the I plotted a 2-3 second snippet. See my demo at the end of the build video for this.

Here is the Demo of them used off the coast of Greenland July 2022 Soundcloud Demo:

Here they are in Dry Ice and Red Hot Metal

They are a great addition to your sonic arsenal. Enjoy! Please comment, ask questions and let us know if you build a set.