In the book The Wind in the Willows, Kenneth Grahame writes: “There’s nothing – absolutely nothing – half so much worth doing as messing about in boats . . .”
If you have not read The Wind in the Willows, buy it and read it to your children (that is, if you can drag the little beggars away from their smartphones). There is an engineering corollary to this, which goes like this: “There’s nothing –absolutely nothing – half so much worth doing as messing about in boats . . . while you study the science of sound travel in the ocean.”
It is a fascinating subject as I will now (I hope) convince you. Let us think about the ocean. The ocean temperature decreases with depth, and the water pressure increases. The salinity also changes. At a depth of about 1 000 m the tempe- rature remains stable. The combination of all these factors creates an area where the speed of sound in the seawater reaches a minimum of about 1 485 m/s.
The result of this is that any sound at 900 m to 1 000 m in the ocean will travel a great distance, since sounds above this and below this are refracted into the area of minimum sound speed. This deep sound channel is known as the SOFAR channel. The SOFAR channel acts as a waveguide for sound, and low-frequency sound waves within the channel can travel thousands of miles.
In 1944, ocean scientists Maurice Ewing and J Worzel tested a theory that predicted that low-frequency sound should be able to travel long distances in the deep ocean. A hydrophone was dangled in the ocean at a depth of 900 m from one ship and a second ship dropped 4 lb explosive charges set to explode deep in the ocean at distances up to 1 449 km away. Ewing and Worzel heard, for the first time, the characteristic sound of a transmission, which, oddly, consisted of a series of pulses, rather than one single pulse, the effect owing to the multiple reflections of the original explosion noise.
The discovery was used by the US and the Soviet Union to track submarines. More importantly, the submarine tracking system also picked up low-frequency blue and fin whale vocalisations, which gave some clue about how the whales could migrate very long distances. Since the speed of sound in water is about four-and-a-half times faster than on land and, since visibility in seawater is limited, echo location is the logical choice for fish navigation underwater.
There is still a great deal of work to do in the field of underwater sound. Very fortunately, this field of study is one in which many of the findings have a defence application and, thus, funding is generally available (well, in other countries, but not in South Africa). All the work that has gone into making ship radar signatures less visible has to also be applied to the acoustic signature of the ship, being the propeller noise, the hull noise, and so on.
However, under some conditions, the acoustic signature can be heard at a great distance but will be masked, to a degree, by wave and wind noise. Thus, a study to determine the degree to which sea noise underwater is composed of rain, wind or ships may well lay the foundation for the US designing a ship to lay mines in the straits of Hormuz or for the US using underwater location to detect another ship doing the same thing.
Further, using hydrophones to measure whale noises, with suitable signal processing, can be used to detect ships entering and leaving False Bay or rounding Cape Point. A suitable database would allow counties to know, exactly, which ships were where, at any time, without radar, in territorial waters.
There is a lot still to be discovered. I wish I was doing the discovering. But there is still time. Sound under water all started with Leonardo da Vinci, who, in 1490, wrote : “If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you.” So, still plenty of time.