Science writer from NY, Lee Kottner wrote about my music and hydrophones with lengthy scientific explanation.

no hyperlinks – original text with links on Cocktail Party Physics

“…. Another example of the intersection of electronics and music, and my new obsession, is Tomoko Sauvage’s water drip performances. Sauvage first came to my attention through a YouTube video of her performing on waterbowls in Paris (It’s kind of a crappy quality video, so I won’t embed it; here’s a better one of her rehearsal with Scottish pop star Momus). She has a set of graduated-sized porcelain bowls filled with varying amounts of water that she plays like a percussion instrument with a couple of wooden kitchen spoons, accompanied by an electronic drone and drum track or electronic shruti box. Same principle as playing a glass harmonium, harmonica or harp (like the wine glasses in the video above): fill a receptacle with water and make it vibrate. The water acts as an amplifier as well as determining what note the receptacle “plays” by how much liquid you make resonate. The water bowls, Instead of being rubbed to make them resonate (as you also do with Tibetan singing bowls), are struck like a xylophone. The cool thing about this method is that the tone can be varied a little by stirring the water, which adds a vibrato. You can hear and see this in the Momus video I linked to above. The struck bowls have a bell-like tone similar to struck singing bowls, one that’s deeper and more resonant than glasses. Don’t try this at Thanksgiving with the jello salad bowl.

Where Sauvage’s work gets back to the ambient is in her waterdrop performances, like this one:
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In this case, she’s using hydrophones to transmit the sound of water dropping into the bowls and hitting the bowls themselves, the water in the bowls moving, the sounds of her pouring water in, and her disturbing the water and flicking the bowls with her fingers. That kind of plooping sound the water makes pouring into the bowl is due in part to a process called cavitation (the making of a cavity), where air bubbles created by changes in pressure in the water oscillate and explode, creating teeny shock waves. Usually it takes a marked change in pressure for cavitation to occur, but fast-flowing water can do the same thing on a smaller, quieter scale than, say, a submarine or ship propeller. On that louder and larger scale, cavitation can actually erode rock and damage metal. In Sauvage’s bowl though, it’s more like blowing water through a straw: noisy but harmless. These are normally sounds you wouldn’t hear well, if it all, without amplification, and a normal mic wouldn’t help much.

Hydrophones were originally developed to collect sounds underwater and transmit them the way land-based mics do, to amplifiers and recording media. This is done with pressure-sensitive transducers “tuned” to the same impedance (how fast sound moves or propagates through a medium) as water, rather than air. Transducers turn the pressure of sound waves into electrical signals that are then decoded by the amplifier. Water is a great acoustic medium because of its density, which gives the sound waves more particles to push around, creating more pressure over the same surface area for the transducers to pick up. Because of this, even faint sounds, like shrimp clicking their little claws to stun fish (remember that shock wave?), can be easily heard underwater with a hydrophone. Sadly, now that we have the tools to hear them, some of those noises and natural songs are being lost in the noise pollution of ship traffic, which is mostly more cavitation noise when it’s not sonar or drilling.

Hydrophones are used in a number of different research areas, from studying the aforementioned sea mammal communication and echolocation and estimating their populations and feeding and migration patterns, to hunting subs, studying sound propagation and visualizing sound wave fields (pdf), and monitoring underwater earthquakes and volcanoes. Whitlow W. L. Au, Chief Scientist in the Marine Mammal Research Program at the Hawaii Institute of Marine Biology, University of Hawaii at Manoa, was recently elected President-Elect of the Acoustical Society of America, which gives you some idea of how closely the two fields are intertwined.

But this is the first I’ve seen hydrophones being used to make music. Is it mermaid music, or music for mermaids?”