RE: [PSUBS-MAILIST] Sonic Transmission in Shallow Waters

["Jay K. Jeffries" <bottomgun@mindspring.com>]

Keep in mind here that they are discussing low frequency (and mention
ultrasonic at the end) SONAR systems using transducers that are matched for
saltwater employment that is backed up with a lot of electronics.  If they
are having difficulties, a hacked together system with improper impedance
matching will have very poor performance.  This is borne out by Jon testing
preliminary saltwater tests.
R/Jay



-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jens Laland
Sent: Tuesday, February 23, 2010 8:24 AM
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST] Sonic Transmission in Shallow Waters

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




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