Hydrophones at depth

Here's a nifty new piece of technology from Stanford University for any ocean-science types: a hydrophone that can be used at any depth which has low-distortion sound detection over a dynamic range of 160dB and a frequency range of 1Hz to 100,000Hz. By contrast, the human ear can hear a range of about 20-20,000Hz, feels immediate and acute pain at about 120dB—chronic damage starts much lower, down about 85dB. (If you want to know what 120dB feels like, stand about 3m in front of an emergency vehicle with its siren sounding.)

Most microphones have a thin diaphragm which vibrates in response to the sound waves hitting it, and this one is no exception. However, when you're detecting tiny pressure changes against the crush of a deep ocean trench, you need something that will not be overwhelmed by the ambient pressure. The answer for this particular microphone was to drill tiny holes in the diaphragm so that the water pressure on both sides of the diaphragm is the same. The holes would have to be small enough that sound waves in the water wouldn't pass right through them without moving the diaphragm, and large enough that the pressure would equalize before damaging the diaphragm as it's brought to depth. The diaphragm itself is about 500nm thick, so it is very fragile. To measure the movement of such a fragile surface, they use a laser. This is highly accurate and also doesn't touch the diaphragm with anything other than light. At the quiet end of the sounds they wanted to measure, and with water resisting the motion of the diaphragm, it moves only about 0.00001nm or so. Fortunately, lasers and mirrors can detect that sort of tiny movement.

Different sizes of diaphragms (and drumskins, and sound boards, and strings, and horn tubes) are most responsive to a particular frequency, so to cover the full range of frequencies they wanted to hear without distortion would be tricky with only one diaphragm. So, they put three different sizes of diaphragm in one microphone to cover the range.