Everything you need to learn about controlling aircraft noise.
Contents:
- Introduction to Aircraft Noise
- Measurement of Aircraft Noise
- Insulation from Aircraft Noise
- Routing, Take-Off and Loading Restrictions
- Noise Control as a System Concept
- Noise Control at Source
- What to do about Aircraft Noise?
- Final Remarks on the Methods of Reducing Noise from Aircraft
ADVERTISEMENTS:
Introduction to Aircraft Noise:
Aircraft have been probably the most dramatic of the man-made noise sources which are heard by the general community, especially in the vicinity of an airport. Although the number and power of aircraft increased gradually in the 1940s and 1950s it was not unit the introduction of jet- powered engines that their potential noise nuisance became widely recognised. In many advanced countries the major objective surveys of air-craft noise levels around airports and companion social surveys of the effects of noise on nearby residents have been undertaken.
There are many thousand people living around. Airports, who are moderately or seriously annoyed by airport noise. No estimate is available of the numbers of people affected by aircraft noise around major and minor airports and airforce bases. In most surveys of noise annoyance, aircraft noise runs a close second to traffic noise in terms of the number of people affected and the extent to which they are annoyed.
As the purpose of an aircraft engine has been to take in air and then generate a thrust by throwing out air behind it, and as noise is generated by the interaction between an ejected airstream and the surrounding atmosphere, it is not surprising that an aircraft engine is noisy.
However, as the mechanical energy which appears as sound has been only a very small fraction of the total energy produced by the engine, it is very difficult to quieten. Despite this, most of the major airport authorities and the airlines take the problem of aircraft noise very seriously.
ADVERTISEMENTS:
Their interest gets accelerated since noise certification in advanced countries whereby every new aircraft of an existing type manufactured after 1976 and every aircraft of a completely new type manufactured after 1969, must satisfy the requirements of a quality control test with regard to noise emission.
In order to begin to discuss how to keep out aircraft noise from a home, or how to influence the take-off procedure and routing of aircraft so as to alter the noise nuisance or similar problems, it is necessary to look at the units which measure aircraft noise. Subsequently, methods of control of aircraft noise including insulation, aircraft and airport operating procedures, planning controls and aircraft design have been examined.
This source of noise pollution is increasing steadily during recent years and, especially close to international airports already constitutes a very serious problem. This problem has mainly arisen because of the widespread use of heavy long-range jet aircraft. Noise made by jet planes has been intrinsically more disturbing than that of propeller- driven aircraft because it is of far higher pitch.
Jet noise has been caused by the violent mixing of the jet of gases from the engine with the surrounding air; it is at a maximum during take-off when the engine must deliver maximum thrust, and falls away rapidly as the aircraft climbs.
ADVERTISEMENTS:
During landing, the main source of high-frequency noise has been the whine of the air compressor and turbine blades as the engine is throttled back. Aircraft pass close to the ground for quite a distance during the landing operation and this noise often constitutes a more sustained environmental nuisance than the intense noise of shorter duration produced during takeoff.
Military aircraft often make annoyance in areas away from airfields because they are to be flown at low altitudes as part of normal training procedures. Little can be done about this; national defence—even is peace-time—will always take priority.
The internal wreckage caused by shriek of siren or the roar of a jet engine has been including gastric ulcers and thymus gland atrophy. With the sonic boom (shock waves produced by an object moving faster than the speed of sound) at our threshold, mankind gets threatened with the most awesome noise pollution. According to Prof. Garret J. Hardin, “Sonic boom has been much worse than noise. Experiencing it is like living inside a drum beaten by an idiot at insane intervals”.
As far as relatively slow-flying piston-engine airliners are concerned, most of the noise comes from the engine and the propellers. The maximum sound radiation emanates from the turning circle of the propeller blades.
ADVERTISEMENTS:
In the case of jet-propelled aircraft, the air sucked into the turbojet engine is compressed and then carried into a combustion chamber. Here fuel is injected and ignited. The hot gas-air mixture flows through the turbine, which drives the compressor, and leaves the turbojet through the outlet nozzle at high speed, thus propelling the aircraft.
Behind the nozzle a turbulent zone is formed in which the thrust stream mixes with the air in the environment. It is the fluctuations in pressure caused in this way that produce the characteristic noise of the turbojet aircraft. As the maximum sound projection falls at an angle behind the aircraft, the jet-propelled aircraft emits the greatest sound intensity for those affected after it has passed overhead.
Turboprop aircraft occupy an intermediate position. In this case the turbine drives not only the compressor but also a propeller, which provides the bulk of the thrust, while the thrust element produced by the released gases is relatively low. The sound comes first from the propeller, and second from the compressor and the gases emitted by the turbine.
The highest sound intensity is produced by the aircraft taking off. In the case of a DC8 with a takeoff weight of 85 tons, for instance, a sound level of 85 decibels will be encountered for a distance of 12 km (7.4 miles) from the starting point in the direction of flight. The sound level then continues at 80 decibels for a distance of some 20 km (12.4 mile).
Recordings show that particular localities in the vicinity of airfields face sound levels (on various days in the week and often for several hours at a time) of up to 100 decibels and where there are regular practice flights, especially with military planes of over 110 decibels. At these levels permanent damage can be done to the hearing.
The first step in the battle against aircraft noise is the installation of sound measuring equipment at individual recording points as widely dispersed as possible. With the aid of the measurement data obtained. Zones subject to different levels of noise stress can be mapped out.
By this means it is possible for air-traffic control to lay down departure routes by which only thinly populated areas in the vicinity of the airfield have to be traversed. In the case of landings, however, for technical flying reasons the range of choices of the terrain to be traversed is limited, making it harder to check noise nuisance in this way.
A sustained improvement in the aircraft noise situation can be achieved in the long run only by the adoption of aircraft with quieter power units such as are already in use for some planes on shorter routes.
The measures adopted to combat the effects of aircraft noise on takeoff and landing must be accompanied by efforts to reduce ground noise. This can be done by erecting sound barriers around repair shops and soundproof walls at airports to shield local residents.
Measurement of Aircraft Noise:
The level of noise created by an aircraft flying nearby increases to a peak before dying away again. Generally the length of time it takes to increase to the peak is less than the length of time it takes to die away. The duration and tone corrected form of the PNdB, called the Effective Perceived Noise Decibel (EPNdB), has been used in the noise certification procedure.
The extent to which anybody gets annoyed by aircraft noise will depend upon the activities which are interrupted, upon the time at which the interruption takes place, upon the frequency with which the interruptions occur, and upon his attitude towards the source.
You will probably be as annoyed by one aircraft which awakens you during the night, as you will be by several aircraft flying over during the day when you are listening to the radio or doing some other activity. The Noise and Number Index (NNI) has been a unit which takes into account some of these variables and which is based upon a survey of annoyance with aircraft noise carried out around Airport.
The most important factors in this index have been the average peak level of aircraft noise in PNdB and the number of planes heard to be creating a level of noise above 80 PNdB(N) some point of interest around the airport.
The formula for calculating NNI is:
NNI = Average peak level in PNdB + 15 log10 N – 80
An alternative to NNI which uses EPNdB in place of PNdB is termed as the Noise Exposure Forecast (NEF).
The formula has been slightly different in this case:
NEF = Average peak EPNdB + 10 log10 N – K
where K = 88 for day-time exposure and K = 76 for nighttime.
The annoyance scores were calculated according to the answers to certain key questions; for example, one point was added to the score if the respondent got annoyed while watching television, another point was added if aircraft noise wakened the respondent or made his house shake or vibrate etc.
The categories of annoyance has been self-rated. NNI could be calculated for aircraft movements during the 12 day-time hours between 6 p.m. and 6 p.m, or for the 8 night-time hours between 10 p.m and 6 a.m. The night-time figure has been multiplied by 3/2 to account for the missing 4 hours (6 pm to 10 p.m.).
Form knowledge of all the important parameters like types of aircraft, their noise ‘footprints’, frequency of movement and routing (including runway usage) and local ground characteristics, it has been possible to predict NNI or NEF contours around any airport.
Noise from individual aircraft, in PNdB, could be measured directly by a sound level meter provided with ‘D’ weighting and adding 6 or 7 to the displayed peak level. Alternatively, if the only available weighting is ‘A’ weighting, to the level in dB(A) should be added 12-14 dB to get an approximate level in PNdB depending on the type of aircraft.
Insulation from Aircraft Noise:
To insulate against aircraft noise it becomes possible to use the traditional methods that are effective against traffic noise, such as acoustic double glazing and replacing air bricks with specially constructed mechanical ventilation systems. But this is not enough since aircraft noise will come through the roof. Note that although these specify noise reduction in dB(A), similar figure would apply for reduction in PNdB.
The roof space can get insulated by laying lead sheeting or a dense mineral wool or sand over the ceiling joists of upper rooms, but care must be taken not to overload the structure. It has been also important to reduce noise transmission through chimney flues.
Measurements have revealed that with good double windows and a sound- attenuating ventilator unit, the first-floor rooms of houses with adequate walls can be having good insulation against external noise of 35 to 40 dB and possibly more, without loss of ventilation. Rooms where upon fires are in use appear to present an insuperable problem. Needless to say, all of this is very expensive. In many countries it is possible to obtain part of the cost of insulation against aircraft noise.
Routing, Take-Off and Loading Restrictions:
Routing exerts a considerable influence on the pattern of noise exposure. The question has been whether it is better to spread the noise burden uniformly around the airport by ‘umbrella’ take-off patterns, or to concentrate traffic along routes that avoid the main built up areas.
So far, around airports, the official policy has been to concentrate routing. The resulting air corridors have been termed as Minimum Noise Routes (MNRs) but they attempt to minimize the number of people affected by aircraft noise rather than reducing the sound itself. This policy relies, among other things, on the ability of pilots to observe marker becomes and maintained the defined courses.
Another noise reduction measure that depends upon pilots has been the specified take-off procedure. At Heathrow this requires that the aircraft be throttled back after gaining 300 m height. Aircraft air monitored at two fixed points after take-off. Aircraft that produce noise above an agreed limit are reported to the airlines concerned but little else occurs as a consequence.
The noise limits in relation to large piston-engine aircraft (110 PNdB during the day, 102 PNdB by night) apply the bigger and more powerful aircraft now flying. The implications of takeoff restrictions for some aircraft have been limits on fuel and numbers of passengers.
As a result of a joint government and industry study proposals from the International Air Traffic Association (IATA), the airlines have modified their take-off procedures for application at all major airports. Aircraft now use nearly maximum power for the first 500 m (1500 ft) of their climb.
This phase is followed by power reduction to normal climb and subsequent acceleration and flap retraction. Such a procedure could be compared with that described for Heathrow whereby an earlier and greater reduction in power meant a consequent stronger reapplication of power later on.
This new procedure causes a decrease in noise over considerable areas some distance from the airport, offset to some extent by a slight increase in noise in a smaller area close to the airport. Currently accepted landing routines employ a 2½ degree descent angle. Three degrees is safe for approaches at most airports.
A continuous gradual descent in which undercarriage and flaps have been put down at the last possible moment enables high air-speeds and avoids the necessity for noisy engine power to overcome drag and to alter the slope of descent. This procedure provides distinct benefits under the glide path at large distances from the air between 10 and 15 miles.
However, a continuous descent procedure has disadvantages for people living under the glide path near the airport (within 4 miles) while the aircraft are descending the last 300 m. For these people a two-segment approach has been better, in which the aircraft employ a steeper than- normal glide slope and make a transition to the standard (continuous) approach path in time to stabilise prior to landing.
However, a lightly different angle of approach has to be chosen for each type of aircraft. Furthermore, the angle of slope of the upper segment is to be chosen very carefully so that the benefit gained as a result of the steeper angle is not more than offset by the greater thrust required on the lower segment. Considerable noise can arise when stacking procedures have been in operation. For example, the noise level under a large jet at the minimum height over one of the airport stacking points is 90 PNdB.
Night flying restrictions are very effective because of increased noise sensitivity at night.
Such was the power invoked in the recent Concorde controversy with the consequent legal wrangling concerning the overall relationship of the federal government to state and local government entities.
Noise Control as a System Concept:
Protection of the public health and welfare from aircraft noise has been accomplished most effectively by exercising four noise-control options taken together as a system:
1. Source control, involving the application of basic design principles or special hardware to the engine/ airframe combination, which will minimize the generation and radiation of noise.
2. Path control, involving the application of flight procedures that will minimize the generation and propagation of noise.
3. Receiver control, involving the application of procedures such as restriction on the type and use of aircraft at the airport, which will minimize community noise exposure.
4. Land-use control, involving the development or modification of airport surroundings for maximum noise compatible usage.
In general, the primary approach for noise abatement has been to attempt to control noise at the source to the extent that aircraft would be acceptable for operations at all airports and enroute. And in principle, aircraft noise can get controlled extensively at the source by massive implementation of technology.
In practice, however, the technological capability for complete control without exorbitant penalties is not yet available and may never be. Symbolic representations of massive noise control would be a “cork” in the exhaust nozzle with 100 per cent performance loss or various “porous corks” with lesser performance losses.
Although the noise would be eliminated or substantially abated, such unreasonable treatment would obviously not be tolerated. Insistence that full protection of the public health and welfare be accomplished solely by source control, therefore, could have the effect of preventing the development of most new aircraft and grounding the existing fleet.
The following sources of noise have been identified as potential noise floors that may be relatively near at hand- jet stream, engine, core, and flow-surface interaction.
Noise from the jet-engine exhaust stream mainly results from the mixing of the high-velocity gas discharge with the ambient. The noise source may be usually defined as a volume distribution of convected quadrupoles whose strength has been proportional to the relative jet-stream velocity to the eighth power.
The absolute noise level for any given velocity has been dependent upon various factors such as exhaust-nozzle size and shape and various influences up-stream of the nozzle, such as geometry, roughness and turbulence scale. Current methods of jet-noise reduction have been involving the use of exhaust-noise suppressors, which break up the main jet and, in effect, alter the manner in which it mixes with the ambient air. Such suppressors are most effective at the higher jet velocities, where the noise is greatest, but get accompanied with significant penalties in thrust, drag, fuel consumption, and airplane empty weight.
The most effective procedure to control jet-stream noise without excessive penalties has been to reduce the jet velocity but maintain thrust by increasing the mass flow. The technique used for turbofan engines has been to increase the bypass ratio. Incidentally, high-bypass-ratio turbofan engine, which are efficient for subsonic airplanes, were developed originally for performance, not for noise control.
The noise levels at the lower jet-stream velocities have been higher than would be expected based on jet mixing noise only. There is evidence that other sources of noise are important in this velocity range. For example, sources generated inside the engine, commonly referred to as core engine noise, may dominate.
Core engine noise may be defined as the noise produced by the gas-generator portion of the gas-turbine engine, either solely, or as influenced or amplified by the fan discharge tail pipe, and any other portion of the exhaust system. Core engine noise is regarded to radiate only in the aft-engine quadrant, and its sources are generated upstream of the tail-pipe exist plane.
Core engine noise does not include compressor-generated noise radiating from the engine inlet nor fan-generated noise radiating from either the engine inlet or exhaust ducts. It may, however, include compressor generated noise transmitted downstream through the engine flow passages or fan-generated noise enhanced by interaction with the core engine noise or with the gas stream.
Flow surface interaction noise gets produced by the interaction of flows with solid surfaces of the aircraft, and can result from propulsive and non-propulsive sources. An example of a propulsive source has been a powered-lift aircraft where the interaction of the jet engine exhaust with the wing and flap surfaces can be significant noise sources.
Non-propulsive noise gets produced by aerodynamic boundary layers or the turbulence produced by air passing over and around the airframe and its various components and is commonly referred to as airframe noise.
It appears at this time that airframe noise has been the limiting source in the sense that there is no technology conceivable that will permit noise levels lower than the self- noise generated by a reasonably clean airframe. Although it has been possible that the noise of the jet-exhaust stream or core engine could “bottom out” at levels higher than those for the airframe, the technological capacity for substantially lowering the levels of the jet and core sources appears to be more promising than for the airframe source.
Path control can be an effective option for substantial reduction of aircraft noise. Furthermore, it is having the advantage that the results are additive those obtained by source control. However, specialized flight procedures are limited because of the need to maintain the highest degree of safety.
Therefore, insistence that full protection of the public health and welfare gets accomplished solely by flight procedures is not feasible at this time and probably never will be. Nevertheless, all aircraft can be flown safely in various modes that generate a wide range of noise levels. And, at the least, those safe modes, which will minimize the generation and propagation of noise, should be identified and standardized.
The major problem with aircraft noise in terms of numbers of people exposed, occurs in the vicinity of airports. This problem could be solved to some extent by the application of various operating restrictions at the airport. Extensive use of airport restrictions, however, would be cost- effective only if all feasible source and path control option have been implemented. Otherwise, airport restrictions may give rise unnecessary damage to the local and national economy.
A concept that merits consideration has been that the airport authorities in some cases, and the FAA in other cases, would impose restrictions on the aircraft operators as needed (curfews, quotas, weight and type limitations, preferential runway use, noise abatement takeoff and approach procedures, landing fees, etc.) to ensure that the airport neighborhood communities are noise-compatible consistent with the requirements of health and welfare.
The restrictions available to the airport operator would be those approved by the Environmental Protection Agency. The highest degree of safety must be maintained, and interstate and foreign commerce requirements must be considered.
After all feasible noise-control measures have been applied to the aircraft—by design, treatment, or modification of the source; by flight and air-traffic control procedures; and by design, location, and use of airport—the noise may be regarded still as a problem at some locations.
In this event, noise-compatible land use is probably the only remaining solution. The land-use control option has been more easily exercised in the siting and development of new airports than as a remedial measure for existing noise- impacted communities. For the latter case, the costs of land- use control may be so high that maximum effort must be devoted to implementing the source, path, and receiver control options taken together as a system.
Control of aircraft noise need several changes. Procedures already in use call for prescribed courses and reduction of power at certain altitudes. Other methods being tested include a study of climb-out profiles, glide slopes for landing, and the design and development of quieter engines and airplanes.
A basic problem is that the safest jet approach to a landing is a long, low approach; but this is also the noisiest. One solution is to rezone the area under the landing and takeoff pattern, but this is costly and seldom solves the problem. Relocation of airports is another solution.
Noise Control at Source:
The idea behind the design of an aircraft engine has been to generate a thrust by throwing-out air behind it. In the turbojet the principal cause of noise is the shearing effect in the interaction between the fast moving ejected air and the relatively still air surrounding it.
The noise suppressor, which was an essential part of the hush-kit retro-fitted on jet aircraft of older types so as to meet NC standards, modifies the mixing process by putting a corrugated nozzle on the jet at the exhaust end. Unfortunately, these devices are exerting a significant effect on the efficiency of a straight jet engine and they are not used on such aircraft. Instead main reliance is kept on the other major component of the hush- kit-the acoustic lining.
The basic form of acoustic lining is a sandwich of a honeycomb nature between the exposed perforated surface and the backing skin. Latest test results on a Boeing 707 fitted with such a kit show a considerable reduction of 14 PNdB at the noise certification monitoring points on landing, with a small penalty in lost operating range, because of the extra mass of only 180 nautical miles. The reduction in takeoff noise is more modest. Another technique of retrofitting older types of engines is to fit a quieter front fan.
However, the most significant advance in engine design from the noise point of view is the by-pass principle. The most recent form of by-pass engines are known as turbo fans. This by-pass engine development was based initially on engine efficiency rather than on noise reduction, but one of its spin-offs has been a reduction in noise levels.
The bypass engine is able to divert some of the thrust to provide a cushion of slower-moving gases between the main jet, which may still be suppressed, and the surrounding air. High bypass engines, as the name suggests, divert a larger fraction of the jet stream than the older low-by-pass engines. So, for example, one of the new by-pass engines with approximately twice the thrust of a conventional low by-pass jet engine generates substantially less noise.
Although the noise of individual aircraft measured as sideline, take-off or landing noise might be reduced by the application of hush-kits, and design improvements such as vertical take-off can result in less noise from a given weight of aircraft, the increasing size of aircraft, e.g., jumbo-jets could well cancel out part of the reduction, so the net change in the noise levels from individual aircraft over the next few years might only be a few decibels.
Furthermore, if the number of airline passengers continues to increase there will be little or no shift in the NNI of NEF contours around major airports. Finally it is to be remembered that supersonic transport uses the original noisy type of jet engine in order to produce the greater thrust required. The main hope with such engine designs lies probably in development of co- annular jet with a faster moving air stream surrounding as slower moving core of exhaust.
What to do about Aircraft Noise?:
Once again, the primary advice to those who are sensitive to aircraft noise should stay away or move away from noisy areas. Those who have a natural tolerance or sense of identification with aircraft may live in the affected area.
Having said that however, there will be those who for many other reasons have to live in affected areas, or people who regard that aircraft noise is an imposition that should get reduced so as to allow them to live again in peace.
Again, some may wish to move away but not feel it fair to stand personally a loss in case due to reduced house value because of the noise, or who cannot afford to move due to high mortgage commitments which the reduced house value may not cover. All these people have to combat the noise.
Final Remarks on the Methods of Reducing Noise from Aircraft:
To reduce noise from jet aircraft during take-off, noise suppressors can be fitted—but these also reduce take-off thrust and increase fuel consumption, drag and weight. The additional operating cost caused by fitting noise suppressors to an aircraft of the size of a Boeing 707 is about & 16500 per annum.
Newer types of jet engine, in which the jet velocity is lower, show a reduction of as much as 12 dB against older engine designs, and with these there is little point in fitting noise suppressors. Modern take-off procedures over densely-populated areas are also planned to reduce noise.
Full power has been used to unable the aircraft to climb as rapidly as possible before it reaches a built-up area, power then gets reduced until the built-up area has been cleared. But such procedures mean that aircraft loading has to be reduced, either by taking on less fuel, fewer passengers or less baggage. This, naturally, tends to increase operating costs. So far it has been found impossible to do much about landing noise, although research in this field is proceeding.
There has been increased commercial pressure to extend night operations so as to improve the profitability of aircraft and airports. Although there is mounting public resistance against this trend, the number of night flights from most airports continues to show a steady increase.
It appears to the authors that many airlines and most civil aviation authorities have been doing as much as possible to reduce the noise nuisance from aircraft; further major improvements can only come from the re-siting of airports away from centres of population, preferably so that both take-off and landing flight-paths have been over the sea.
For the present, the only way to avoid the detrimental effect of noise originating at large airports is to provide individual sound-proofing for houses, hospitals and schools in their vicinity. Openable double-glazed windows, complete with a mechanical ventilator and equipped with a noise attenuator, can reduce aircraft noise by about 40 dB, as against a figure of 20 dB with normal single windows.
Conclusion:
The general word recession could mean a decline in aircraft noise. The threat of a fuel crisis in the next ten to twenty years is beginning to make people think of alternatives to flight. People do not usually fly in order to fly; they fly to travel a large distance in a short time.
Advances in telecommunications and increases in the cost of travel may reduce the demand for business travel and advances in railway technology will help to reduce medium- range air travel. The ground effect machine e.g., hovercraft, may reduce the seaport transfer problem (a good reason for flying over water) by permitting direct communication with inland areas.
The technology of aircraft engines has shown a significant improvement in power to noise ratios and the larger aircraft reduce the total number of flights for a given number of passengers. All this leads to a certain optimum about the future of aircraft noise but improvements will not happen by themselves. Continued pressure is required on those who have the power to reduce it.
The Supersonic Transport and Sonic Boom:
Whenever a solid body travels at a speed in excess of the speed of sound a sonic boom gets produced which can be heard up to 80 km from the point of origin. The actual pressure exerted has been found to vary with the distance above the ground. A supersonic passenger aircraft flying at a height of 12,000 m can produce a pressure jump at ground level of up to 100 N/m2 (127 dB) which may not bring about actual damage buildings, but can still be an unpleasant experience for many people.
Supersonic passenger aircraft also bring about other side-effects including the destruction of ozone in the outer atmosphere, with consequent increase of harmful radiation. The construction and sale of supersonic aircraft has been at present a matter of national prestige and economics; but from the environmental standpoint these aircraft are most undesirable.
The supersonic transport (SST) brought noise pollution problems. A supersonic airplane is one that travels faster than sound. When an aircraft travels at a velocity greater than the speed of sound, there is an enormous increase in air resistance. A shock wave is created that can be extremely energetic, depending of the size, speed, and rotate of the aircraft.
When a shock wave strikes things below, it can have a jolting and devastating effect. The shock wave, or sonic boom, may spread over an area of 10 to 80 miles. Although many cases of damage to structures caused by sonic booms from relatively small military aircraft have been reported and numerous damage claims have been filed, the destructiveness and nuisance effects of large commercial supersonic aircraft have not been fully evaluated. Objections to the development and commercialization of an SST are based on the possibility that their shock waves might become intolerable.