[TarJak]

Ask your mate about this if any of it is unclear.

A 37.5 kHz (160.5 dB re 1 μPa) pinger can be detectable 1–2 kilometres (0.62–1.24 mi) from the surface in normal conditions and 4–5 kilometres (2.5–3.1 mi) in good conditions. A 37.5 kHz (180 dB re 1 μPa) transponder pinger can be detected 4–5 kilometres (2.5–3.1 mi) in normal conditions and 6–7 kilometres (3.7–4.3 mi) in good conditions. Transponder 10 kHz (180 dB re 1 μPa) range is 7–9 kilometres (4.3–5.6 mi) in normal conditions and 17–22 kilometres (11–14 mi) in good conditions.

Underwater acoustic propagation depends on many factors. The direction of sound propagation is determined by the sound speed gradients in the water. In the sea the vertical gradients are generally much larger than the horizontal ones. Combining this with a tendency towards increasing sound speed at increasing depth, due to the increasing pressure in the deep sea, causes a reversal of the sound speed gradient in the thermocline, creating an efficient waveguide at the depth, corresponding to the minimum sound speed. The sound speed profile may cause regions of low sound intensity called "Shadow Zones," and regions of high intensity called "Caustics". These may be found by ray tracing methods.

At equator and temperate latitudes in the ocean, the surface temperature is high enough to reverse the pressure effect, such that a sound speed minimum occurs at depth of a few hundred metres. The presence of this minimum creates a special channel known as Deep Sound Channel, previously known as the SOFAR (sound fixing and ranging) channel, permitting guided propagation of underwater sound for thousands of kilometres without interaction with the sea surface or the seabed. Another phenomenon in the deep sea is the formation of sound focusing areas, known as Convergence Zones. In this case sound is refracted downward from a near-surface source and then back up again. The horizontal distance from the source at which this occurs depends on the positive and negative sound speed gradients. A surface duct can also occur in both deep and moderately shallow water when there is upward refraction, for example due to cold surface temperatures. Propagation is by repeated sound bounces off the surface.

In general, as sound propagates underwater there is a reduction in the sound intensity over increasing ranges, though in some circumstances a gain can be obtained due to focusing. Propagation loss (sometimes referred to as transmission loss) is a quantitative measure of the reduction in sound intensity between two points, normally the sound source and a distant receiver. If I_s is the far field intensity of the source referred to a point 1 m from its acoustic centre and I_r is the intensity at the receiver, then the propagation loss is given by[1] PL=10log (I_s/I_r). In this equation I_r is not the true acoustic intensity at the receiver, which is a vector quantity, but a scalar equal to the equivalent plane wave intensity (EPWI) of the sound field. The EPWI is defined as the magnitude of the intensity of a plane wave of the same RMS pressure as the true acoustic field. At short range the propagation loss is dominated by spreading while at long range it is dominated by absorption and/or scattering losses.

An alternative definition is possible in terms of pressure instead of intensity,[13] giving PL=20 log (p_s/p_r), where p_s is the RMS acoustic pressure in the far-field of the projector, scaled to a standard distance of 1 m, and p_r is the RMS pressure at the receiver position.

These two definitions are not exactly equivalent because the characteristic impedance at the receiver may be different from that at the source. Because of this, the use of the intensity definition leads to a different sonar equation to the definition based on a pressure ratio. If the source and receiver are both in water, the difference is small.

The simple outline without all the science, is that the pinger is quite quiet (deliberately so), therefore picking it up "thousands of miles away" is not actually that easy. I'm not saying its impossible, however its very unlikely even given modern classified technology. They transmit on specific frequencies that make them easier to locate with pinpoint accuracy when you are on top of them. Not from thousands of miles away. If they were loud enough to be heard that far away they would actually be harder to pinpoint because they would saturate sensitive hydrophones that are too close.

You RN mate may know more about the science than I do, but unless he and his colleagues know where the FDR and CVR are, then he doesn't know any more about this than the rest of us.