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Aviation History
1959
1959 - 1791.PDF
10 FLIGHT, 14 August 1959 Noise in the Jet Age By H. R. BROADBENT PEOPLE around London Airport are primarily troubled bynoise from aircraft: aircraft taking-off, landing and testing.There is an undercurrent of fear of one crashing on thefr houses, particularly on the line of the runways and under theregular flight-paths; but most people protect themselves with the cloak of "it won't happen to us" and push fear, for themselves andtheir children, into the background. The primary nuisance is noise. It interrupts thought, conversa-tion and wireless. It wakes people up and keeps them awake. It does no good. Protests have been made in varying degrees, initiallyby those close to the airport; but gradually, with more power in aircraft and greater noise, people further and further away haveadded to the clamour for noise reduction on the ground and in the air. Now, with the advent of the jets, this unwilling audience hasgrown enormously. There is, however, a secondary source of irritation which coloursthe whole background of the approach to the primary nuisance and may indeed be responsible for wrong thinking in the Ministryof Transport and Civil Aviation. Local residents, when they make their protests to the Ministry, are never satisfied that the answersare complete. There appears to be a missing factor. One of the residents' associations refused to take part in a B.B.C. discussionon the subject because, they said, answers not given in free dis- cussion were "frequently ambiguous and sometimes misleading."The association concerned has had many years' experience in dealing with the Ministry; and, whilst to the inexperienced theirremarks may seem to be unduly antagonistic, it cannot be said that the results of official propaganda and the effect of the measurestaken have been successful. The Minister and his officials, on their part, find themselves inthe main the foster-parents of a bawling child which they cannot control. The original plans for the layout of the runways and themaintenance area were made on maps with a complete blank out- side the airport. In consequence, there is a runway directly inline with an established housing estate only 2,400ft from the threshold and a maintenance area less than 2,000ft from a wellbuilt-up area. The Ministry has been committed to a programme of expansion which has led to a conflict between its official com-mitments and its personal desires to avoid being a nuisance. Aviation and its attendant industries, magnified enormously dur-ing the war, carried into peacetime an aura of romance which has since been well exploited by the advertisements for air travel. Tothe operators of aircraft and to the providers of airports, there could be no other than a common incentive to ensure that airtransport should grow and that London Airport should become a main centre in the web.Financial commitments, with an annual deficit of over £5,000,000 on the airports of Britain, added pressure to this desire.It has been no wonder that protests from local residents were staved off with politenesses and work on noise reduction relegatedto a low priority. In addition, since air transport is international, the Ministry has been faced in the past not only with protestsabout noise from people local to London Airport, but also with a contrary heavy volume of reaction from change, both in public andin private, from the air centres of the world. Local residents are told, in consequence, that if conditions are made too onerous thetraffic will go elsewhere and London Airport will sink in importance.Further, the aircraft industry, with its military research and building programmes cut by the growth of the long-range missiles,pushes for greater subsidies to research into the development of civil aircraft. Larger and more expensive aircraft have been andare to be built with greater power and, it is said, greater noise. With all this as a background the Minister finds himself facednow with a larger and larger audience for the aircraft he allows to operate. It should be clear that it is the Minister who permits thenoise to be made. Parliament, twelve years ago, after a debate which showed its doubts about the wisdom of the Clause, licensedair operators through the Air Navigation Act (1947) to make as much noise as they liked so long as it was produced within thelimits of the Air Navigation Order and Air Navigation (General) Regulations obtaining at the time. Fundamentally, therefore,people around the airports have no legal standing for their pro- tests. Action for nuisance has been barred, and the goodwill of theMinister and his officials and the operating companies, urged by public opinion through the Press and Members of Parliament withconstituencies adjoining airports, has been practically the only source of effort in noise reduction. Sound insulation can dealsufficiently with the silencing in passenger and crew cabins, so there is little incentive from that angle.It cannot be said that the protests have been unavailing. The statement of Sir Miles Thomas, a former chairman of B.O.A.G, is evidence. He said: "We aircraft operators could well considerinjecting a new factor into our forward aircraft specifications and refuse to buy machines which do not conform to strict noiselimitation." But his advice may not be followed, and the suspicion of residents around the airport that research and the applicationof research to aircraft noise are well down in the programme of work on aircraft and at London Airport remains. The attitude of playing down the effects of noise is understand-able, but it is one which, in the opinion of a growing number of people, must change. Under present legislation, however, it wouldseem to be difficult to bring greater pressure to bear, except by the use of the one safeguarding sentence in the Air Navigation Regu-lations which reads as follows: — "Regulation 230 ... the conditions under which noise and vibration may be caused by aircraft (including military aircraft) on Government aero- dromes, . . . shall be as follows ... . "... ,. (a) ... -•• \. ;.;....•• (b) ... - \.- - - (C) (i), (ii) & (iil) . . . and such special conditions, if any, as may be prescribed as respect - any such aerodrome as aforesaid."The vagueness of this sub-section appears to be its own advant- age, but in defining such conditions the Minister faces a furtherdifficulty—one which has haunted every discussion and led to more than one misunderstanding—in the assessment of the noisefrom aircraft. Noise is subjective. It is by British Standards definition "soundundesired by the recipient," and by American definition "un- wanted sound." In argument, therefore, there is a perpetual refer-ence to personal reactions in which local residents who are dis- turbed are very clear in their opinion on the sound coming fromthe aircraft, whereas people in the Ministry, and the operators (whose bread and butter comes from the source of the sound) donot find it so unwelcome. There is no difficulty in making an objective scientific appraisalof sound. It can be split up into its various frequencies or fre- quency bands, be measured in terms of sound-pressure levelsabove a reference magnitude and be read on an instrument. The unit of sound pressure level is the decibel (di>), which is a log-arithmic function related to other levels by the equation . - - n=20 logio—' • , . -.-•••- p2 where the sound pressure level is in decibels and pi and pa arcsound pressures in dynes/sq cm (microbars). The reference magnitude for stating sound pressure levels inair is 0.0002 dynes/ sq cm. The comparison between two sound-pressures can be madewithout resorting to the reference magnitude. If p r is the refer-ence sound-pressure and pi and p% two other sound-pressures, the two sound-pressure levels (s.p.l.) m and W2'are: — ' . ' ... wi=201ogio— ": P na=20 logio Pr pi If there are, therefore, two sound-pressure levels of, say, lOOdband 94db, the ratio of sound pressures ':-: -^ '-•• ,••-:'•-••;-• £1=20 doo—u)lm_ 10»-3 = 20 " * ' Pt A difference of 6db in sound-pressure levels means that thesound pressure is double. Figures of sound as sound-pressure levels are, however, uselessto the layman. He may well be told that two sounds which he knows have a different effect on his senses and nerves, Qne beingmore unpleasant than the other, have the same sound pressure. He feels he is being misled and distrusts further information fromthe same source. Sound pressure-levels in isolation are of no use in discussion on noise. It was not long before the difference between objective measure-ment of sound as sound pressure and subjective reaction to the sounds became obviously necessary. The stage was reached whenthe loudness of a sound as measured by the human ear was intro- duced into the acoustic vocabulary and tests to compare sounds ofequal loudness were made with the human ear as the comparing instrument.
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