trying to list any proof I can find
here are some theoratic principle explaned or active/passive sonar:
these are document about Anti sub warfare in general:
here are some intresting qoates I found:
MAD is short for â€œMagnetic Anomaly Detectorâ€. Usually deployed as a probe, MAD is towed either behind a helicopter, or aircraft, and based on the fact that any submarine is basically a large mass of steel and other metallic alloys, concentrated in large amounts in an environment otherwise free of such materials. Consequently, the submarines cause fine deviations in the Earthâ€™s magnetic field. These deviations can be detected â€“ and even tracked with the help of a MAD. Although this detection method is meanwhile increasingly problematic â€“ to no small degree also because of an increasing number of shipwrecks on the bottom of most seas (since shipwrecks can cause a similar deviation and the Earthâ€™s magnetic field) â€“ meanwhile very precise maps of such deviations are available, and they can be taken into account. The largest disadvantage of the MAD remains therefore its limited range: the aircraft or helicopter deploying its MAD-sensor has to fly very low and slow over the area where the submarine is suspected, in order to use it effectively.
The differences in water temperature at different depths form the so-called â€œthermal layersâ€ (or â€œthermocyclinesâ€), the borders of which also bounce sound beams. In specific parts of some seas and oceans, these differences are so massive, that they enable even large submarines to hide in one thermal layer, or â€“ better said: bellow them â€“ then the thermocyclines are so massive they bounce sounds from any kind of active sonars, or completely block the sounds from reaching the passive sonar.
On the contrary, in the oceans there is a certain layer that is perfectly â€œtransportingâ€ sound waves. This is the so-called â€œdeep sea sound channelâ€ (DSSC). The layer on top of this layer has a too high temperature and bounces back the waves sent within the DSSC. The layer below, has a too high pressure and hence bounces the waves back too. By this way, a wave sent within the DSSC will be bounced back by these layers from one another, this will form a sinusoidal movement of the sound wave and transport it for very long periods over several thousands of miles. This layer is mostly located at depths between 800 and 2.000 meters, but is frequently depending on temperature and pressure. As an example what a DSSC can do: in WWII, some bombers and airplanes used to carry a depth charge that was set to explode within the DSSC and in the USA and UK there were several stations with hydrophones inside this layer. By taking bearings from an explosion of such a depth charge, the position of the crashed aircraft could be determined and a rescue team could be sent outâ€¦.Today, surface ASW platforms equipped with VDS can lower these into this layer enabling them to detect submarines over immense ranges. Of course, any decent submarine-skipper knows this as well, and will attempt to avoid operating within the DSSC.
The Sound wave is determined by the â€œgainâ€ strength of the emission and the frequency - which is dependent on the wavelength For frequency a simple rule of thumb can be used: the higher the frequency the less of it will protrude the water (meaning the shorter range), but in turn this is making the frequency easier to concentrate - or â€œaim the beamâ€ (meaning more accurate position fix). The situation is directly opposite when it comes to low frequencies. All such details and factors fit into the submarine hunting process and procedures: the submarine is most likely to be detected at a long range by low frequency devices, which will provide a blurry â€“ i.e. approximate â€“ position. Once the hunters come closer higher frequencies will be used to track the submarine down and for attack.