Low-cost Hydrophone and Ultrasonic Transducer

Do you want to record dolphins or whales speaking? Or build an underwater acoustic communication system? Ok, we're going to teach you ‘how to’.

Let’s start with the main thing: antenna. If in everyday life we use speakers (such as in your laptop or car) for sound emission and a microphone for sound recording, then I hurry to please you: sound transmitting underwater (we say “radiation”) and sound recording are often performed by the same device, which is called an underwater acoustic (hydroacoustic) antenna, or hydrophone (if it is a receiving-only device), or transducer if it works both ways.

In the overwhelming majority of cases, a hydroacoustic antenna consists of one or several piezoelectric elements: plates, disks, rings, tubes, spheres, hemispheres, etc.

Piezo elements have the so-called piezoelectric effect. If an alternating electric signal is applied to an element, the element begins to oscillate, and if the element oscillates, for example, by an acoustic wave, then an alternating electric signal starts to be generated on it.

Therefore, the piezoelectric element converts an electrical signal into acoustic waves (mechanical vibrations) and vice versa - acoustic waves into an electrical signal.

As the saying goes: theory without practice is dead! Let's not waste time and make a pair of hydroacoustic antennas.

Materials that we need:

• a pair of piezo buzzers Ф35mm (we bought 10 pieces for $1.5 on Aliexpress)
• a 10-meter piece of RG-174 cable
• two jack 3.5 mm stereo connectors
• copper / brass / stainless plate 50x100 mm wide 1-2 mm thick
• epoxy glue
• silicone sealant (non-acetic)
• solder and flux
• alcohol for degreasing
• any two resistors with nominal values ~100Ω and 470-1000 kΩ (we took 0.25 W MF25)
• two diodes 1N4934
• nylon thread

Instruments:

• drill and drills Ф3mm and 2.5 mm (to drill copper plate)
• hacksaw or dremel (to cut a copper plate)
• sandpaper 200-600 grit (to clean copper plate)
• knife, wire cutters (for stripping wires)
• soldering iron or PCB rework station
• dental spatula for leveling sealant

It’s just not a good idea to connect a piezo element to a sound card, laptop or tablet directly.

Firstly, the piezoelectric element can accumulate a large enough charge that can damage the electronics when connected.

Secondly, when connected to a line or microphone input of a sound card, you need to protect the input cascade of the sound card.

In order to prevent the unconnected antenna from accumulating charge, we put a resistor of 0.5-1 MΩ (R1) in parallel to it.

In the receiving antenna to limit the maximum voltage you can assemble the simplest threshold limiter from the diodes D1, D2 and the resistor 100Ω (R2). As diodes, we used 1N4934 and as resistors R1, R2 we took MF25 (R1 470 kOhm).

Please note that if you plan to connect the receiving antenna to the microphone input (and not to the line one), then you will additionally need a capacitor C1 with a nominal 0.1 .. 1 uF, otherwise, the power supplied by the sound card to the electret microphone will be short-circuited through the diode D1.

Piezo elements themselves need to be glued to metal plates with epoxy. It will lower the resonant frequency of the piezoelectric element (as the unsuspended mass is added).

Also, being glued by one side to a rigid metal plate, the piezoelectric element will not be able to contract and stretch and it will have to bend. That is what we need.

• We cut two square plates 50 x 50 mm and drilled holes for the cable (3 mm in diameter) and two holes for fastening the cable with a thin nylon thread, it turned out like on the photo
• Antennas got two pieces of 3 meters from the purchased 10-meter piece of cable, the rest was left in reserve
• We wind the cable into the hole, solder its central core to the metallization layer of the piezoelectric element, and the screen to its metal base. In parallel, as agreed, we solder a resistor of 470 kΩ.
• We clean the other end of the cable and assemble the connector: solder the central core to the central contact (the very tip of the connector), leave the middle one intact, and solder the connector body to the cable sheath.

I always forget to put the connector body on the cable and I have to re-solder everything two times. Do not repeat my mistake).

After soldering it is very important to clean the flux - especially on the piezoelectric element. Otherwise, over time flux will eat the soldering.

So, we have prepared two antennas (one of them has a threshold limiter). Now it's the time to knead the epoxy and wear latex gloves.

Before gluing the piezoelectric elements to the copper plates, both should be thoroughly sanded and degreased with alcohol (ethyl or isopropyl) or acetone.

Do not use anything else! Gasoline or kerosene leave greasy traces that impair adhesion.

It is worth recalling that all work with alcohols, acetone, and epoxy should be carried out in a well-ventilated room with your hands and eyes protected. Do not neglect the safety rules!

We saturate the nylon thread that holds the cable to the plate. For gluing the piezoelectric element to the plate use just a bit of epoxy glue. Do not overdo it! The epoxy should not get to the top, otherwise, it can destroy a thin layer of piezoceramics during polymerization, plus the epoxy deteriorates in water.

The result should be something like on the photos. Usually, epoxy completely polymerizes within 24 hours. We, for example, left our antennas until the next day.

When we arrived at the laboratory in the morning, we connected the first antenna (without a threshold limiter) to the headphone jack of the laptop. If you turn on the music and bring the antenna to your ear, you can make sure that at least the audible frequency range it reproduces quite well. There is even a hint of bass, the result of the copper base.

So now we have an acoustic transmitting antenna, but still not a hydroacoustic one. To fix this, we should degrease the antenna again and cover it in a thin layer of sealant.

Important note: Do not use the acetate-containing sanitary sealant! The acetic acid contained in it will corrode the solder joints, the cable and the metallization of the piezoelectric element.

We recommend Kim Tek liquid rubber for boats and yachts. DIY-ers from the United States can use the excellent polyurethane compounds from the Smooth-On company instead of sealant.

For our convenience, we fill the medical disposable syringe with sealant first, and then apply it to the piezoelectric element and solder joints.

After applying the sealant, we level it with a dental spatula or with what it is convenient (even with a finger). In the end, we got it like in the picture.

You should not make a layer of sealant too thick - the antenna will lose sensitivity. A layer of 1 mm is absolutely sufficient. Carefully protect solder joints, resistors, and diodes with sealant.

You can cover the back side of the plate with a sealant - we did it on one antenna.

If you move the resistors and diodes closer to the cable, then the piezoelectric element will be much more convenient to smear with sealant and the layer will be smoother.

After the completion of this kind of sculptor's work, we again leave the antennas for 24 hours....

And congratulations! Now you have two hydrophones!

Now you can check how good are newly built antennas by connecting it directly to your laptop, tablet or phone.

Unfortunately, not all devices are friendly to a underwater acoustics. Most of all modern sound cards have low-pass filters in microphone input, cutting everything above 15 kHz. But some of the laptops do not have such filters.

These hydrophone and transducer that we’ve built are just a beginning: we plan to publish a series of Instructables about underwater acoustic communication & navigation, please let us know if you are interested!