8 September 2011
Having successfully created hydrophone recordings of San Francisco Bay, the next logical step was to experiment with active sonar. The idea was program a cell phone to emit 4000 Hz pulses once a second while simultaneously recording an audio file of the entire experiment. A pair of PC speakers and a large battery would amplify the sound. An external hydrophone would listen for the echo returning from the bottom of the bay.
Keeping this contraption dry while towing it across the bay was a challenge. My first attempt filled up with water very quickly and sank. I hauled it out and redesigned the seal. The second attempt also flooded, albeit more slowly. The third attempt was water-tight.
An important consideration was that the sonar pod had to be slightly denser than water. Noise is the enemy of any kind of faint signal, and I didn't want a lot of surface wave action being picked up. So the goal was to add ballast until the pod had slight negative buoyancy, then let it cruise just under the surface as I towed it across the bay with my kayak.
Unfortunately there was a bug. The rope was tied to the pod the same way that a ribbon is tied to a birthday present. This is not an escape-proof cage. About 300 meters out into the bay the pod flipped around and popped out of the rope harness. It promptly sank to the bottom of the bay, at least 15 meters down. It's still down there, happily pinging away and recording data until the batteries eventually die. (With any luck it landed on one of the Navy's anti-submarine hydrophones and is giving someone a headache.)
Painful lesson learned. I've bought a replacement cell phone and will try again in a few weeks.