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One of water's great features is its ability to absorb many kinds of radiation.  For example, ultraviolet light, the kind of light that gives your a sunburn and tries really hard to destroy your DNA, can only penetrate a few centimeters below the ocean's surface.  Visible light fares better than UV, but visibility in clear water is limited to only about 100m.  While water's ability to absorb radiation allowed life to evolve on our planet (thanks water!), it also means that mammals like us who rely on visual cues are limited in the water.

Because water is much denser than air, water is very good at transmitting sound, especially at low frequencies.  This is why whales, which evolved from land-dwelling ancestors, evolved a remarkable ability to make and interpret sounds.  Through a long, convoluted process, our lab is leading the marine mammal monitoring work for UMaine's offshore wind project.  Because of the role that sounds play in the lives of whales, one of our main tasks will be to monitor the sounds emitted by the wind turbine and calculate the acoustic "footprint" of the turbine.  

We have purchased a special microphone, called a hydrophone, that is designed to be used in the water and have been making some recordings of sounds here in Portland Harbor.  Although our neighbors might disagree, one of the most interesting sounds in the harbor this summer is the sound of the pile driver being used to build GMRI's bulkhead (time lapse photos here).  Earlier today, Pete and I went down to the dock next door and made some recordings before and during a pile driving session. Before the pile driver started, the harbor was fairly quiet, other than some grinding being done on a boat next to us. Here's what is sounded like: test3_background.wav.  And, here's what the sound looked like:

This kind of visualization is called a spectrogram.  It shows the intensity (indicated by the colors) of the sound at different frequencies (vertical axis) and at different times (horizontal axis).  The intense green band shows that the grinding sound is strongest at about 0.55 kHz (550 Hz) and that the sound has regular overtones at higher frequencies.

Now, here is the pile driving: test3_pile_operating.wav
You'll notice that the sound is much louder and is concentrated at the very lowest frequencies.  The intensity decays continuously to higher frequencies and also decays after each strike.  It's hard to hear, but each strike produces an echo (small green blobs at the bottom, for example, just before the 6s mark).  Because lower frequencies transmit better through the water, the echo only appears at the lowest frequencies.  Similarly, the sound we recorded in the water is much lower frequency than the sounds our ears heard.  Here's Pete's iPhone video:
and here's the corresponding spectrogram:
There's clearly more sound at higher frequencies; however, the lack of signal at the lowest frequencies is most likely due to the iPhone's microphone "rolling off" at these lowest frequencies.

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This page contains a single entry by Andy Pershing published on June 23, 2010 4:20 PM.

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