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Old 27th Mar 2014, 08:40
  #8333 (permalink)  
Pontius Navigator
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Originally Posted by Erwin Schroedinger
Some posts on this thread contradict the above.

So which part(s), if any, of the above quotes are incorrect to the extent that on-or-above sea surface searches with appropriate equipment won't locate the FDR when within the ranges stated?
Not sure exactly to what contradiction you refer. Is it range propagation? You mention 'above sea surface'.

Are you inferring that a 'microphone' or hydrophone not in the water can detect the ULB? Looking at the cross media transmission first, the speed of sound in a dense medium (water) is around 4800 feet per second and around 1100 feet per second in air. A wave or sound beam will refract at the media boundary. At certain angles it will be reflected rather than pass through. The physics example is a light beam striking a prism and 'total internal refraction.' IIRC, a beam of light passing from a dense medium to a less dense medium will be refracted away from the normal.

However the sea is not an homogenous mass of water. Temperature and salinity vary with depth. The water body has well defined layers; it is possible to even see and feel these layers when scuba diving. The speed of sound will vary with salinity (hence density) and temperature. Sound waves on striking these layers will either reflect or pass through depending upon the vertical angle between the emitter and the hydrophone.

It follows that a hydrophone directly above the emitter might hear the ULB but in most of the figures you have quoted the signal strength may decline too much for surface capture.

The first thing then is to determine the bathythermal layers and place the hydrophone in the same layer as the emitter. Next is to lower the hydrophone so that it would be as close as possible in the vertical to the emitter to maximise its horizontal range.

You would then have a sweep width of between 4 km and 44 km depending upon water conditions and the figures you quoted being applicable.
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