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Aviation History
1936
1936 - 0819.PDF
348 FLIGHT. APRIL 2, 1936. SILENCE WITHIN How the Cabins of Modern Aeroplanes are Sound-proofed : Problems to be Solved : Whispers and Snores SOME of the difficulties encountered in attempts to silence extraneous aeroplane noises were touched on in a recent article (Flight, March 12, 1936). It was seen that, although the use of slow-running, airscrews, cleaner aerodynamic design, and muffled exhausts has brought about a tangible improvement, a vast amount of work re mains to be done. The "silencing" of cabins is a simpler task, but even so has presented, and still presents, a multitude of widely varying problems. It is not long since wads of cotton wool formed almost the entire protection of a passenger's ears against the engine and airscrew noise which swept in through the thin fabric walls of cabin aeroplanes. In many machines even shouted conversation was next to impossible, and it is safe to say that the booking receipts of operating com panies suffered very considerably as a result. The noise experienced during an actual flight was deterrent enough, and the painful deafness which followed often overshadowed any pleasant memories. The aircraft operator knows only too well the difficulty of getting something for nothing. If he wants a thoroughly- comfortable, quiet aeroplane he must sacrifice precious payload; and that, actually, is what he has been doing in recent years. But such sacrifices on the altar of silence are doing much to retain patrons of the air lines, besides attract ing new ones, and are deserving of yet greater consideration by designers and operators. From the Qround Up It is a highly unsatisfactory procedure to build a transport aeroplane and then to call in a sound-proofing expert. The whole problem of noise reduction should be tackled in the early stages of design. Ample allowance can then be made for the lagging of cabin walls with appropriate materials, for insulation and for anti- resonance treatment. The closest colla boration is necessary between those responsible for structural design and ;m expert, or experts, in sound-proofing, ventilation and upholstery. Some indication of the improvements which can be effected in a noisy aero-" plane by the application of modern pro cesses is given by thcjollowing instances. When the first Boulton-Paul P.71a feeder-line biplane (two Siddeley Jaguars) built for Imperial Airways was first flown fully furnished, but without sound-proof ing, it proved itself a very noisy aero- PLANE OF / \ AIR _J SCREW J \ /@ r__ •f It 90 90 \]90 90 \E)\ -39-69\ * 87^67 -K<i X KSVxWVSAW 7S 73 73 75 73 73 72 74 72 72 72 73 m\ 72 73 f^f] 70 71 72 72 70 69 70 Z? 69 63 79 72 71 71 72 YrA 71 72 72 73 70 70 71 UA 69 70 70 71 [i<?] 70 \r75 \\J2 ) ' \J~j3~\ 70 g 'pS\ \7oi!e \j/ Baggage tJ plane, giving an audiometer reading of 94-95 decibels. Rumbold and Co. were given a free hand to remedy this, and succeeded in getting the reading down to 74 decibels —a distinctly creditable figure. The Monospar S.T.12 (two Gipsy Majors) was similarly reduced from 94 to 76 decibels. This was an unusually tough proposition, for not only was the fuselage fabric covered, but the tips of the airscrews were abnormally close to the cabin walls. The cabin of the Douglas transport gives a reading of from 69-75 decibels when the machine is cruising at 75-80 per cent, power. Its complete Sperry sound proofing system weighs, roughly, 250 lb. The big Martin China Clipper is said to be even quieter, giving a reading of 65 decibels. Audiometer readings, of course, are taken when a machine is in flight; other wise they would be false owing to the vibration experienced on the ground. Noise increases with speed, but decreases very considerably with height. The position of engines and airscrews in relation to the passenger cabin is obviously of the greatest importance. Exhausts should be led above or below a wing _ to blanket off the noise, and airscrew tips should be at least 10-12 inches away from the cabin. The biplane with engines be tween the wings (happily an obsolescent arrangement) is undesirable because the noise is trapped between the upper and lower planes alongside the cabin. It IS advantageous to keep passenger seats out of the plane of the airscrews. In a num ber of machines, including the Handley Page 42 and Short Scylla class machines cf Imperial Airways, lavatories, kitchms or luggage compartments may be placed 'fl that position. It is generally agreed that the easiest type of cabin to silence is that of a metal monocoque machine: the hardest is the A diagramatic representation of the noise level distribution in the cabin of the Douglas D.C.i cruising at 75-80 per cent full power and carrying twelve passengers but no mail nor baggage.
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