J B Ollerhead

Department of Transport Technology, Loughborough Umverslty of Technology

A~rcraft no~se ~s a major nmsance m residential communit ies around a~rports If the a~r transport ~ndustries are to meet the ever ~ncreasmg demand for atr travel, determined efforts are reqmred now to reduce the burden of norse upon these commun)ties. Signffmant engine noise reductions have already been achieved tn the latest generation of wide-bodied aircraft, and further reductions are being forcast by the engine manufacturers. Regardless of whether there are justif iable grounds for th~s optimism there are alternatwe steps to be taken. But the problem ~s basically an economic rather than a technologmal one - how much does no~se reduction cost and how much can we afford to pay?

The various costs of aircraft noise, both monetary and social, are discussed )n relation to )ts effects upon people. Al though an economic analys=s of the problem is feasible, ~t ~s doubt fu l whether our understanding of the relattonshtps between physmal no,se levels and human reaction =s yet adequate for such purposes. Planning methods for estimating the extent of communi ty noise nutsance are presented, and it is shown that consideration should be given to out ly ing regions exposed to relat ively htt le mrcraft no~se

I n t r o d u c t i o n - - The price of a i rcraf t noise

Possibly the biggest ploblem facing the air t iansport industry today is that of noise Thloughout the developed counmes of the world the continued growth of air transportation ~s threatened - a~rport development has slowed to a crawl and the fate of advanced technology tl ansport all el aft, especmlly m the supel somc (SST) and vertical or short take-off (V/STOL) categories, hmges on the abd~ty of their manufacturers to make substantial cuts

m noise radlaUon. However, the aircraft engmeer can only accomphsh so much. A halving m subjecUve loudness reqmres a tenlold reductmn m acousnc power output . Although such reducuons can and have been achieved m the latest series of wide-bodied jet aMmers, it is unreasonable to suppose that further technical feats of this magmtude wall be forthcoming at regular mtervals. Indeed, it can be argued that the first few decibels are the easiest further reduction becomes more and more difficult as the noise goes down.

130 Appl,ed Ergonomics September 1973

The technical problems of aircraft noise reduction wall be d~scussed later m the paper. For the present, it is tmportant to stress that the problem is basically an economic one. There are numerous ways ]n which the aeroplane and the commumty can be made compatible, aeroplanes can be made quieter by sdencmg or the fitting of quieter engines. They can be flown m and out of a~rports in such a way that no~se nuisance is mmtmlsed at all txmes. The distance between the airport and the commumty can be increased by relocating the airport or by the acquisition of adjacent land for compatible use. Houses and other buddmgs can be effectively sound-proofed against aircraft and other sources of noise. If the worst comes to the worst, people can even be compensated for property depreciation and 'loss of amenity' due to h~gh noise levels. Needless to say, each of these alternatives ]s very expensive and the questions are very smaply: how much could or should be spent to get rid of the noise, and who paysq

The situation is dlustrated in Fig 1 which shows the cost and benefits of air transportation to the operating country. The mare benefit to the national economy is derived through the business user whose productivity is increased by air transportation. The general pubhc, therefore, benefits mdtrectly through general economic expansion. In addition to the business users themselves, two further special segments of the public must be identified, the private users and the res]dentml communities near to airports. The private users, who travel by air for pleasure, derive social benefits which are difficult to quantify m monetary terms but which must at least equal the fares paid. The air transport industries, including the a]rhnes, the supporting services and ultimately the aircraft manufacturers, derive their income prtmanly from the two user groups. However, their operations are effectwely subs]d~sed by the a~rport commumt~es. In the language of economics this factor ~s an 'externahty', a cost of production not actually borne by the producer - ]n this case the 'socml cost' of environmental degradation caused by the exastence of a~rcraft and a~rports.

Such externallt~es are nothing new. S~mflar s~de-effects have accompamed practically every development since the beginmng of the mdustrml revoluUon. But in most instances the socml costs have been distributed reasonably equitably between those people who derive direct benefit. What ]s unusual about aircraft noise ~s that Its costs are normally recurred by a very small proportion of the (Indirect) beneficmnes. Clearly there are two possible solutions to the problem - to ehmmate or at least reduce the socml costs themselves, or to compensate those who are forced to bear them, obtaining payment for e~ther action from the industry (and therefore indirectly from the user)

The monetary costs revolved are surnmarlsed for a hypothetical case m Fig 2. Two components are ~dentlfied, each plotted against an increase of overall nmse reduction (on some form of decibel scale). This ~s the effectwe reduction achieved, possibly by a vartety of nmse control approaches. The curve from the origin, which rises w~th nmse reduction, ~s the cost associated with doing something about the nmse, for example, any or all of those poss~bdltles already mentioned. The decreasing function, on the other hand, represents the costs of n o t

domg anything about the remaining noise. These are much

more difficult to quantify but they include such considerations as property depreciation due to noise, removal costs for people who can bear it no longer, the social costs of suffering the noise - lost time, lost efficiency, loss of health, and general loss of an acceptable environment - all factors which many people would regard as grounds for compensaUon.

Perhaps of considerable slgmficance also ]s the reduction of growth and revenue wtuch the air transport industries would otherwise enjoy ff their operations were less ob]ect]onable. All these contribute to the costs of noise alleviation. Although the two components m Fig 2 are hypothetical, it is conceivable that they would add to form a sum which exhibits a distinct mlmmum at some fixed value of 'noise reduction'. In a perfectly free market it ~s reasonable to suppose that events would take care of themselves to ensure an opttmum noise environment at all times.

In reahty this ~s not the case and external constraints must be imposed ff a reasonable balance of interests ~s to be maintained. To achieve these we must first understand the relationships between the complex factors revolved and then apply our knowledge to maxamlse the benefits obtained from a gwen expenchture on noise control. At the same time we must guard against what might be

i National economy J Indirect I[ benefits I Increased

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F~g 2 Costs of aircraft noise reduction

Apphed Ergonomics September 1973 131

ca-ll-e-d an 'overbfll'. There is at the present time a real danger that over-anxaety for the preservation of the environment wdl result m no~se control expen&tures which are difficult to justify m terms of the benefits reaped.

The effects of aircraft noise on residential comm un ities

Although aircraft noise has far-reaching effects upon many human activities, particularly in schools, hospitals, and work areas where a quiet worlang environment is important (Stockbndge and Lee, 1973), consideration is restricted here to hving condmons m and around the home.

In the home environment, aircraft noise ~s primarily a nmsance. Although there can be httle question that no~se can contribute to ill-health in a variety of ways, by far the most widespread effect of a~rcraft no~se is that ~t annoys people. Although any definmon of annoyance must remain somewhat arbitrary, ~ts meaning for present purposes ~s illustrated m F~g 3.

The &rect effect of noise ~s to cause &sturbance, that ~s, it &stracts and interferes w~th human activity whether this be work, rest or play. At mght-t~me it may also prevent or disturb sleep. The factors affecting disturbance include the physical characteristics of the noise and ~ts relat~onshlp to the pre-existing 'ambient ' noise. They also include the nature of the actw~ty interrupted, the mdwldual's degree of concentration upon it, h~s hearing acuity and his general noise sensmv~ty.

The mdtrect effect of no~se ~s to cause annoyance which may be defined as a human response to disturbance. Whether or not an mdlwdual is annoyed by a disturbance depends of course upon the seventy of the chsturbance, but perhaps even more important Is his attitude towards the source of the no~se. In the case of aircraft noise, it has been established that a person's reaction to it is h~ghly mfluenced by his fear of aircraft crashes. Also of known ~mportance are his attitudes towards the a~rports and a~r transportation m general, especmlly regarding whether or not he receives any direct personal benefit, and upon his opmmns about officml concern, that is whether the responsible authorities are worried about the noise and whether or not they are trying to do anything about it.

These intervening effects are grouped under the general heading of 'SOClO-psycholog~cal factors'. They may be expected to have a greater influence upon annoyance than disturbance and an even more s~gmficant effect upon complaints. A~rport operators are usually forced to rely upon complaints to assess the nature o f their noise problem, mdeed ~t Is only the complaints which reveal the existence of a noise problem m the first place However, ~t ~s widely recogmsed that complaint activity ~s an unrehable guide to true commuruty feehng This ~s because complaints tend to emanate from the h~gher social classes, the more influential members o f the commumty who are more articulate and more famflmr with appropriate channels for complaint. They are particularly sensltwe to 'external' pressures, poht~cal events and coverage by the news medm. Complaints mvarmbly increase whenever changes are made or threatened and &e away again soon afterwards.

For th~s reason, it has been usual m commumty noise studies to concentrate upon the measurement of annoyance,

Sooo- psychological factors

i

F~g 3 Commumty response to norse

Noise exposure

• - Disturbance

,~ Annoyance

,~ Complaints

. . I I Protests and l l A, r transport ,n~eres~S~darnag ¢ cla,rnsl- ~

Cost of oo,se reduct,on t ICosts I

,I, J, - ,i, /, Tremomlngl ' [ A,rpor't , P |no,se I

Fig 4 Nmse control model

a response wtuch is considered to be 'central and general' to the aircraft noise problem and relatlvely independent of SOClO-economlc status.

Fig 4 is a block schematic &agram showing the xmplementatlon and effects of alternative steps to reduce the impact of aircraft noise,

Noise control expen&tures by the a~r transport industries are shown divided between numerous alternaUve actions. The use of quieter a~rcraft and operaUonal restncnons results m less noise exposure around a~rports Separating the people from the no~se by &stance or physical barriers reduces the &sturbance caused by an exastlng noise pattern. Improved pubhc relations and the payment of compensauon can substannally reduce the resulting annoyance felt and this m turn will dnnmlsh pressures on the industry to do something about the noise. This chain of events forms a closed loop for which an eqmhbnum state m~ght be calculated. To do this would require a reasonable quantitative understan&ng of the relationships depicted m Fig 4. Although tbas exists for the physical aspects of the problem, the weak hnks are the relationships between physical noise exposure and community response. Although a great deal of effort has been expended m the search for an appropriate scale of noise w~th which to calculate ~ts effects upon people, th~s part of the problem is still poorly understood.

Noise scaling techniques

Before considering the more complex problem of estimating the total impact of airport noise upon the commumty, xt Is as well to review the related but more basic matter of measuring the noise of a single aircraft. This m itself has caused dlfflculnes enough which, although largely resolved, can appear rather confusing to the non-speciahst.

For most everyday noise measurements the acoustician relies upon his sound level meter which allows him to measure decibel levels on one of several scales. The two major ones are the 'linear' and 'A-weighted" scales The

132 Applied Ergonomics September 1973

linear scale g~ves an overall measurement of total noise energy but the A-scale makes due allowance for the fact that the human ear vanes in its sensitivity of response to sounds of different frequencms. To rate sounds in subjectively meaningful terms, they are normally measured in dB(A). This i s the case for road vehicle and industrial noise for example.

The aviation commumty, however, developed what it felt was a more accurate scale called 'perceived noise level' (PNL) which is measured m units of PNdB, like the dB(A) scale this accounts for the ear's variable frequency sensltwlty but also attempts to smaulate the rather comphcated way in which the ear integrates loudness as a function of frequency. There is no stmple-to-use meter for PNL - It has to be calculated with pencil and paper or by computer. However, even PNL proved inadequate when aircraft noise certification requtrements (U.S. Federal Axaatlon Regulations, 1969, and International Clval Avtatlon Orgamsatlon, 1971) were specified and for this purpose an even more complicated scale known as Effective Perceived Noise Level (EPNL, units EPNdB) was derived. Ttus takes specml account of tones or wtustles m the sound and also the way in which noise level vanes with time.

For certlficatmn purposes, of course, it is Important to use the best scale available, regardless of complexity. In fact, the improvement of EPNL over dB(A) is small, typically of the order of 1 dB, thus as we shall see, there is little to choose between any of these scales when consldenng the matter of commumty response.

The noise to which airport commumtles are subjected is, of course, much more complex than a single flyover sound and it cannot be described In terms of a single sound level meter reading. In the UK. a social survey was performed around Heathrow airport m 1961, the results of which have formed the basis for most subsequent planmng decisions (McKennell, 1963). For this survey, a technique was developed which allowed interviewees to be ranked accorchng to their expression of annoyance with aircraft noise on a scale of 0 to 6 between no annoyance and the very highest degree of annoyance observed. Through comparisons of average 'annoyance scores" w~th various noise parameters, the Noise and Number Index was derwed (Wilson, 1963), given by the equatmn

NNI -- LpN + 15 lOgl 0N- 80

The term LpN is the average peak perceived noise level of all aircraft sounds (the peak level is the maxamum value heard) and N is the number of aircraft heard during a 12 hour day. The constant 80 ensures a zero NNI value in areas where the average annoyance ~s zero. All sounds which do not reach 80PNdB are omttted from the calculation.

Fig 5 shows how well the Noise and Number Index gwes a linear relationship with average annoyance data. The experimental results are derived from two surveys, one at Heathrow London and the other at Schlphol, Amsterdam (Kosten, 1969) and the annoyance scores are normahsed as percentages of the maximum possible. This chagram seems to support the NNI concept as a reliable mchcator of community feehngs.

Noise and Number Index is in fact only one of numerous noise rating formulae, both national and International,

which are used for scahng aircraft noise. However, although there has been much debate about the relative merits of these alternative procedures, it is fairly clear that there IS little to choose between them (Ollerhead, 1972)

The basic chfficulty m the search for formulae hnkang noise and annoyance is that people vary enormously In their feelings about noise. Fig 5 only shows averaged results. The consaderable variation of lnchvlduat annoyance scores about the average can be seen from Fig 6 which depicts the percentile distributions of scores based on data from two Heathrow surveys (McKennell, 1963, and MIL Research, 1971 ). The distribution is found to be approximately normal (Gausslan) and the varlablhty, expressed in noise exposure terms, is equivalent to the standard deviation of 20NNI. That is, there will be as much variation of opinion amongst a

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Ftg 5 Relationship between average annoyance response and NNI

5 Percentoge of s a m p l e / / / / / / /

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Fig 6 Distr,but,on of annoyance scores

, j c

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<~

Applied Ergonom=cs September 1973 133

group of real people hvmg at a fixed noise level as there would be between an unaglnary group of identically behaved people normally distributed over a range of noise with a standard devmtlon of 20NNI. Clearly, the vanabdny between people is conszderable.

Fig 7 shows the percentage of people registering various degrees of noise annoyance as a function of NNI. Two sets of experimental data are plotted, taken from the Heathrow surveys of 1961 (McKennell, 1963)and 1967 (MIL Research, 1971). The curves are the theoretical normal distributions of annoyance scores corresponding to Fig 6. The agreement is considered to be reasonably good over most of the range. Curves corresponding to scores of 1 to 5 are shown. The methods by which these scores were obtained wall not be described here but to give some idea of their meamng, scores of 2, 3 and 4 correspond to self-ratings of 'a httle annoyed', moderately annoyed' and 'very annoyed'.

An analys~s of the survey results revealed that between annoyance scores of 3 and 4, aircraft noise began to emerge as the most dlsagreeable factor affecting an mdwldual's hying conchtmns. Based upon th~s and other related observatmns, a score of 3.5 was chosen as a critical value dwldmg the senously affected people from the not-so-affected and unaffected people. Fig 8 has been derived from Fig 7 by shifting the curves half a point. The centre curve thus becomes the critical hne of 3-5. The chfferent regmns so formed have been labelled as annoyance categories A to F and the descnpUons of these categones, also hsted m F~g 8, have been derived from a number of surveys performed m the USA and Europe in addmon to the London studies (Ollerhead, 1972).

To apply thas formulation, noise 'footprints', contours of equal NNI, are computed at 5 or 10NNI intervals and supenmpesed on a populatmn map of the regaon surrounding the airport. For each interval, the number of people falhng into each annoyance category may then be estimated and totalled for the entire regmn.

The following general conclusmns may also be noted

1. As no~se exposure increases by an amount NNI, the number of people who record a pamcular annoyance score increases by roughly 2NNI%. That is, for every 10NNI increase m nmse exposure the number of people expressing any pamcular degree of annoyance ~s increased by around 20% of the total number exposed.

2. Accepting a score of 3-5 as critical, the number of people who are seriously annoyed at any exposure level is approximately 2(NNI - 20) percent of the total.

3 A change m exposure equwalent of about 10NNI is reqmred to cause a 'category change" In annoyance, for example from 'a httle annoyed' to 'moderately annoyed" or to be annoyed by a fu{ther kind of d~sturbance. Roughly 20% of people fall into each category at any exposure level. It is interesting that a change of I 0NNI (10dB) corresponds to a doubling of loudness. It also nnphes that no~se reductmns of this order are required to produce a noticeable Improvement in the noise chmate. To achieve a significant improvement, reducUons of about 20NNI would be reqmred.

134 Apphed Ergonomics September 1973

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IO 20 30 4 0 50 60 70 Noise and number index NNI

AnnoyonceJ category! Feehngs about aircraft noise

Not annoyed, practJc011y unaware of mrcraft no~se

A little annoyed, occasionally disturbed

A

B

Moderately annoy edj disturbed by v=bratton, interference w~th conversation and TV/rad~o sound, may be awakened at mght

Very annoy e_d, considers area poor because of a i rcraf t nolse~ ts sometimes start led and awakened at mght

Severely annoyed, finds rest and relaxation dasturbed and ts prevented f romgomg to sleep, considers a i rcraf t noise to I~ the major disadvantage to the area

Finds norse difficult to tolerate, suffers severe dlsturbonce feels like moving away because of alrcraft norse and ~s likely to complam

Fig 8 Strat i f icat ion of feehngs about mrcraft noise as a functton of norse exposure

Aircraft noise generation

It must be accepted that the best way to tackle the atrport noise problem is to reduce the noise at source, that is, to make aircraft qmeter. At the present tune there are around 2000 jet aircraft in serwce with the world alrhnes. If these could be silenced, the nolse problem would disappear at most mrports. There is every reason to

believe that this might be considerably less expensive than gnplementing alternatwe approaches to noise control at each individual airport. It Is important to understand, however, that noise is fundamental to the operation of an aircraft and very difficult to reduce.

Jet noise The most obvaous source of aircraft noise on the

ground, and certainly the only one o f concern to people, is its power umt. In particular, the source o f noise which has caused most problems to date, most recently m the case of Concorde, is the turbojet engine exhaust. Jet exhaust noise falls into the general category of aerodynamic noise which originates solely from the turbulent mot ion of air and exhaust gases.

Even if the gas exhausting from the jet nozzle is a perfectly smooth or 'laminar' flow, it meets a region of relatively slow-moxqng air with a velocity difference equal to as much as two or three tLmes the speed of sound. The result (Fig 9) is a shear layer containing a chaotic mixture of whirhng eddies, millions of little whirlpools which grow and move and collapse at very tugh speed. It is this turbulent region which generates the characteristic roar of the jet engine, an intense random noise with energy distributed over the entire audible frequency range. This noise is greatest on take-off where the thrust is at a maximum and the velocity differential (between exhaust and ambient air) is a mmtmum. A very important feature of jet noise is that it is highly direcnonal with a major proportion of the energy rachatmg at about 45 ° to the jet axas (Fig 10).

This is fortunate for the passengers of a jet aircraft, particularly when the engines are mounted near the taft (eg, VC-10, Tndent). Dunng take-off in these aircraft very little jet roar is noticeable in the cabin. It is, however, less fortunate for people on the ground. As the aircraft climbs steeply the noise is actually radiated towards them.

Jet exhaust noise is very dependent upon the exit gas velocity, the total acoustic power being roughly proportional to its eighth power (V a ). This is why the early 'pure' jet engines with no by-pass flow were very noisy. Mditary after-burning jets are even noisier due to a combination of higher exhaust velocity and combustion noises. The by-pass fan engine which is installed in most modern passenger aircraft and the high by-pass ratio fans such as the Rolls-Royce RB211 represent orders of magmtude improvement m noise characteristics.

The reason for ttus is that the average exhaust velocity of a fan engine IS much lower than that of a turbojet. Also the high speed primary flow from the turbine exhaust is completely shrouded by the low speed fan exhaust so that it encounters a much reduced velocity differential (Fig 11).

The problem with supersonic aircraft is that they cannot make use of the low velocity fan engines. In simple terms, the engine exhaust velocity must always be greater than the forward speed of the aircraft to maintain thrust. Concorde. which fhes faster than the exhaust velocity of the large fan engines, therefore requires high speed turbojet engines to power it.

Propeller, rotor and fan noise Although propellers, helicopters, rotors, fans, and

compressors sound rather different to the human hstener,

Lain=nor core Region of maxfmum noise r -- . . . . \ gene ra t i on . . . . . . ~ - "

Nozzle Turbulent m~x~ng region

Fig 9 Turbojet noise generation

Fig 10 Jet noise d l r e c t i v l t y

Low speed exhaust from by-pass fan

by c F 0

Fig 11 By-pass jet noise generation

their noise characteristics, from a physical pomt of view, are very similar - the major chfference being simply one of frequency. However, there are some slight chfferences in the noise generating mechanisms. The high by-pass fan engine can be regarded simply as a turbo-propeller engine m which the propeller is enclosed by a circular shroud, and indeed the large modern engines are beginning to sound a little like the earher power units. The noise problem is in fact passing from the exhaust to the fan itself, as in the case o f the turboprop, the bulk of the noise is generated by the propeller or fan

Like jet noise, propeller noise is also generated aerodynamically, but in this case it is largely the interaction of the air with the blade surfaces which causes the noise rather than the turbulence itself. As the propeller blade rotates and passes a fixed point the air is first pushed aside and then returns to fill the void. These

Applied Ergonom=cs September 1973 135

coherent Osclllanons radiate harmonic noise, the famdlar regular pulsatde note of propellers. However, at normal propeller speeds it is the actual thrust and torque forces acting upon the air which generate most of the noise. In particular, unsteady pressures acting on the blades due to inflow disturbance, interference with the fuselage, atmospheric turbulence and the wakes of the blades themselves are very effective noise generators. Like jet noise, propeller noise is very senslstlve to velocity, this time the tip speed of the blades.

Helicopter rotors generate noise in precisely the same way, but here the problem is aggravated by the fact that the flow over the blades is very non-umform. The rotor is essentially a sideways moving propeller so that, relative to the air, the forward moving blade is travelling faster than the rearward moving blade. This gives rise to some very high fluctuating pressures on the blade surfaces. Also, it is possible for the forward moving blade to approach the speed of sound, at which point there is a sharp increase in the noise output.

Fans, compressors and turbines generate noise in a stmllar manner to propellers but there are some additional noise generation mechanisms. The most Important is the presence of stator or guide vanes between the multiple rotor blade rows. The flow interaction between the rotor and stator vanes is an additional source of large pressure fluctuations on the blade surfaces; this, in fact, is the major source of noise m most jet engine compressors. In order to reduce noise in modern engines, the separation between the blade rows is made as big as possible and the inlet guide vanes are omitted.

The effect of the duct is twofold. In the first place it gwes rise to additional noise generation by creating turbulence on the up-stream side of the rotors, thus increasing the unsteady blade pressures. Secondly, it modifies the way in which the fan radiates the noise into the atmosphere. Favourable duct propagation often can be used to mlnlmlse the noise which escapes, and further acoustic treatment to absorb sound energy can be attached to the duct surfaces. This techmque has been very successful in reducing the fan noise radiation in B747 (Jumbo-jet), L1011 (Tnstar) and DC- 10 aircraft.

The most effective way to reduce the noise output of all rotating machines is to reduce their rotational speed. However, since this Is usually contrary to the interests of aerodynamic efficiency, a compromise has to be made, this compromise is normally biased towards the lowest operating costs.

Piston engine noises Although piston engines can be effectively silenced,

this involves a lot of adcht]onal weight and aircraft engines are normally relatively unsflenced. Consequently, m piston engined aircraft, the engine exhaust is often a dominant source of noise. However, because its charactenstlcs are very similar to those of the propeller nois~, the two are often very difficult to distinguish. Nevertheless, the engine exhaust is a major source of noise m light aircraft

Engine noise reduction The whole purpose of an aero engnne is to exert a

thrust force upon the aircraft, this m turn requires a momentum exchange with the surrounding air. It is this

very process which generates noise to a degree which depends heavily upon the typical velocities involved, either of the air itself or of moving surfaces on the engine. Basically, there are two ways to reduce the noise output - by reducing the velocities, and thus reducing the acoustic energy generated, or by preventing it from escaping to the surrounding air. The first is normally accompanied by a loss of performance, the second by an increase of weight.

Either result has a profound effect upon the operating efficiency of an aircraft. From an econormc point of view the effects of even modest reductions of noise can be disastrous. For the most part the noise reductions achieved m the large new 'alrbuses' have not involved any significant penalties of this land. Further reductions almost certainly will.

Application of noise planning methodology Use of the methodology outlined in the previous

sections will be illustrated by application to the situation erastmg at London Heathrow airport m 1967. The analysis is based upon the distribution of people throughout the different noise strata at that time Wlttnn an area of 1550 km ~ (600 miles 2) 32kin (20 miles) north to south,

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w norse areas (tess than 35 NNI)

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I L I B C D E F Low Degree of annoyance High

D~stnbut~on of annoyance, Heathrow 1967

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FIg 13 Distnbutton of seriously annoyed people, Heathrow 1967

60

136 Applied Ergonom,cs September 1973

48 km (30 miles) east to west, centred at the airport.

It is common pracUce in the UK, when mapping airport noise xmpact, to construct the 35NNI and 50NNI contours only. The assumption underlying this pracUce is that below 35NNI no slgmflcant noxse problem exists, whereas exposures of 50NNI and above are generally regarded as unacceptable. Between 35 and 50NNI, &scretlon should be exercised m planning buil&ng

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Fig 14 Effect of awcraft noise reduct ion on annoyance around Heathrow (assumes 1967 popula t ion dB tnbu t l on throughout)

60

construction or other developments.

However, for the 1967 Heathrow survey, exposure levels down to ONNI were mapped and Fig 12 shows the interesting result that except at the very highest level of annoyance (category F), there is considerably more annoyance m the low noise areas below 35NNI than m the high noise areas. This is samply because although the fraction of people affected decreases at lower nmse levels. the total number of people exposed increases. The true difference is probably even larger than ln&cated by Fig 12 since the 20NNI footprint covers large tracts of Central London outside the 1550 km 2 box studied.

The effect is perhaps more clearly illustrated m Fag 13 which shows the dIstnbutaon of seriously annoyed people, le, categories D, E and F, as a funcuon of noase level. Out of a total population of nearly 3 mflhon, nearly 500 000 are seriously annoyed However, most of these live m the intermediate noBe zones between 20 and 40NNI. Approxamately 200 000 of the seriously annoyed live m areas above 35NNI, yet nearly 300 000 lave below 35NNI It is clear that the usual neglect of the low noise areas leads to a very mlsleadmg impression of the real extent of the problem. The error xs hkely to be particularly large at new airports such as Maphn (Foulness) where "noise planning" as based on mmlmlslng the number of people exposed to more than 35NNI. In this case, the proportion of senously annoyed in the areas outside the control zone may be very much greater

In an attempt to foresee what m~ght be accomphshed at Heathrow by future aarcraft nmse reduction, R~chards (1972) used the present techmques to examine the effects of successive step reductions of 10NNI. The results are shown in Fig 14. We see that for each 10NNI reduction, the number of seriously affected people is cut by approxmaately 50%.

Slgmficant reductions of aircraft no~se, by 10 and 20dB, are certainly wxthm the bounds of technical feasxbdlty. However, It ~s ~mportant to recognlse that the benefits of such technologxcal achievements may be slow to materlahse An overall reductmn of 10NNI for example might be achieved by reducing the noise of all aircraft using an airport by 10dB. However, ff only a frac taon of the aircraft are treated, the net benefit mL"-~at be substantmUy less.

This may be seen in Fig 15 which shows the reduction

0

c - I 0 0

2 -20 z

2O

per a i rcraf t

' ' 6 ' ' - 3 % 20 4 0 O 80 IOO Percentage of aircraft treated

Fig 15 Effect of equal ly silencing all aircraft upon c o m m u m t y noise levels (assumes all awcraft m~tmlly equal ly noisy)

A p p h e d E r g o n o m = c s September 1973 137

O ~ N N I r~ uctlon

Av~age reduction requwed ~N~

- 30 / for treeted ~rcreft \

/

I I I t

0 20 40 60 80 I00 Percentage of awcraft treated

F~g 16 Effect of silencing notstest a=rcraft upon commumty no~se levels (assumes distributed no~se levels)

m NNI obtained when any fraction of the aircraft xs silenced by varying amounts. It may be seen that NNI reductions are very much less than the a~rcraft noise reductions until a very high percentage of the aircraft have been treated, say more than 90%. However, this does assume that all aircraft are lmtlally equally noisy.

A more reahstlc sltuatmn may be found m Fig 16 which is based upon the assumption that the initial noise levels are normally distributed with a standard deviataon of 10dB. In thas case, only the noisiest of the aircraft using the airport are treated. The two curves show the aircraft attenuauon reqmred per aircraft and the NNI reduction achieved. For example, to reduce the total noise exposure by 10NNI, approximately 35% of the aircraft would have to be silenced by an average of 12dB. For a drop of 20NNI, approxamately 80% of the aircraft would have to be silenced by an average of 20dB. A comparison of Figs 15 and 16 clearly illustrates the practical need to concentrate initially on the particularly noisy offenders If airport noise control ~s to bring effecuve rehef to airport nelghbours.

Acknowledgements

The paper stems from research sponsored by the Directorate of Operations Research and Analysis of the Cwd Aviation Authonty. Thanks are also due to Prof E. J. Rachards, Vice-Chancellor of Loughborough Umverslty, for h~s keen interest and advice.

References

International Civil Aviation Organisation, 1971 Annex 16 - 3arcraft Noise, 1st Edition, August.

Kosten, C.W. et al. 1969 "Aircraft Noise Abatement", Enghsh Translation -

NASA TT F- 12093.

McKenneli, A.C. 1967 Aircraft Noise Annoyance Around London

(Heathrow) Airport, General Office of Informauon, 55,337.

MIL Research Ltd. 1971 Second Survey of Aircraft Noise Annoyance Around

London (Heathrow) Airport, HMSO, London.

Ollerhead, J.B. 1972 Estimating Community Annoyance due to Airport

Noise, Loughborough University TT 7203.

Riehards, E.J. 1972 A Ten-Year Look at our Noise Environment,

Presldentral Address, Society of Environmental Engineers, London, October.

Stockbridge, H.C.W., and Lee, Mary. 1973 ApphedErgonomws, 4.1,44-45, The psycho-social

consequences of aircraft noise.

U.S. Federal Aviation Regulations, 1969 Part 21, Ceruficatlon Procedure for Product and

Parts, Part 36, Noise Standards, Aircraft Type Cemficauon, December.

Wilson, A., and the Committee on the problem of noise. 1963 "'No~se", Final Report, HMSO, London.

138 Applied Ergonomics September 1973