With the EEGsynth we are not only working with EEG data but occasionally also with data from trees, from microbes and from cod fish. For the cod fish project we collaborated with Rebekah Oomen and John Andrew Wilhite-Hannisdal on the sounds that cod make during the mating season, using recordings of the “grunting” and “drumming” that were made in previous years at Flødevigen Research Station.
For the Science Festival in Gothenburg we are considering to take it one step further and integrate live video and possibly also sounds from the cod fish that are swimming in the large basin in the Sjöfartsmuseet Akvariet. Recording the sounds to use them live requires hydrophones, i.e., underwater microphones. Commercial hydrophones that are commonly used for marine research purposes are rather expensive, but luckily it is not too difficult to make a hydrophone yourself from a piezo element. I largely followed the process described by Felix Blume. Below I will detail some specific considerations, design steps, and modifications that I made to the original.
I ordered a bunch of 27 mm diameter piezo elements from Amazon. Since I did not have nicely fitting bottle caps, I designed a small round housing that exactly fits the piezo elements. I used my Prusa to 3D-print the housing in PLA, a bioplastic made from corn starch. Copper tape on the inside helps to prevent pick-up from electromagnetic interference. I found out experimentally that the copper tape needs to have a good connection to the copper of the piezo element so that they together form a Faraday cage, otherwise there was still a lot of 50Hz hum.
I ordered two instrument cables of 9 meter each (one red, one blue) and cut them into a 3 meter and a 6 meter section. The stripped cables were then soldered to the Piezo elements.
To ensure that the hydrophones will properly sink, I placed two nuts in the housing and completely filled it with silicone gel. As the hydrophones are to be used in an aquarium, I bought special Seachem silicone sealant in the local tropical aquarium fish store.
I had planned to use a coat of Plasti Dip Multi-Purpose Rubber Coating according to the instruction, but a review of the safety sheet revealed that Plastidip is made from petrol derivates and contains substances that pose a safety hazard to water and marine life. I don’t know whether that is in the solvents which evaporate during the drying process, or also with the remaining solid coating itself. Given the intended use in an aquarium with a closed water recycling system, I don’t want to take a risk and decided to keep the hydrophones bare with the PLA bioplastic and the copper of the piezo element exposed.
With the four hydrophones we will be able to do a four-channel recording of the sound that we record inside the fish tank and play it back spatially on a quadrophonic speaker array.
