Hydrophone

8 August 2011

The surface of water forms an acoustic barrier; sounds in the air aren't heard underwater and sounds underwater aren't heard in the air. To find out more about what is happening underwater, I built a hydrophone.

'Built' might be overstating it a bit. Actually I just found an iPhone headset on the street that had been run over a few times. I clipped off the ear buds leaving just the microphone. A combination of Saran Wrap, bathtub caulking, rubber cement, some tape and a twist-tie made the microphone waterproof. Finally, I attached a lead weight to make sure the microphone would sink.

Yesterday paddled out in a kayak to the middle of San Francisco Bay, plugged the hydrophone into my Android phone and pressed 'record'. The result was a cacophony of sound. There was a little bit of wave action, but that was mostly drowned out by man-made noises. I was able to clearly identify two of them.

The first is a large cargo ship that cruised by. Although nearly silent above the surface, the engines make a lot of noise underwater. Here it is as it sails away:



SFBay-Ship.mp3 (209 kb, 26 seconds)

The second is a BART train rolling through the Transbay Tube directly underneath me, heading between Embarcadero and Oakland. According to the schedule, at this moment there were two trains in the tunnel traveling in opposite directions. Any San Franciscan will immediately recognize this sound:



SFBay-BART.mp3 (315 kb, 39 seconds)

On a related note, here's what the Transbay Tube looks like from the driver's perspective and here's what it looked like when it was being built.

Special thanks to City Kayak for putting up with the weird and wonderful things I do with their equipment.

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