Practical noise control at international airports

241

PRACTICAL NOISE CONTROL AT I N T E R N A T I O N A L AIRPORTS W. GRACEY

B & K Laboratories Ltd., Hounslow, Middlesex (Great Britain

(Received: 10 April, 1968)

SUMMARY

The success of any noise control project at an International Airport depends not only upon the monitoring capability but largely upon co-operation between the Airport Authorities and Airlines involved. Although this paper outlines the technical aspect of a highly suitable system, the legal and political problems should be borne in mind.

HISTORICAL

With the coming of the turbo-jet aircraft, noise in and around civil airports became a major problem. In the early nineteen-sixties, there were no international recommendations for assessing noise nuisance from aircraft. Here in England, guidance came in the form of the Wilson Committee Report (1963), 1 while in Sweden (1961) 2 and Switzerland (1963) a similar official reports were produced. In America K. D. Kryter had been working on the problem for some years, and as early as 1959 had introduced the PN dB (perceived noise level) concept. This method covers piston engined aircraft as well as jet aircraft. (See Appendix I.) This method forms the basis of an I.S.O. Recommendation No. 507. The advantages of such a recommendation is that a well-defined measure of accepted noise level may be laid down, and yet still leave scope for local variations of the limits. However, the present day tendency is to "standardise" on 110 PN dB by day and 100 PN dB by night in built-up areas.

PRACTICAL NOISE CONTROL

Unfortunately the siting of most airports in Europe is not ideal from the noise aspect. Applied Acoustics (1) (1968)--Elsevier Publishing Company Ltd., England--Printed in Great Britain

242

W. GRACEY

NOISE FIGURES FOR CARAVELLE

TABLE 1 III wrrn SILENCER

AND R A

527 ENOmES

Engine setting

8050 rpm

7650 rpm

7500 rpm

7350 rpm

(start)

400 500 600 700 800 900 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200

120 117 115 114 113 112 111 109 107 106 105 104 103 102 100 99 98 97

130 129 127 126 124 122 120 118 116 115 113 112 111 110 108 107 106 105

114 111 109 108 107 106 105 103 102 100 99 98 96 94 93 92 91 90

126 125 123 121 119 118 116 113 111 109 108 106 105 103 102 100 99 98

113 110 108 107 106 105 104 102 100 98 97 96 95 94 93 91 90 89

125 123 121 119 118 116 114 112 110 108 106 105 104 102 100 99 98 97

111 109 107 106 105 103 102 100 99 97 96 94 93 92 91 90 88 87

122 120 118 117 115 114 112 110 108 106 104 103 101 100 98 97 96 95

A i r c r a f t noise can be r e d u c e d by the following: (1) (2) (3) (4) (5) (6)

K e e p i n g aircraft as high as possible when passing over restricted areas. Restricting p o w e r settings. K e e p i n g aircraft as far as possible laterally from residential areas. L i m i t i n g n u m b e r s o f j e t movements. A c o u s t i c insulation o f buildings. I n t r o d u c t i o n o f practical noise limits.

In the first case, i.e. keeping aircraft as high as p o s s i b l e when passing over residential areas, this is closely b o u n d u p with item ( 2 ) - - r e s t r i c t e d p o w e r settings. These in t u r n d e p e n d u p o n gross weight at take-off. The m o s t satisfactory f o r m o f c o n t r o l is to specify the m a x i m u m levels in P N dB a l o n g the flight p a t h to the residential area, the m i n i m u m height a l o n g this p a t h a n d the rate o f climb over the area. This allows the airlines involved to establish their own p r o c e d u r e s to give m a x i m u m e c o n o m y , safety a n d a d j u s t m e n t s for meteorological conditions, c o u p l e d to a low percentage o f noise violations. l t e m ( 3 ) - - k e e p i n g aircraft as far as possible laterally f r o m residential areas, o r as it is sometimes k n o w n " M i n i m u m N o i s e R o u t i n g s " requires the aircraft to fly a n d

243

NOISE CONTROL AT INTERNATIONAL AIRPORTS TABLE 2 NOISE FIGURES FOR D E - 8

WITH SILENCER AND J T

4A-9 ENGINES

Engine power Start

~ 400 500 600 700 800 900 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200

~

~

121 119 117 115 114 113 112 110 108 107 106 104 103 102 100 99 98 97

133 131 129 126 125 123 122 119 116 114 112 110 109 108 107 105 104 103

10,000 Ib

.~ 115 113 111 109 108 106 105 102 100 98 97 95 94 93 91 90 89 88

~

~&

113 111 109 107 106 104 103 101 100 98 97 96 95 94 93 92 91 90

126 124 122 120 118 117 115 113 110 108 106 105 103 102 101 99 98 97

8000 Ib

.~ 108 106 104 102 101 99 98 96 93 91 90 88 87 85 84 83 82 81

6000 Ib

~

~

.~

~

~

107 105 103 101 100 99 98 96 95 93 92 91 90 89 87 86 85 84

121 118 116 114 113 111 110 107 105 103 101 100 98 97 95 94 93 92

102 100 98 97 95 94 92 90 88 86 84 83 81 80 78 77 76 75

102 100 98 97 96 94 93 91 90 88 87 86 85 84 82 81 80 79

116 114 112 110 108 107 105 103 100 98 96 94 93 91 90 89 87 86

.~ 98 95 93 91 90 88 87 84 82 80 79 77 76 74 73 72 70 69

approach over low population areas. This technique depends upon any radius of turn not being too tight for the aircraft concerned, and also the dependence on point source navigation aids, i.e. beacons. Checking on aircraft flight path again calls for remote monitoring preferably by microphone technique. Item (4)--number of jet movements by day and night, is of course, subject to consultation of the bodies involved. Acoustical insulation of buildings does bring relief to those indoors, particularly if taken to the length of full air-conditioning. Practical noise limits will not be effective unless backed up by accurate noise level monitoring in a method agreed by all parties.

AIRCRAFT NOISE MEASUREMENT

The measurement of aircraft noise and describing it in physical quantities is not a ditiicult problem. All that is required is a top quality microphone to transduce the acoustic information into electrical signals, and well-defined filtering and recording instruments. This instrument is readily available and complies with the inter-

244

W. GRACEY TABLE 3 4 wrrl-I SILENCER AND R A - 2 9 ENGINES

NOISE FIGURES FOR COMET

Engine setting 8000 rprn (start)

~'~

~

~

200 250 320 400 500 600 700 800 900 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200

. . . 117 115 113 112 ll0 109 108 106 104 102 101 100 99 98 97 96 95 94

. . . 131 128 126 125 123 121 120 117 115 113 112 110 108 106 105 104 103 102

7550 rpm

. . . 113 111 109 107 105 103 102 100 98 96 94 93 92 90 89 88 87 86

620(06300 rpm

(landing)

.~ . . .

7350 rpm

~

~~ ~-

. . . 113 111 109 108 107 105 104 103 101 99 98 97 95 94 93 92 91 90

. . . 127 125 123 121 119 117 116 114 112 109 107 106 104 102 101 100 99 98

.~ . . .

~~ . . .

110 108 106 104 102 100 98 96 94 92 90 88 87 86 85 84 83 82

. . . 111 109 107 105 104 103 102 100 98 97 96 95 93 92 91 90 88 87

~

125 123 121 119 117 115 114 112 110 108 107 105 103 102 100 98 96 94

.~

107 105 103 101 99 97 96 94 92 90 88 86 85 83 82 81 80 79

~

~

104 102 100 98 95 93 92 91 89 88 86 84 83 -

123 121 119 117 114 112 110 109 107 105 102 99 97

4~ .~

105 103 101 99 96 94 92 90 88 86 83 80 78

-

nationally accepted precision specifications. All electronic instruments however are objective and the subjective interpretation, i.e. correlation to annoyance, is a complex problem. (See Appendix II.)

HEATHROW AIRPORT

Noise monitoring began at Heathrow in 1958, when jet aircraft were introduced. This early work was purely on a research basis and it was not until early 1960 that the Noise Monitoring Unit was set up, to observe the 500 jet aircraft departures a month from Heathrow operated by 11 airlines. The figure at the moment is approximately 7000 departures a month and 45 operators. To begin with, the method was to monitor along the flight path in the residential area encountered during take-off. Late in 1961, a working committee was set up with the airlines out of which came an important agreement--the Minimum Noise Routings.

245

NOISE CONTROL AT INTERNATIONAL AIRPORTS

The Noise Monitoring Unit had by this time an effective mobile service operating a twenty-four hours a day watch at Heathrow, each vehicle equipped with a sound level meter, tape recorder and two-way radio. Passing aircraft were taped and logged and at the end of each watch the tape was analysed through an " N " filter. After detailed discussions, on the 9th August 1966 B & K Laboratories Limited were instructed by the Board of Trade to install a pilot scheme "Automatic noise monitoring system for London (Heathrow) Airport". The objects of the installation were as follows: Twenty-four hour watch. 360° monitoring not only on the flight path but also in other residential areas. Instantaneous values of dB (A) and dB (N) as required. Retention of full frequency information for future analysis, PN dB, proof if need be. Channel, date, time, correlation. Ability to calibrate the measurement immediately following the recording and/or when required. The instrumentation now installed at Heathrow employs the following components:

S_

[

I Noiselimil I

WeLghted r~se response CH ident t iEne

Tm'~esiqnol

I

4

~

responseCl-t ident. time

[ _ Time si0nol

Two ~ e l

noise reco~ng (hneor) CH.ident.time

Fig. 1.

Schematic--Heathrow

noise m o n i t o r i n g system.

The acoustic information is received at the remote measuring point and is transduced by means of purpose designed outdoor microphone system.

246

W.

GRACEY

Fig. 2. The outdoor microphone system.

The microphone itself is of the condenser type and is protected environmentally by a windscreen and raincover, with the result that, if left switched on, the performance is virtually unaffected from - 3 0 ° to +60°C. The amplifier section and trigger circuits are built into a weatherproof cabinet as shown.

Waterproof cobinel

Windscreen

I

O

'

I

Output

I Amplifi~ trier Rain Cover andelectrostatic Actuator

--7 Power

Microphone and Cathode Follower

Suppl)

ZColibrc Osc.

i

.....

I

1 °e'°' unit .,,J

Fig. 3. Schematic of remote monitoring system. The purpose of the amplifier is to feed the signal to the transmission lines (G.P.O. music circuit) at 0 dBm matched into 600 dB. The amplifier is transistorised throughout and has a gain of 30 dB. The trigger circuit is based on the "A" curve, in other

247

NOISE CONTROL AT INTERNATIONAL AIRPORTS

words, the input spectrum is sampled and weighted such that only "doubtful" levels from the annoyance point of view are passed down the transmission line. Being "level controlled", the channel remains open until the signal fails well below the pre-determined limit; the calibration signal is then triggered. The calibration oscillator in the amplifier assembly is fed to an electrostatic actuator built into the microphone rain cover. This voltage produces a known force which acts upon the microphone diaphragm and is comparable to a sound pressure. The actuator is adjusted to produce an equivalent SPL of 90 + 1 dB by the injection of an AC voltage of 215 V at 500 Hz. The time duration of the calibration signal acts as channel identification. i I013

0

0

~7 rn r~ 0 1 3

0

0

013

0

013

rn 0

0

0

Cl ~

I 4D

i.-i

b~

'

-

0

0

0

~ F

0

0

0

0

I',II--~

_ _ ~

0

0

0

------~--',

.

.

I

i

.

.

----,-4-- ~

i----I-r:

J i

10

'17':

m

10~

Fig. 4.

0

0

: ',',',',

0

O0

C

q--4

"

. . . . .

I ( ~ ~,r~

__X¢,--~ 17-

i1

0Pl123

0

i!;ii

.

.

0

_ .~ L. . . i . ~. - - * ' - - 4 - ~

'

.... -•

0

~--V" in

1000 "

i

I

I i :li!

i--FV - - ~

ii~

10000 "

" ' ~ " ~ " ~ " ~• :

~: ~ ~"

Overall frequency response of r e m o t e m e a s u r i n g system.

Thus the noise level and spectrum is faithfully reproduced and transmitted over the land lines. Of course, with outdoor systems of this type the stability and life expectancy are of prime importance. Here the stability should be better than +_ 0.5 dB, coupled with a lifetime on the electronics side at least 20,000 hours. The microphone cartridge can deteriorate in very corrosive atmospheres and the figure of 20,000 hours may be taken as a minimum. G.P.O. music circuits have been used in the Heathrow system as they offer a uniform frequency characteristic over the bandwidth of interest (see Fig. 5), reasonable signal-to-noise figures and a high degree of reliability and stability. Alternatively ordinary telephone lines or twisted pair up to 6 miles could have been used, as the amplifier has built-in provisions for a certain adjustable compensation of the high-frequency loss in long cables. Radio links are of course another possibility.

248

w. GRACEY

O

0 0 0 ; 0 0 0 0 0 0 0 0 0 0 ~ O 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 O O O D

0 ~ 0 0 ~ 0 0

[

I - -

.

.

.

.

~,~ .... ~ h

OP 1123

"7ES

_~-7~_~$2

~

-

t-- i x-r-----

m

g

m

10

m

sm

100

~

~

to

1000

~

sm

~

10000

~

~ A ~.,,.,A

.... ii 0

,

c ~

c.

Fig. 5. Typical characteristic of a G.P.O. music circuit. On arrival at the control centre the signal is then fed into the noise limit indicator. The noise limit indicator is best considered as a multi-channel amplifier whose bandwidth of weighting is controlled by plug-in filters, in our case "'A" or "N'" curves. Following the amplifier is a quasi-r.m.s, rectifier circuit. It provides full wave rectification and gives accurate r.m.s, indication for complex waveforms. This signal is now capable of energising a relay which then triggers the B.K.E. I 1 logic unit. The signal is also available for lights, klaxons and other warning devices. The trigger level for the noise limit indicator may be pre-selected on the front panel, i.e. if acceptable levels are 110 PN dB by day, and 100 PN dB by night, these may be set up twice a day. The function of the B.K.E. 11 logic unit is that of "signal sorter", i.e. when a "'violation signal" triggers the relay in the noise limit indicator, the B.K.E. l l transfers the full signal to the magnetic tape recorder and the level recorder. In the case of the tape recorder the sequence is as follows. Firstly the signal xs recorded; the duration of this record is entirely dependent upon the level and in no way time controlled. When the signal falls below the predetermined limit, the calibration signal comes up, the duration of which gives the channel identification. (As previously described, the calibration signal is derived by electrostatic actuator techniques at the remote microphone position.) Immediately following the calibration signal, the B.K.E. 11 feeds the G.P.O. speaking clock onto the tape, thus we now have full frequency, level, time and place information permanently retained. The level recorder in the meantime is fed with the dB (A) level, channel identification and calibration immediately following. Date and time information are stamped on the record on receipt of a command pulse from B.K.E. 11. As can be seen from block diagram Fig. 1 the B.K.E. 11 can handle two signals simultaneously; in the unlikely event of three violation signals being present or

249

NOISE CONTROL AT INTERNATIONAL AIRPORTS

S.P.L. de/Z x IO'~.ioor

P

~{b)

120 HO (o)

,

IOO

90 801 7C

30 40

50

I00

200:500400 500 Altitude m

I000

2000

Fig. 6. Acoustic noise produced by Caravcllc I l l during its take-off. 80( Take-off procedure: Steepclimbto IO00m. Take-off weight:43.5t, 20 kt

70,

heodwind

~

J

/

J

~

f

i

60q 50, 40O

,

I. //I ' ~" /" !j /,,-

)

4

6

I

Toke-off procedure ~ Sud Z',v,oho Toke-off welght~)7'5 t, lOkt toilwind

30O 20C ~oc

t

Take-off

{

2

3

5

L

7

8

9

I0

Horizontal distance km

Fig. 7. Take-off profi|¢s for two different take-off procedures at standard temperature.

250

w . GRACEY 800

I

700

/

,/ /

600

5OO

500

250

E

~

~00

40C

200

300

30¢ /

/

Aircfof! speed

/

/

20C

150~

2OO

/

/ I I

IOC

I00

3

4

5 6 HO~ZOntOI dlslonce

7 km

8

9

50 0

I0

Toke-off

Fig. 8.

Take-off profiles for Caravelle 11I with different loads.

800

i

7OO

600

1

." /

Temperolure

/

~

/

500

50C

4o(:

40C

30C

30C -;

!2C

200

/

/

/ ioo /

20C

I0 ,S

I00

5C'

I i I Toke -off

Fig. 9.

2

3

4

i

i

6 7 Hor~zontGI dlSlGnC~ ~,m

~5C ~c

i I0

Take-off profiles for Caravelle III at different ambient temperatures.

251

NOISE CONTROL AT INTERNATIONAL AIRPORTS

arriving during analysis, the B.K.E. 11 selects the largest signal and inhibits the second. Statistical information is also available by means of a modified Bruel and Kjaer 4420. In this case, the count is broken down into two classes; one count gives the number of violations and the second count indicates the total time of the violations, per watch, per day, week, etc. Although primarily designed as a noise control system, the instrumentation may be used as a basic research and development tool. In this way a maximum PN dB value can be assigned to each measurement condition, and curves such as Figs. 6, 7 8, 9 and 10 may be constructed. If research is the aim, then information is required over and above that provided by the installation. Air temperature in °C, relative humidity, atmospheric pressure, and windspeed are measured at the remote microphone point. In addition, data to be noted are: type and model of aircraft and engine, aircraft gross masses (take-off weight), aircraft figurations (flap and landing gear positions) airspeeds, aircraft height, power settings, and maximum powers. 80C

I

700

1

//i

600

50O

//S!

400

oo

300

200

t50

i

/

20C

Ioc

250

I00

/

///,n

l1

l

..~,.

3

1

I

4 6 Horizontol distonce km

i

7

I

i

8

'

I

9

oo

50

i

I0

Toke- off

Fig. 10.

Take-off profiles for CaraveUe III under different ambient wind conditions.

OTHER INTERNATIONAL AIRPORTS

Outside the U.K. there are at least 20 major international airports at which noise is recognised as a significant problem. None have quite the problems that there are

252

w. GRACEY

at Heathrow, where there are large numbers of jet movements, residential areas almost up to the boundary on some sides, approach routes over the metropolis, and no areas of water over which aircraft could be routed. Most airports nowadays have "minimum noise routings" in order to keep the aircraft as far as possible from built-up areas. Some restrict or even ban jet movements by night and others devote considerable attention to Public Relations work. Few so far have installed an automatic noise measuring facility. 140 ,3o

Noy$ PN dB L30

:~o'2° "~ ~ ~-~-. ~ - - -25o~~ ~,,.~ ~-zoo.~

120 I10

.

Z

I00

9C

,"o_

_

~

~

~o-

~

/ - 80

~ 60

~

:

70

-3-

60 50

~v"

30

• 40

30

m

I0 o

20

5

I00

2

Fig. I I.

5 I000 Frequency in Hz

2

5

I0.000 20.000

Equal "noisiness" c o n t o u r s .

APPENDIX I

The PN dB concept is basically a measure of the loudness of a noise, taking some of the "annoyance" effect caused by the noise into account. It is normally called a

NOISE CONTROL

AT INTERNATIONAL

253

AIRPORTS

measure of "noisiness". To calculate the P N dB value of a noise, the noise should be frequency analysed by means of an octave band analyser. From the sound pressure level measured in each octave band (re 2 x 10- 4 pbar) the noisiness of the band sound pressure level is found by means of the chart. The total noisiness (in Noys) is then found by adding the noisiness from the individual octave bands according to the formula: Nto t = Nmax + 0"3(ZN -- Nmax)

Here Ntot is the total noisiness (in Noys), Nm=x is the noisiness of the "noisiest" octave band and ZN is the sum of the noisiness of all the octave bands. When the value of Ntot is found in Noys this value can be converted into PN dB by means of the scale also shown.

APPENDIX

II

For monitoring or other purposes for which the highest precision may not always be demanded, there is need for a suitable direct reading instrument. The sound level meter with the " A " weighting which is at present often used goes some way to meeting this need. This practice is based on the experience that, for take-off noise, the difference between perceived noise level and sound level A is roughly the same for aircraft of the same class at about the same distance from the start of

i6 12 iO

~0

iO 12 14 16 ~8 2

3

4

5 6 7 891C

2

3

4

5 6 7 89K) HZ

2

3

Fig. 12. Weightingnetwork curve

4

5 6 7 89104

2

254

w. GRACEY

w~

,!

Figs. 13 and 14. Examples of microphone mountings. .

x

"

I

\ I

---~___

LANGLEY /I

//

/---I

/ ,-

/ ./"

//

,/k

/

.,,. /

YIEWSLEY ~1 ~. , /

\'l ~

/

WEST /I DRAYTON /

/

-~

/ ~

~

'

"--~

it

"~ "~,

~

t

I

I I I I / ~ / / SOUTHALL l

I

./

:

HARLINGTON~ .... i

-

!

I

, 1/ i_ ~ " ' ~ . . . . I

-

NORWOOD //

/" i.~

!

k\

f,

\

\

HESTON

~

l

! !

Fig. 15.

Noise contours around London airport.

NOISE CONTROL AT INTERNATIONALAIRPORTS

255

the take-off roll. For example, an examination of the octave band spectra for the noise at about 4 miles from take-off of about 100 jet passenger aircraft, of 9 types currently in use, showed sound level A to be, on the average, about 12 dB lower than perceived noise level, about 90 per cent of the difference lying within + 2 dB of this average. A similar examination for propeller aircraft, including 12 different types, showed an average difference of 14 dB, about 80 per cent lying within + 2 dB of this average. It should be recognised, however, that on occasions wider variations can occur, values up to about + 4 dB having been encountered in the above examination. For other conditions, e.g. at smaller distances from take-off or for landing, the average difference between perceived noise level and sound level A will be different. For the measurement of sound level A, the use is recommended of a high quality sound level meter having the "A'" weighting as close as possible to the values specified in the I.E.C. Recommendation for Sound Level Meters, and using the slow response time of the meter. In the tables on pp. 242-244 some results obtained from measurements reported by Bolt, Beranek and Newman Inc. (U.S.A.) are given.

REFERENCES 1. Committee on the problem of noise, final report, Cmnd 2056, H.M.S.O., July 1963. 2. Flygbuller sore samhalisproblem statens offentliga utredninger, Stockholm, 1961. 3. ~ b e k S z n p f u n g in der Schweiz, Bericht der Eidgenossischen Experten Kommission an den Bundersrat, Government Printing Office, Berne, 1963,