QUIET AEROPLANES.

New Zealand Herald, Volume LXVII, Issue 20626, 26 July 1930, Page 8 (Supplement)

QUIET AEROPLANES.

SOUND-PROOFING CABINS. The deafening roar of the motor and the noise of a propeller churning the air at nearly 2000 revolutions a minute must cease to be a necessary evil in flying, and aeronautical engineers in the United States are working on the problem of building quiet aeroplanes. Encouraged by the success that has attended the efforts of motor-car manufacturers to muffle their engines, aeronautical designers have attacked the problem from two angles, first, by muffling the engine and the propeller, and, second, by insulating the cabins of pianos so as to shut out the noise. The problem is considerably different in an aeroplane from that in a motor-car, particularly because the plane's propeller must be quieted, whereas for tho car only the motor must be considered. More than two years ago the aeronautics branch of the Department of Commerce in the United States undertook to sponsor experiments in sound-proofing aeroplanes. Reporting recently on the progress of tests conducted at the Bureau of Standards, the Department of Commerce said that principles had been worked out both for shuting off tho noise at its source and for keeping tho noise out of tho cabins. Three Essential Features. Th« three requisites for sound-proofing materials laid down by the engineers are, first, that they must shut out sound; second, that they must absorb sound; and, third, that they must bo light. In their experiments they sought the material that had the most efficient combination of these three characteristics. Lightness is a most important consideration, since every part of an aeroplane mrist be as light as possible. Many materials were available that served admirably as sound insulators, but were useless in aeroplane building becauso of their weight. Starting their work by classifying noises according to strength, tho Bureau of Standards experimenters adopted tho "decibel" system of comparing intensity of noises. On the ascending scale of noises, the rustle of leaves in a gentle breeze is given at 10, the range of speech in ordinary conversation is about 35 to 65, the din in a New York subway is 80, and that of an ordinary aeroplane is 100. Padded Walls Used. To make an aeroplane comfortable for its passengers, noise should be brought within tho limits of ordinary conversation. Testing various methods of shutting out sounds from the cabin, engineers found that the best results were obtained with a double wall of the usual materials, with a padding in the space between. An air space within the cabin walls, it was found, is detrimental to sound insulation. Materials used for filler included hair felt, balsam wool, cotton and dry-zero blanket, the latter being the most efficient. The dry-zero blanket is a fluffy, resilient kapok fibre, a two-inch thickness of which weighs about one-fifth of a pound per square foot. In addition to insulating against noise, it is an excellent heat insulator.