ULTRASONIC WAVES.

Auckland Star, Volume LX, Issue 40, 16 February 1929, Page 10 (Supplement)

ULTRASONIC WAVES.

TO FIND OCEAN DEPTHS.

DETECTING UNDER-WATER

PERILS.

FATAL TO SOME FISH.

Sound vibrations that the human oar Is capable of hearing range from about fifteen 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 l>cyond this point was neglected by experimenters.

Now this region has been shown to he of peculiar interest. Waves above the audible are used to find the ocean's depth from travelling steamers, for under-tfater 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 6tudy of these high-frequency short sound waves, a stujy of what is called ultrasonics, was instigated by the sinking of the Titanic in H»l2. It Ms tlicn proposed to use high-frequency sound waves for detection of icebergs, so that disasters of this sort might be avoided. At that time iLewie 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 6ound of short highfrequency and corresponding shortwave- length when directed to the bottom of the ocean, are returned a» 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 1t travels, it is a simple calculation to find how far the tound must have travelled to make- the return trip, and thus to find 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 ako used for detection of submarines, mines, etc.

A directed beam under water may he 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 of 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,")00, the ear will hear a sound of 000 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 ai\? present ultrasonic waves the oil will creep up the glass, and at the end, which is made about the diameter of a needle, the oil is sprayed olf as invisible vapour. If touched at this needlelike end the glass will burn the fingers. The point will bore its wav through wood or even glass.

Ail undcr-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 receivers 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 motion 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."