‘Active Sonar’ as it is now called, makes use of the sound echo principle. Just as a person hears the echo of his hand clap in an empty hall, and some of the sound returns to his stereo-hearing system — his ears — so the same system is used in Asdics. Not only the direction from which the echo returns, but the distance it has travelled can be computed, for we know the speed of sound in water just as we know it in air. In addition the relative speed of approach can be assessed from its doppler effect.
As water is a very good conductor of high frequency sound, echoes from small objects in water can be heard for 20 miles or more in the right conditions, as every whale knows. However, there are snags. Just as light is ‘bent’ or refracted as it passes through glass to air or from air to glass, so sound is refracted as it passes from water at one density to water of a different density. Water in the oceans and ponds varies very much in density due to changes in salinity, temperature and pressure (depth). It remains for long periods in these separate density layers which are very clearly defined when the water is calm and tideless.
The path of a sound beam, therefore, seldom travels in straight lines in water, and, like a stick is ‘bent’ when it is half in and half out of the water, sound appears to come back to the listener from an entirely wrong direction. So that if the sound from an Asdic transmitter under the hull of a destroyer strikes a very well defined layer or interface between waters of different densities, the sound will be refracted or reflected, depending on its angle of incidence to the interface. If it is refracted, it may eventually come back to the Asdic ‘transducer’. If it is reflected — as if from some giant mirror under the surface — it may never return. In both cases the sound will appear to ‘come back’ from the wrong direction.
The cold waters of the North Sea and Atlantic are usually mixed by tides and wind, and are not particularly well warmed on top by the sun. In the Mediterranean, however, the situation is entirely different as the Captain of U-73 knew perfectly well when he sank Eagle. The Mediterranean waters are full of well defined layers at different densities, with the warmest at the top of course — for there is little or no tide and in summer there are no continuous strong winds nor surface drift. There is also one further important difference — just where Rosenbaum was hunting, a few hundred miles east off the Straits of Gibraltar. For here, the cold water from the Atlantic, which flows in through the Straits to compensate for heavy evaporation during the summer, undercuts the warmer water already in the Mediterranean ‘pond’, and stays in its well defined layers for huge areas of the western Mediterranean. The Navy knew of the effects of changes in density in 1939-41, but had no idea just how marked their effects were in the Mediterranean compared with the Atlantic or North Sea, because they had not carried out hunting exercises in the Mediterranean between the wars. After the war, they carried out extensive bathythermograph ‘dips’, to establish the extent of these marked changes in density.
The problem is not now so great, for we have ‘dunking sonar’ to get below such layers. But in WW II, the layering effect in the Mediterranean gave the Germans an advantage which they exploited to good effect. The only answer possessed by the Royal Navy at that time was to provide more and more A/S destroyers and manufacture variable-depth depthcharges. In the case of ‘Pedestal’, even 24 destroyers and 12 A/S Swordfish of 813 and 824 Squadrons were not enough.