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Topic: The deeper the louder (Read 598 times) |
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BillRoh
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The deeper the louder
« on: 2002-10-28 17:20:40 » |
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Ok everyone - here is something that I have not bothered to look up yet, and I hope you all, the interested people anyway, ponder what happened to me before looking for the explaination themselves. I'm gonna try to find information on this phenom after this post, but I have my guesses.
Saturday night my brother, a friend and myself decided to night time SCUBA dive in an area very seldom frequented by other divers - an isolated area. After scrambling about 50 meters over rock, we hit the water at about 9:00 pm on a neap tide (half moon rising at about 10:00). As we headed for our designated compass location, we started to hear a deep droning sound. It's difficult to describe - it went something like this:
starting at about a 440 hz tone, pitch would drop to below hearing level in about a second - maybe 8 or 10 hz, then would cycle for a long time like the drone of a big engine going very slow, maybe a cycle every .5 seconds. whum, whum, whum, whum, whum, whum at about 40 to 50 hz in pitch, with undertones in the 20 to 30 hz range. At about 35 feet, the bottom of this area, there were places where this sound was so loud that it would rattle our teath and bones - like standing next to railroad tracks when train cruises by.
At first, all of us thought that a big fishing boat had planted itself right above us, or was perhaps dragging a big net. nope. At one point we had all stopped over an area of ground that seemed extra loud. The rocks were vibrating, our bones were vibrating, if I had to guess at a DB level, i would put this sound in the 115 area. Rock concert loud. Finally, all of us decided to go take a look up top - so we started to ascend. At 20' it was getting softer, at 10' it was just decernable, and the surface you could not hear it at all - and like we expected it was pitch black out, and no vessels in sight.
The interesting things:
1> The sound was quite regular and cyclic.
2> The sound pressure level was more that I have ever heard - but akin to a train
3> none of the animals seemed to care - they all slept on the bottom like fish do, right over the area.
4> There were calm seas
5> Aside from the initial dropping tone, all other pitches were very low freq.
6> The sound could not be heard in less than 10 feet of water (20 feet from the source). Since low freq is omnidirectional, the source had to be very close to where we were under water. If one were to make a 50m x 50m grid, there were several hot spots, all with a accoustic range under 20 feet. In air, sound disipates at a steady rate of 6db per doubling of distance from the source starting at 1m. I do not know this formula for sea water, but usually sound travels better in the ocean than in air. I am confused as to how the sound completely dissapated over such a short distance.
7> Upon returing the next day, I did not hear the sound at all.
well, there is your mystery for the day. If I can't figure it out, I'll have to assume it was god - the answer to all unsolved problems.
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BillRoh
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Re:The deeper the louder
« Reply #1 on: 2002-10-29 16:09:06 » |
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I just can't believe that no one here has a clue as to what I heard. I have been looking for info on the subject and have found nothing. I have listened to dozens of under water recordings and again, nothing. I am beginning to think that I have found something interesting - and if possible, undocumented. If that is the case, there is a paper here for someone.
Well, if anyone gets any ideas - please speak up.
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