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Great stuff Jens! Gotta run to the dentist and get a root canal...wanna go in my place? David Bartsch > Date: Tue, 23 Feb 2010 14:23:56 +0100 > Subject: [PSUBS-MAILIST] Sonic Transmission in Shallow Waters > From: laland@artematrix.org > To: personal_submersibles@psubs.org > > Here is a summary from the book PHYSICS OF SOUND IN THE SEA (1969), > Chapter 6: "Shallow-Water Transmission" (i.e. sonic transmission, only): > > In shallow water, a distinction must be made between transmission over > "mud" bottoms (which resembles deep-water transmission) and transmission > over all other bottom types. No significant differences has been > discovered in sonic experiments between any of the other bottom types, > including "mud and sand". > > Over sloping bottoms, a significant dependence on refraction pattern has > been observed: with downward refraction transmission tends to be poor, > while in isothermal water it is as good as in deep water. > > Over level bottoms, with isothermal water or in the presence of downward > refraction, the attenuation is very small for frequencies below 2 kHz > (ref., the HBH-2 should be operated below it's resonance frequency of 3 > kHz). > > In the portion of the sea fairly near to the surface, which is the region > of interest in sonic listening (in our case), the absorption coefficient > probably depends on highly variable factors, such as bubble content; thus > large deviations may be expected to occur quite frequently. > > There appears to be little correlation at sonic frequencies between > transmission loss and refraction conditions, depth of the water, and > surface roughness. With strong upward refraction, an increase of > attenuation with increasing sea state has been observed, undoubtedly > caused by the poor reflectivity of a rough and aerated surface. > > At short ranges, out to approximately the range equal the depth of the > water, image interference maxima and minima have frequently been measured. > > Reliable information on sonic transmission is scanty and is less > consistent than the information on the transmission of sound above 20 kHz. > > Sonic transmission has probably been a more difficult field for > investigation than supersonic transmission, because of the low directivity > of most sources of sonic sound. > > Best regards, > Jens Laland > > > > > > > ************************************************************************ > ************************************************************************ > ************************************************************************ > The personal submersibles mailing list complies with the US Federal > CAN-SPAM Act of 2003. Your email address appears in our database > because either you, or someone you know, requested you receive messages > from our organization. > > If you want to be removed from this mailing list simply click on the > link below or send a blank email message to: > removeme-personal_submersibles@psubs.org > > Removal of your email address from this mailing list occurs by an > automated process and should be complete within five minutes of > our server receiving your request. > > PSUBS.ORG > PO Box 53 > Weare, NH 03281 > 603-529-1100 > ************************************************************************ > ************************************************************************ > ************************************************************************ > Hotmail: Free, trusted and rich email service. Get it now. |