ULTRASONIC WAVES.

Otago Daily Times, Issue 20708, 4 May 1929, Page 24

ULTRASONIC WAVES.

TO FIND OCEAN DEPTHS. DETECTING UNDER-WATER PERILS. Sound vibrations that the human car is capable of hearing range from about 15 a second to' about 20,000 a second. Beyond this point the ear is no longer a detector, and it is perhaps • because of this that until recently the whole region beyond this point was neglected by experimenters. Now this region has been shown to be of peculiar interest. 'Waves above the audible are used to find the ocean’s depth from travelling steamers, for under-water signalling, for iceberg detection and so on. These, waves have been found to kill fish and other small animals, and produce chemical reactions, and the investigation of the uses of such waves has really just begun. STUDY BEGAN IN 1912. The development of the study of these high frequency short sound waves, a, study of what is called ultrasonics, was instigated by the sinking of the Titanic in 1912. It was then proposed to use high-frequency sound waves for detection of icebergs, so that disasters of this sort might be avoided. At that time Lewis Richardson, an English scientist, clearly outlined the whole procedure that is now used for detection of rocks, icebergs, submarines and wrecks, but the methods in use for production of ultrasonic beams of sufficient intensity for the purpose were not then available. REACH OCEAN’S DEPTHS. Beams of sound of short highfrequency and corresponding shortwave length when directed to the bottom of the ocean, are returned as if reflected from an almost perfect reflector. If the time it takes .the sound to go to the bottom is carefully measured, and knowing the speed at which it travels, it is a simple calculation to find how fdr the sound must have travelled to make the return trip, and thus to fijid the depth of the ocean. It is possible to map very small changes in contour of the ocean bed as the ship proceeds, and, unlike the use of a sounding lead, does not require that the ship come to rest to take measurements. The reflected sound method, for finding depths is also used for detection of submarines, mines, etc. A directed beam under water may be used for a secret dot-dash system of telegraphy, or by superimposing the voice on the beam it may be used for telephony by a method analogous to broadcasting. It may be asked how the wave can be detected if it is ot too high pitch to be heard. The system is electrical and is known as heterodyning. A wave of nearly the same frequency as that to be received is produced at the receiving end and the ear hears a frequency which is the difference between these two. Thus if the incoming wave has a frequency of 200,000 vibrations a second and that produced at the receiving end is 200,500, the ear will hear a sound of 500 vibrations a second. FATAL TO FISH. ( Recent experiments, while perhaps of a less practical nature, are probably more interesting. Particularly is this so of the experiments on animals in which it was found that fish could not survive exposure to these short waves of high intensity. It is believed by some bacteriologists that this is due to breaking down the membranes of the cells. If a long, tapered glass rod is dipped into oil in which there are present ultrasonic waves the oil will creep up the at the end, which is mads about the diameter of a needle, the oil is sprayed off as invisible vapour. If touched at this ncedle-like end the glass will burn the fingers. The point will bore its way through wood or even glass. An under-water ultrasonic beacon to guide ships coming into harbours was installed in the port of Calais, France, and proved successful. Ships suitably equipped can pick up ‘ the beam and follow it past obstacles. It is limited to a straight channel. Messages can at the same time be sent from ship to shore, or from ship to ship. “An incidental result is that the revivers at the short station have been able to detect vibrations from a shoal of pebbles situated off the shore not far away,” says Professor Boyle. “It is thought that these vibrations are due to the rolling and colliding of the pebbles against one another by the niotion of the water. The explanation is a reasonable one, since the dimensions of the pebbles would be about right to cause in one another elastic vibrations of ultrasonic pitch.”