AERIAL SOUND GUIDING.

Dunstan Times, Issue 3135, 18 September 1922, Page 7

AERIAL SOUND GUIDING.

The greatest drawback to a_erial traffic is fog or that low visibility which prevents the pilot from (seeing a. safe landing r\ice. 'Against this search-lights are almost powerless. At the great Croydon aerodrome stations are being established which will locate the wandering aeroplane, and, by its sound, enable it to. reach safety. This "sound ranging" will doubtless be mutual, the ground stations rapping out their waves to receivers carried by the invisible ma-

chine. Hitherto this method of location has been regarded' as merely partial. Even in a fog the air continues to move m streams and strata in varying directions and 1 of varying density. The sound waves are therefore distorted, and cannot be regarded witli complete certainty and' reliance.

During the war sound ranging on land and sea was carried to a fine art. A microphone and its electrical equipment lowered into the sea off the Isle of Wight recorded the exact location of depth charges dropped by a French destroyer near its own coast. There are records, too, that by the combination of a pair of stations 100 miles apart, the position of air explosion could be established to within half a mile at 201) miles range. Our war-time sea-patrols marked the location of a submarined wreck or minefield by dropping a depth charge, and leaving the in-shore hydrophones to couple up the sound-waves and issue the necessary warnings to shipping. On land the location of enemy batteries was possible, even in the infernal riot of a general barrage. The microphone equipments were so complete that a masked gun 20 miles from the trenches could be discovered with surety. The air is not nearly so good a sound carrier as water. Hitherto it has been found difficult to locate moving aircraft by the collection of their sound waves. The direction is comparatively easy, but height and distance alike are vague. The sound of depth-charge or gun is a sudden impact, or departure from the normal, easy to identify and record. The hum of an aeroplane, however, is low in pitch, and the sound Avaves are long. Indeed, they form a more or less continuous stream, and must be located by strength rather than incidental impact. Tin comparatively low speed of sound through the air (1 mile in osec. on the earth)" means that between the emission of the wave and its arrival at the operator's tahlc the machine has moved considerably, and may have changed direction. "The track," says Sir AViiliam Bragg, our leading authority on sound waves, "can be plotted, but can never he quite brought up to date." It would seem, therefore, unless new discoveries have been made, the u>o of sound-waves as an aid to mist-bound aeroplanes may not be without its perils.

The location of an aerodrome needs the most careful calculation and an elaborate accuracy even in clear weather. To plunge down hi semi-darkness, guided by mere sound-waves which may be interrupted by a puff of wind, a denser strata of the atmosphere, would soem to savour of the reckless.