After building my original hydrophone project, I thought about ways to approach the project from a different perspective. I wanted a stereo recording and a compact setup that was waterproof. Here are the results of a lot of experimenting and redesign: The Hydrophone Drop Rig. I built a few variations that run the gambit of splashproof, to full on: Take it snorkeling or diving with you. It uses the same Piezo Cylinder that is in my original hydrophone build. That project is in use globally by several nature recordists, and was involved in a hydrophone shoot out at Skywalker Sound. It bested all the ones they had including a $3K one that is no longer made. Another improvement in the "ease of build" part is the mold we are using. It is a silicone egg shaped mold, normally used to make chocolate candy eggs. In short we are:

Molding a piezo element in plastic resin
locating the electronics near the recorder
Putting the recorder and electronics in a waterproof case

The Underwater Drop Rig has the same acoustic properties and sound quality as my first instructable. It is a great project for a science fair and investigating amphibians and other water borne animals to see if they actually produce vocalizations. https://www.npr.org/2022/11/03/1133418579/turtles-sound-acoustic-communicate

Supplies

Parts/Supplies

Piezo Transducer: https://www.steminc.com/PZT/en/piezo-ceramic-cylinder-26x22x13mm-43-khz

Cases:

Scuba Choice: https://www.amazon.com/gp/product/B006UKNLBE/

Pelican Case 1050: https://www.amazon.com/Waterproof-Case-Pelican-1050-Micro/dp/B001PYL1BM/

Plano Waterproof Case: https://www.amazon.com/dp/B003JFOZH2/

Wire: Mogami W2524: https://www.redco.com/Mogami-W2524.html

Wire: Mogami W2947: https://www.redco.com/Mogami-W2947.html

Wire Mogami W2696 (3.5mm Jack wiring) https://www.redco.com/Mogami-W2697.html

Printed Circuit Board: https://www.jlielectronics.com/diy-accessories/dual-hydrophone-pcb/

Resin for Molding: https://www.amazon.com/gp/product/B00E3ZJ9XW/

Silicone Mold: https://www.amazon.com/gp/product/B077R98SD8/

Epoxy: https://www.amazon.com/J-B-Weld-ClearWeld-Syringe-25mL/dp/B09C6R4VSQ/

Liquid Electrical Tape: https://www.amazon.com/gp/product/B0000AXNOD/

Silicone Grease: https://www.amazon.com/Waterproof-Silicone-Grease-1-Pack-88693/dp/B000DZFUPC/

3.5mm audio cable: https://www.amazon.com/gp/product/B004G7Q53Y/

3.5mm Audio Jack: https://www.redco.com/Rean-Neutrik-NYS231BG.html

9 Volt Battery Clip: https://www.amazon.com/Battery-Connector-Plastic-Experiment-Equipment/dp/B08SL9X2YC/

Velcro Tape: https://www.amazon.com/Adhesive-Industrial-Strength-Strong-Strips/dp/B097TSBJS7/

Recorders:

Tascam DR-05X https://www.amazon.com/gp/product/B07N3FC7N3/

Sony Voice: https://www.amazon.com/Sony-ICD-UX570-Digital-Recorder-ICDUX570BLK/dp/B082QL6KLG/

Sony PCM A10: https://www.amazon.com/Sony-Portable-Studio-Recorder-PCM-A10/dp/B07NF17GDJ

Step 1: How a Hydrophone Works

The heart of our hydrophone is a piezo cylinder. Piezo materials exhibit the property that when compressed, produce an electrical signal. And oppositely, when they have a voltage applied to them, physically expand and contract. Thus, they make great transducers. Sound travels in water like it does in air, by having the molecules bounce off of each other as the sound wave travels. With one big difference; water is a liquid and is not compressible. Air is a gas and is compressible. The molecules of water are much closer together, thus the speed of sound is faster. In air it is about 340M/sec.That goes up to 1480M/sec in water, or 4.35 times as fast. Remember that for later.

Back to our cylinder. We need to electrically insulate the cylinder and protect it. To do this, we will embed it in plastic resin. To minimize any changes in acoustical properties, we need to embed it in something with the same density, or as close as we can get, to water. Density affects the speed of sound in a liquid or solid. We minimize refraction and acoustical changes in the incoming sound by matching the density. I found a great casting resin for this in my previous hydrophone instructable, which we are using again here. I spent 20 years in the Navy on three different submarines. This let me occasionally hang out in Sonar with headphones on, listening to the ocean. For a deep dive into Navy hydrophones see this: https://apps.dtic.mil/sti/pdfs/AD1068326.pdf

Modern hydrophones using piezo transducers need a high impedance buffer to prevent loading down the signal. This minimizes any change in frequency response. They are either embedded in the “Wet end” like my previous instructable, or cabled to the dry end, where it is located near a recorder. That is what we are going to do in this project.

Step 2: The Electronics

I tried to make the electronics portion as simple as possible. It consists of one OPA1642 dual FET input Operational Amplifier (Opamp). In order for the opamp to work correctly, it needs a split voltage power supply. We are emulating that by using a virtual ground that is halfway between VCC and ground. That is formed by a resistor divider with two 47KΩ resistors and a 22uF filter capacitor. The signal from the transducer is coupled into a 1Meg resistor to ground to develop the voltage from the piezo cylinder. This also sets the input impedance and low end frequency response. The piezo element has a capacitance of 6600pF, thus our 3dB point is 24hz. Then it goes into a textbook unity gain buffer. Finally, it is coupled out to the recorder via a 22uF capacitor in series with a 47Ω resistor. This prevents the DC from our virtual ground passing through to the recorder. The 47Ω resistor also keeps the opamp stable when driving longer cable runs. The whole circuit is powered by a nine volt battery. An alkaline one will run the circuit for several days continuously. Caution: there are rechargeable LiPo 9V batteries out there. They have a switching boost converter in them that is noisy. I tried a few with poor results. We are using shielded coaxial cable to connect the transducer to the circuit and a short 3.5mm cable to connect the electronics to the recorder.

I also learned that we need a ground connection to the water. This prevents noise and buzzing. We are building a “stereo” rig here so we only need to connect one of the hydrophones to the water. This entails soldering a short 18-12 gauge solid copper wire to one of the shield wires on the Piezo cylinder and extending it beyond the mold.

Step 3: The Case

This was a big challenge to solve. I had someone ask how my original hydrophone could be so good while being so inexpensive compared to a commercial $5000 setup. The answer is that the commercial one is designed to be submerged for months at a time, and rated for 1000 ft of depth. I ended up trying several case styles and ideas. Basically we are taking a pelican style case that is IP67 rated and another smaller one that is meant to hold credit cards and car keys while diving then modifying them. By modifying, I mean drilling holes for wires to go through and sealing them up with epoxy. Yes, this totally voids the warranty of the case but hey, that's part of the fun. I am a navy Submariner and my last tour of duty was overseeing the Navy’s deep submergence unit and the USS Dolphin. I am quite familiar with how the Navy handled hull penetrations. For our purpose, the case post epoxy will be just fine. We are also not taking these to extreme depths. The dive one has been tested to 60 ft and the Pelican on to about 12 ft. We are also going to use silicone grease on the O-Ring to help maintain a seal. I’ll show you how to test them prior to use. The case needs to be big enough to hold the recorder, the PCB, a 9 volt battery, and the wires. The Pelican case is large enough to hold a bit bigger (and less expensive!) recorder and a USB battery, allowing for several days of recording. I also looked at a Pelican “knockoff” case from Harbor Freight. It is good for splash resistance, but I wouldn't submerge it for 24 hours. The three I tested before drilling holes leaked over 24 hours.

Note: Leak check your cases before drilling holes in them.

The tricky part:

Drilling holes in the case and sealing in the wire penetrations was a bit more of a challenge than I thought. I have a bunch of 22 gauge shielded Teflon insulated wire from a previous life. It is wonderful, durable, and did I mention it is insulated with teflon? Mil Spec Type E stuff. After multiple attempts to seal it, I realized that the teflon prevented silicon seal, epoxy and other sealing methods from sticking to it. Thus eventually allowing water in. When testing for leakage, I did 24 hour leak checks in my pool. What I learned was, you need the outer insulation to stick to the sealing material. I ended up using Mogami instrument cable and, for some thinner cable, Mogami SVHS video cable. Remember S-Video cables? Two coaxial cables next to each other. They are easily split, easy to strip and work with. More importantly, both have insulation that seals well and sticks to epoxy and silicon sealant. Cleaning everything with a wipe down of Isopropyl alcohol right before applying the epoxy and silicone seal.

Step 4: The Build

I put together an assembly video with all the steps in it here:

Here are the steps we will follow:

Cut the wires to length. I used 4-5 feet for the dive case, and 8-10 feet for the Plano/Pelican case version.
Strip, prep, and tin the wire. You need to center the wire and the Piezo Cylinder for embedding.
Solder one end of the wire to the piezo cylinder.
Center conductor to the outer part of the cylinder
Shield or ground the inner section. You want to have center and shield lengths such that wire is centered above the cylinder.
Solder a short 3-4” solid copper wire to one of the shield connections. Form a small half loop in one end and solder that close to the shield outer insulation junction.
Embed the cylinder in the resin using the egg shaped mold. See the video or my previous instructable for how to do this.
Prep the PCB. See the photos and video for the best length. This consists ot attaching the signal out 3.5mm jack and the nine volt battery clip.
Cut the 3.5mm molded cable in half, then strip and tin the inner red and white wires. Or make your own with some Mogami W2697 and a 3.5mm male jack.
Cut the 3.5mm wire to length (4-6 inches) and prep the shield and center conductor.
Solder the wires to the Signal "A", "B" and ground. I used a short wire to connect the two grounds of the incoming cable to the single ground connection on the PCB.
Solder those to the PCB.
Solder the 9 Volt battery lead to the PCB: Red to VCC and Black to Gnd
Prep the case by drilling two holes in it for the wires.
Pelican case: On one side, about a half an inch apart from each other. See Photos. I built one with the wires coming in on the lid, this is cumbersome opening and closing.
Dive case: Drill holes on the bottom below the edge that is molded up. Look through the case while holding it up to some light. You will see the internal supports. Mark the hole location to avoid those internal ribs. My first build used the top because I thought it would be easier. When you screw on the lid, you are twisting everything around inside as it tightens up. Using some 220 grit or finer, rough up the area around the holes.
Bring the hydrophone wires into the case and connect.
Strip and prep them for connection to the PCB.
Solder the two shield wires together and use a short wire to connect to the ground connection.
Do not seal or glue anything yet! We need to test it first.
Connect a nine volt battery plug in the PCB to the mic input on the recorder and headphones.
Press record or record pause.
Rub the molded part of the hydrophone. You should hear that in the headphones. You may also hear 60hz hum. That is OK as when these are in use, there won't be any interference once submerged or out in nature.
Epoxy the wires into the case. This is done in a few steps.

Pelican style case:

Clean the area around the holes and wire with isopropyl alcohol; both inside and outside. You don't want any oil, dirt or residue to mess up the bond.
While wearing nitrile gloves apply a bead of silicone seal around the inside of the case to seal the wires in. This will prevent the epoxy from dripping when we use that on the outside of the case. The easiest way to do this is to apply glob around the wire itself, then pull the wire a bit from outside the case. This will pull some of the silicone seal into the holes.
Position the case so that the area is flat and even where the epoxy will go. Use something to hold the wires stationary while the epoxy hardens.
Cut some index card stock to allow to build a frame around the wires so that we can fill it with about ¼”- ⅜” of epoxy. Use hot glue to hold the card stock in place and seal the bottom edges.
Mix up enough epoxy so you can fill in the "moat"
Fill the area. I used the small stir stick to help spread it out. DOn't worry about bubbles in the epoxy, that did not cause any problems.

Dive Style Case:

Clean the outside of the case with isopropyl and the area of the wires that will be sealed with epoxy.
Mark the spot on the wires on the outside of the case where we will glue them. A sharpie works well for this, although a bit hard to see.
Pull the wires back up through the case so we can get to them from the inside side of the case. See the video for more on this.
WIpe the wire clean with isopropyl alcohol. Clean the sharpie marking off, that will tell you the rest is clean.
Wearing Nitrile gloves, apply a bead around the wires that is larger than the holes drilled in the case.
Pull the wires back through so that the silicone seal meets the holes inside the case and has a small amount of it seal off the holes.
Let this setup for 24 hours
Make a "moat" with card stock and hot glue. I estimated the curve and cut a few pieces of an index card untill it looked right. Hot glue is your friend here.
Position the case upside down so that we can fill in the area around the wires with epoxy.
Mix up equal parts of epoxy and pour into the area.
Let this setup for 24 hours.
Coat the PCBs with liquid electrical tape. This insulates and protects it
Place wax paper, a grocery bag, other protective material below the PCB to ensure that the liquid electrical tape doesn't drip on anything you don't want coated. And it will drip!
Brush on thick and coat one side of the board and let dry.
After the first side is dry, flip the board over and coat the other side.
Let the whole thing dry for 24 hours.
Leak check the case. I carefully took off the O-Rings and put a light coating of silicon grease on them. This is one extra step in sealing the case when closed and every bit helps.
For the Dive Case, I filled it with 2 Oz fishing weights
For the Plano and the Pelican cases I used small rocks.
Submerge them for 24 hours in at least a foot or more of water.

DO NOT SKIMP ON THE 24 HOURS!!!!

Step 5: Setup and Use

To use these, you want to connect the nine volt battery. A fresh one will last a few days Then plug in the 3.5mm jack to the recorder input. Set the recorder settings to 24 bit Wav and 96Khz. The smaller sony only goes to 16bit 44Khz but hey, those are CD specifications and perfectly fine. For the Sony A10 and the Tascam, set the gain to about 30% You can increase the levels in post and really they will only clip when tapped hard or banged into each other. The Sony “Voice Recorder” doesn't have gain settings the way the others do. It has settings for “Music” and “Voice”. I set mine for Voice and Medium. It also can be set for Mic in or Audio in. Set it to Mic. The Sony A10 only lets you adjust the gain for external mics when there is an actual jack plugged into it.

Common Sony Things:

Ensure it is set to record to the external memory card NOT the internal memory.
Pressing Record TWICE goes into record pause, letting you listen to everything while you set things up and test.
Turn off any recording filters or auto gain settings.

Common Tascam Things:

When you first press record, it goes into record pause mode. You have to press record twice to start recording. If the LED is blinking, you are not recording. If it is solid red, you are.
Turn off the backlight in settings. When you are running this off of USB power, it stays on all the time, even if set to turn off after a minute. You really don't want your drop rig glowing all night long somewhere attracting attention to it.

Common to all:

PIP power (This is low voltage 3-5VDC for external electret microphones) I leave it on so that I wont forget when I use the recorders with an external microphone and wonder why they don't work. The Tascam lets you turn it on and off, the Sony’s do not.
Set the date and time on your recorder! This really helps for overnight recordings to know when things were captured.

Make sure they are recording before you close the case! I have lost a few recordings this way! The sony immediately starts recording when you press the record button. The Tascam goes into record pause and the LED blinks. Once you know you are recording, close the case. The Pelican and the Plano have positive snap levers. The DIve Case has the lid screw on. Do this before you are at the water's edge lol!

The fun part:

Remember the speed of sound in Water? if we space the hydrophones 4-5 times the width of our ears we can get interesting stereo that works out to about 80cm or so. That is the minimum distance apart we should shoot for, but there are no rules here. Experiment and have fun. Put them in a bowl full of water and drop in a piece of sidewalk chalk. Drop in a piece of Dry ice. Then take the recordings into Audacity and slow the sound down. It turns really ethereal and quite unique.

The Plano/Pelican case build can easily be hidden at the edge of a pond or lake and left for a day or two. The Tascam will break the recordings into about 1 hour chunks that are 2Gig each (there are settings for recording size).

The Dive style case can be tie wrapped to a dowel or carbon fiber tube (or any thing really) that can be brought with you snorkeling or diving.

These things are really fun to play with and explore your aqueous surroundings. I'm planning a follow up video on uses as there are so many possibilities and different techniques. Have fun and maybe you can capture a never before heard sound.