After reading this article you will learn about:- 1. Subject Matter of Aircraft Noise 2. Sources of Aircraft Noise 3. Sound Pressure Levels 4. Piston-Engined Aircraft 5. Engine Noise 6. Jet-Prop Aircraft 7. Sonic Boom 8. Airport Noise 9. Importance of Noise and Number Index (NNI) 10. Equivalent Continuous Sound Level 11. Ground Running.
Contents:
- Subject Matter of Aircraft Noise
- Sources of Aircraft Noise
- Sound Pressure Levels of Aircraft
- Piston-Engined Aircraft
- Engine Noise in an Aircraft
- Jet-Prop Aircraft
- Sonic Boom
- Airport Noise
- Importance of Noise and Number Index (NNI)
- Equivalent Continuous Sound Level of Aircraft Noise
- Ground Running Aircraft Noise
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1. Subject Matter of Aircraft Noise:
There has been a substantial increase in the noise nuisance from subsonic aircrafts over the last two decades. There are several reasons for this. In the first place, noisier jet-engined planes have progressively replaced the earlier piston-engined and turbo- prop types. The latter types of planes, although still noisy, were relatively quiter in comparison with jet-engined aircraft.
Secondly, there has been a steep rise in the number of civil aircraft. This has resulted in a considerable increase in the number of air movements. When compared to developed countries of Europe and America, the number of civil and military airports in India and other developing nations is not very large.
The main noise disturbance due to aircraft is confined to a radius of about 16 km around the airports. But there are many people who work or live under the flight paths connecting air ports. For these people, the noise from the aircraft passing overhead is inescapable.
Aircraft noise is variable and intermittent. It is not continuous as in the case of road traffic noise. There are peak noise levels when aircrafts are flying overhead, or are taking-off and landing at the airports. The peak frequency varies with the number and the types of aircraft, and the operational height.
2. Sources of Aircraft Noise:
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The chief producers of noise in the aircraft are various components of the propulsive system. These include the engine, propeller or rotor, jet engine compressor, turbines and jet flow, etc. The noise generated by the motion of the aircraft through the air is confined to boundary layer noise which is really an aerodynamic phenomenon.
This, in turn, is mainly responsible for the secondary noise generated by the skin vibration, observed particularly inside the aircraft in flight.
The contribution of this noise to an outside observer is almost entirely masked by the propulsion noise. Other pure sources of aerodynamic noise are the static pressure fields moving with very low flying aircraft, and shock waves and sonic booms generated by supersonic flight. These two (i.e., static pressure fields and sonic booms) are separate phenomena.
3. Sound Pressure Levels of Aircraft:
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Typical overall sound pressure levels are generated by four distinct types of aircraft. These are:
(a) Propeller driven aircraft,
(b) Jet-engined aircraft (for transport),
(c) Jet fighter, and
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(d) Jet bomber.
Each of these four types may undertake three different climb out positions, viz.:
(a) Steep climb,
(b) Normal climb, and
(c) Shallow climb.
Typical noise levels for these twelve possibilities are shown in Figs. 1-4. These diagrams would establish the sound pressure levels for aircraft in level flight, this time as functions of slant range instead of the distance from take-off.
These are shown in Fig. 5 for various aircraft. But neither of these two types of sound pressure level measurements convey any specific information about the sound “footprint” (actual distribution of sound on either side of the flight path). This can only be given by actual sound pressure contours plotted with the help of measurements taken at a large number of different points.
4. Piston-Engined Aircraft:
The main sources of noise with the piston-engined aircraft are the engines and propellers. Among these, the propeller noise is often predominant except at fairly low tip speeds and/or low power levels.
The propeller noise consists of two components:
(a) Discrete tones at the blade speed frequency and its harmonics, and
(b) Vortex noise or broad-band noise which predominates above about 1000 Hz. At very low tip speeds, the vortex noise can exceed the noise due to rotation of the propeller at all frequencies. Mathematical analysis of these two components of propeller noise is possible. It becomes complex, however, if forward speed and blade load distribution are taken into account.
The sound field for the vortex field generation is directive. It is characterized by a four-lobe pattern, as shown in Fig. 6. In this diagram, the arrow indicates the direction of flight of the aircraft. The sound pressure level under the angle of maximum radiation is usually of the order of 4 dB higher than the space average.
This angle (of maximum sound radiation) is roughly 25-30° inclined backwards from the plane of propellers (shown by a disc in Fig. 6).
The vortex noise level can be calculated with a reasonable degree of accuracy (+ 10 dB) from the empirical relation
Lp = 10 log10 [1016 k Ab(Vo.7)6] dB, (1)
where Lp = Vortex noise level in dB,
Ab = total area of propeller blades (in m 2),
Vo.7 = velocity of blade element at 0.7 radius (in m/s),
k = an empirical constant
Similarly, peak frequency fmax of the vortex noise can be obtained from the following formula: fmax = 0.13
Hz,
where Vh = helical tip speed of the propeller (in m/s),
R = radius of the propeller (in m).
The typical contribution of vortex noise to the overall noise of a piston- engined aircraft can be judged from Fig. 7, which refers to a specific diameter of the propeller and specific input horsepower.
5. Engine Noise in an Aircraft:
The engine noise in an aircraft is derived almost entirely from the pulsating exhaust flow. Its fundamental frequency is, therefore, equal to the average cylinder firing frequency of the engine. For the direct analysis of sound output in this case, the engine is treated as a simple source. But this approximation does not hold good for harmonics.
In addition to Eq. (1) given earlier, the overall sound power level Lp can also be estimated directly from the following empirical relation:
Lp = 125 + 10 log10Wt dB (ref 10-13 watts), …(3)
where Wt = total horsepower delivered by the engine.
By treating the engine as a simple point source, the sound pressure level Lr at a distance r is given by
Lr = Lp – 10 log 10 4 Tπ/r2 (ref 0.0002 microbars) …. (4)
For low powers, in general, there is a typical decrease in the sound pressure level for each successive octave of 2.5-3.0 dB, as shown in Fig. 8, with a leveling off at higher frequencies. For engines operating at maximum power (particularly large engines), the power level is substantially the same in all octave bands, at a level of about 7-8 dB below the overall level.
Reduction of Engine Noise:
The standard method employed to reduce exhaust noise is silencing. The application of silencers is, however, limited to straight-through type silencers to minimise backpressure. Moreover, the use of silencers is also likely to modify the sound spectrum of the engine noise.
It may suppress some frequencies and amplify others, depending on the relative phases of the individual exhaust pulses as they meet the exhaust system.
In addition to exhaust noise, the engine may also generate noise by engine-excited vibrations. These can be transmitted through the engine mounts to the airframe. This, however, is only likely to affect noise levels inside the aircraft.
This type of noise is generally negligible in the aircraft of modern design where adequate attention is given to isolation of the engines at the point of mounting. In older types of aircraft, however, engine vibrations may be responsible for vibrational waves being set up in the fuselage.
The frequencies of these vibrations are, in general, below 600 Hz and they decrease with increasing distance from the source. The usual treatment of such engine-ex- cited vibrations was to stiffen the forepart of the fuselage (on single-engine aircraft) or the region of the plane of the propeller (on multi-engine aircraft).
Modern treatment for the engine-excited vibrations, however, is to suspend aircraft engines on rubber-in-shear mounts so as to decouple the six modes of translational and rotational vibrations, with a natural frequency for the suspension of less than 50 Hz. For all vibrations above this natural frequency, attenuation increases by 6 dB per octave.
6. Jet-Prop Aircraft:
In this type of aircraft, the propeller is driven by a turbojet instead of an internal combustion engine. When this is the case, the jet exhaust contributes only about 10% of the total thrust and is thus working under comparatively low power level conditions.
As a result, jet noise is usually far less significant than propeller noise. In spite of this, it may appreciably modify the overall noise spectrum by “filling in” between any discrete frequencies which may be present.
Jet-prop aircraft is appreciably quiter than the combination of a propeller and a piston engine for equal thrust. This difference in the noise levels of two types of aircraft is of the order of 10 db. Under particular conditions, and particularly at low thrust levels, compressor noise may predominate.
It is easily identified as a high pitched whine due to fan noise radiated from the engine intake. This noise is most noticeable to an observer positioned in front of the engine. To an observer on the ground, therefore, compressor whine is a noticeable characteristic of a jet-prop aircraft approaching to land.
7. Sonic Boom:
The phenomenon of sonic boom occurs due to the shock waves produced in supersonic flight, i.e., flight of aircraft at speeds larger than the speed of sound in the air. In supersonic flight, the shock waves generated in the near field to the aircraft are of a complex, multi-peaked pattern.
These develop into a more clearly defined double-peak pattern in the intermediate field and finally into the well-known “N-wave” pattern in the far field.
The N-wave shock pattern with its separate bow and tail shocks is the “pressure signature” of the aircraft in supersonic flight. It can be expected to sweep over the ground at the same velocity as the aircraft and is responsible for the sonic boom and associated phenomena.
The separation between the two waves (bow and tail shocks) is determined by the length of the aircraft. For a large aircraft, two separate booms may be heard.
For the smaller aircraft, on the other hand, the two waves may be sufficiently close together for the ear to hear a single boom, although there are still two distinct pressure pulses. Moreover, a larger aircraft will not only extend the time duration of the N-wave but also increase the over-pressures.
Usually the duration of the ground pressure wave for a small fighter aircraft is of the order of 0.1 seconds. The same for a supersonic bomber may be around 0.3 seconds. The SST (supersonic transport) aircraft, on the other hand, may produce a sound pressure wave of 0.3 to 1.0 second duration, depending on their length.
Besides length, the duration of the pressure wave is also modified by the altitude of the flight path of the aircraft relative to the ground. Although the characteristic N-wave still reaches the ground, any departure from the horizontal flight will shorten the distance between the low and tail pressure peaks and, consequently, the duration of the pressure pulse.
Instead of the idealized linear form given in Fig. 9, both bow and tail shocks diverge with distance. As a result, the far field “pressure signature” may depart to some extent from the standard N-wave configuration.
8. Airport Noise:
Noise nuisance near an airport is usually characterized by the “Noise and Number Index” (NNI). Contours based on the NNI are now generally accepted as providing a convenient and relatively straightforward method of assessing the annoyance caused by aircraft operating in the vicinity of an airport.
The Noise and Number Index (NNI) of a point on the ground is defined by
NNI= Lap + 15 log 10 N-80
Where Lap= average peak noise level of aircraft,….(5)
N=number of flights per day.
The derivation of this index makes it necessary to perform the computation for an average 12-hour day in the busy period of the year.
The pattern of noise exposure of the area around an airport can be obtained, therefore, if the following information is available:
(1) The flight paths of arriving and departing aircraft,
(2) The number and types of aircraft on those flight paths (for the prescribed period of calculation) and
(3) The noise level emitted by those particular types of aircraft.
9. Importance of Noise and Number Index (NNI):
The NNI is particularly valuable for assessing the potential noise nuisance likely to result from future developments and extensions of the existing airports or from the construction of new airports. When the NNI is used for this purpose, the effects of changes in various parameters can be assessed simply by changing the inputs to the calculation.
NNI, is therefore, a very good diagnostic, as well as predictive, method of noise assessment.
Subjective reaction to the NNI, as determined by objective measurement, can be summarized as follows:
(a) In areas of NNI greater than 35, aircraft noise begins to become a significant reason for discontent with living conditions. In areas where NNI is greater than 55, aircraft noise can be considered as intolerable.
(b) In areas between 40-60 NNI, no major residential development should be allowed, but in fill development may be allowed subject to adequate sound-proofing being incorporated into the design of the dwellings.
(c) In areas of greater than 60 NNI, no major residential development should be allowed.
This grading of airport noise nuisance against NNI should be considered carefully in the context of planning for residential development. It offers guidance to a planning authority when considering applications for such development.
10. Equivalent Continuous Sound Level of Aircraft Noise:
The measurement of equivalent continuous sound level (denoted by Leq) takes into account the actual duration as well as the number of individual- occurrences of aircraft noise. This important parameter was used by the (former) Federal Republic of Germany for the measurement of aircraft noise.
For this purpose, Leq values are plotted in contour form. An Leq of 75 dB(A) defines the boundary of “Protection Zone 1”, while the area between this and an Leq of 67 dB (A) is known as “Protection Zone 2”. Inside the Leq 75 limit, noise abatement measures (soundproofing, etc.) are considered necessary and the construction of residential buildings is prohibited. Outside the Leq 67 contour, on the other hand, it is assumed that the noise would not disturb the general population.
Individual aircraft, in general, have different effects as sources of noise when:
(a) Taking off,
(b) Landing, and
(c) Running up engines on the ground.
The last of these can also be considered as an “airport noise” since the noise source is stationary and thus remains at a constant distance from any individual observer at a fixed point.
Noise produced by the first two conditions (Taking off and Landing), on the other hand, can readily be expressed by any of the following:
(a) A single-line curve representing overall sound pressure level at various distances from the source.
(b) Spot values of overall sound pressure at a particular distance from the takeoff point, in line with the runway or otherwise co-ordinated.
(c) Sound spectra determined in octave bands at specific points related to landing and/or take off paths.
(d) Spot values of overall sound pressure or octave band analysis during fly-over at specific heights.
(e) Overall sound pressure contours establishing a complete fly-over pattern from the point of takeoff.
(f) Overall “sound rating” contours taking into account the number of aircraft operating during a specific period, e.g., as expressed by NNI or Leq.
Of these six methods of expressing the taking off and landing noise of individual aircraft, only second and fourth can be readily undertaken by individual measurement with simple instrumentation.
An idea of the noise levels produced by various types of aircraft can be obtained from Table 1. This table shows the average and maximum noise levels obtained by independent measurements of aircraft noise at London’s Heathrow airport.
11. Ground Running Aircraft Noise:
Ground running is an essential feature of airport operation. This is especially true for airports where maintenance is carried out. To some extent, the introduction of jet aircraft has eliminated much of the noise associated with ground running prior to take off.
From the view-point of noise nuisance, a nice feature of jet aircraft is that they do not need lengthy running up period necessary for aircraft with piston engines for the cylinders to reach their correct operating temperature.
But jet aircraft do have to be ground run to test them after maintenance work. When this happens, they are much noisier than piston-engined aircraft. Ground running of jet engines requires an attenuation of the order of 20-30 dB to achieve satisfactory noise suppression.
The arrangement for this purpose (noise suppression) should ideally consist of an exhaust muffler and an intake silencer. Both of these should be fitted with an acoustically tight seal tailored to fit the concerned aircraft.
When this is not possible, the other alternative is to do the ground running in a specially designed test cell (for individual engines removed from the aircraft) or in an acoustic run-up hanger (for complete aircraft). Such systems, however, involve highly elaborate constructions and considerable cost. The employment of such systems, therefore, is largely confined to major aircraft maintenance bases and development areas.