Consequently, they must be equipped with soundproofing devices being efficient and durable to withstand the high temperature and the aggressiveness of the burnt gases to limit their impact on the sound environment. Sur Noogle. Protection of workers against noise. Preservation of acoustic environment. Limitation of noise emissions in energy sector. Testing rooms acoustics. Acoustic comfort in buildings. Noise reduction device for heat engines exhaust. The motion can be very complex—for example, in the case of a panel on a machine.
Consequently, the coupling between the moving structure and air, required to generate noise, depends on details of the motion as well as frequency.
Generally, low-frequency vibration is less efficient in generating sound than high-frequency vibration. Sound reaches the ear by propagation from the source to the receiver and can be complicated by reflections from nearby surfaces as well as atmospheric conditions outdoors.
Some motion is unavoidable; for example, fan blades must move and vehicle tires must rotate. In many cases the function of the machine is unrelated to the noise generated. An example is the mechanical suspensions that attach airplane engines to an airplane but also allow the transmission of vibrations to the fuselage. This transmission into the fuselage and subsequent radiation of sound can be and is minimized by good design—which can also save money by reducing wear and fatigue.
This chapter is concerned with new technologies in materials and systems to reduce noise, the modeling and analytical tools used to design products for reduced noise, and experimental methods of gathering and interpreting data to test and determine how much noise is generated by different product designs. It will be immediately obvious that there are enormous disparities among programs, facilities, and resources for addressing noises of different types.
For example, although engineering tools may be available for reducing aircraft noise and highway noise, the former has been deemed a national priority, while the latter has received less attention. Resources allocated for noise reduction are not always commensurate with noise exposures and impacts. Many tools for designing and developing quieter products have become available in the past few decades, driven largely by increases in computational power and reductions in computational costs.
Even so, access to new tools is as uneven as the allocation of resources; corporate budgets for capital equipment are generally tight and there is competition between departments for available funds. Furthermore, organizations that are doing only routine testing of products according to national and international standards find expensive new tools hard to justify.
Thus, even though noise mechanisms in aircraft, automobiles, rapid transit and trains, consumer products, and industrial machinery are fundamentally similar, the availability and application of tools for addressing them are not.
The question is whether ways can be found to give industry and academia access to these tools for the benefit of manufacturers, workers, and the public. Noise from aircraft includes both noise from airplanes and noise from helicopters.
At commercial airports, airplanes are the major noise source and will be emphasized here because of the widespread annoyance issues that have affected the quality of life for many persons. Noise from helicopters is also an important issue and affects people living near heliports and in densely populated areas where helicopter flights are not uncommon.
One important issue relates to noise metrics; the impulsive character of the noise requires that metrics in addition to the widely used day-night average sound level DNL be used to assess its effects on people. The noise heard when an airplane flies overhead comes from many sources, but the main contributors are engine noise and airframe noise. Airframe noise is produced mostly by airflows around lifting and control surfaces, such as flaps and slats, and around landing gears.
The relative contribution of these sources depends on the engine and airframe designs and the operating conditions. For example, during takeoff, when the engines are at full thrust, jet noise is the largest contributor to the noise signature of an aircraft.
At approach, when the engine is throttled back, noise comes more from the airframe. Other sources, such as the fan, are significant contributors during both takeoff and landing. Typical noise sources for a fixed-wing aircraft are shown in Figure The noise received by an observer depends on the sources and propagation effects. The noise sources for a propeller-driven aircraft are shown in Figure The aeroacoustics community has made significant progress over the years in understanding and reducing aircraft noise.
Figure shows comparative contributions from different noise sources for s and s engines. The figure, which originally appeared in Rolls-Royce a , shows that the development of the turbofan engine and reduction in noise from individual engine components resulted in smaller, more evenly matched noise contributions from engine sources SBAC, Over a period of 30 years, these improvements, coupled with advances in aircraft aerodynamics and weight technologies, have reduced aircraft noise by about 20 dB, which corresponds to a reduction in noise annoyance of about 75 percent EU, The new Airbus A, the largest commercial aircraft ever produced average of passengers , has takeoff and approach noise levels comparable to those of heavy road traffic, a lower noise level than in an underground train.
The noise footprint of the A is about half that of older, large commercial aircraft Rolls-Royce, b. Both the United States and Europe have first-class aero-acoustics test facilities. Anechoic flight simulation facilities, the most useful for testing both jet noise and airframe noise, are available on both sides of the Atlantic on a rental basis. Source: Posey Source: Burley Langley researchers have also used the Dutch anechoic wind tunnel to make helicopter noise measurements.
Both the United States and Europe also have access to state-of-the-art flow measurement equipment including particle imaging velocimetry and modern phased microphone array systems. Most of these facilities have been described in great detail by Ahuja The FAA focuses on the impacts of noise on communities, while NASA focuses on noise at its source—namely, aircraft engines and airframes. Source: Rolls-Royce a. However, industry will have to integrate the research results into production-ready aircraft designs.
Most of the goals of AST were met by However, because of an anticipated annual increase of 3 to 8 percent in passenger and cargo operations well into the twenty-first century and the slow introduction of new noise reduction technology into the fleet, the global impact of world aircraft noise is expected to remain essentially constant until or perhaps and thereafter begin to increase. These programs have validated new, advanced noise reduction technologies, including nacelle inlet acoustical treatments and chevrons on engine exhaust ducts.
Source: Herkes Copyright Boeing. All rights reserved. After rigorous testing, including measurements taken on the ground, in the passenger cabin, and on the airframe Herkes, , many noise reduction technologies, including nozzle chevrons, spliceless inlet linings, extended lining locations, and redesigned wing anti-icing systems see Figure , as well as smooth fairings to reduce landing gear noise see Figure , have been incorporated into existing airplanes and designs for future Boeing airplanes.
A third phase, QTD3, is in the planning stages at Boeing. Over the years the FAA has defined requirements for the reduction of aircraft noise emissions in terms of stages 1—4. The metric for describing the noise emissions is the effective perceived noise level in decibels EPNdB , and well-defined microphone positions are used for the measurement.
Note that this is quite different from the immission metric DNL used to describe the effects of aircraft noise on communities.
In addition to reducing noise, NASA expects dramatic improvements in the emission and performance of these aircraft. Driven by increasingly stringent noise requirements and strong competition from the United States, Europe has set. Source: Collier and Huff For example, as shown in Figure , the Advisory Council for Aeronautical Research in Europe ACARE has set a goal of a 50 percent reduction in noise annoyance relative to their counterparts for aircraft entering into service in This is equivalent to a EPNdB reduction in the day-evening-night averaged sound level from fixed-wing airplanes.
Along with the noise reduction, there must be a 50 percent reduction in specific fuel consumption again relative to engines introduced into service in Source: EU Over the years a number of ambitious programs for reducing aircraft noise and, more recently, reducing aircraft emissions have been launched in Europe. The program addressed both engine noise including jet noise, fan noise, compressor noise see Figure and landing gear noise and airframe noise see Figure Technologies for reducing jet noise included the ultra high bypass ratio fan; low-noise core and fan nozzles designed to improve the mixing of exhaust and bypass flows; internal and external exhaust plugs; and technologies to attenuate fan noise, including a zero-splice passive liner, active noise control technologies, and a negatively scarfed intake design to reflect fan noise away from the ground see Figure In flight tests the negatively scarfed fan was shown to reduce perceived noise by about 2.
Acoustical liners have traditionally been constructed from two or three pieces to facilitate manufacturing and maintenance. The change has resulted in a 4- to 7-dB reduction in fan-tone noise on takeoff and a 2-dB reduction in fan noise on approach Coppinger, An active noise control system was also successfully demonstrated SBAC, The system consisted of microphones mounted in the fan duct and actuators mounted on the stator vanes.
To reduce landing gear noise, some new low-noise designs for the nose and main landing gears were investigated. Ultimately, the noise was reduced by shielding the gears from each other and aligning them in the direction of the flow. Two aligned nose landing gears were demonstrated to be as much as 3 dB quieter than two independent gears Coppinger, SAX culminated in a revolutionary concept design for a very quiet aircraft see Figures and The concept design includes an airframe and engines designed specifically for a steep, low-speed climb and a low-noise approach that reduces both the amount of noise generated and the ground area of noise exposure.
Some of the noise reduction technologies are listed below:. Ahuja, K. Source: LEMA Source: SBAC Source: The Jet Engine , Reprinted with permission from Rolls Royce, Copyright Silent Aircraft Initiative. SAX is predicted to achieve a reduction in noise of 25 dB based on current standards and also a reduction in fuel consumption of about 25 percent for a typical flight.
Although these results are impressive, the SAX is a concept design only. Further work must be done to confirm the feasibility and develop and validate the novel technologies. The partners, major stakeholders in the European aviation industry, include all major engine manufacturers, Airbus, and equipment makers, as well as innovative small businesses, universities, and research centers.
By the end of VITAL, there should be a noise reduction of 8 dB per aircraft operation and an 18 percent reduction in carbon dioxide emissions, compared to engines in service prior to To reduce engine noise, very high bypass ratio engines with novel low-noise, low-speed fan designs are being studied. One of these designs, the contrarotating turbo fan, is shown in Figure VITAL also plans to demonstrate a low-pressure compressor and turbine technologies designed for low noise and weight and compatible with the novel fan designs.
The goal of the Clean Sky Initiative is to create a radically innovative air transport system with a reduced environmental impact based on less noise and gaseous emissions and better fuel economy.
The specific objective is to reduce carbon dioxide emissions by about 40 percent, nitrogen oxide emissions by 60 percent, and noise by 50 percent in time for a major fleet renewal in The approach is to conduct an overall assessment of individual technologies at. The technical and geographical scope of a typical team is shown in Table The program is organized around six technical areas, called integrated technology demonstrators ITDs , that will 1 perform preliminary studies, 2 select research areas, and 3 lead large-scale demonstrations either on the ground or in flight.
In the next 20 years, newly designed aircraft are likely to be introduced at a rapid rate. These aircraft will likely be based on current aircraft but designed to achieve significant reductions in noise and carbon emissions. In the longer term beyond , further reductions in noise and carbon emissions are likely to require the development of entirely new aircraft and engine configurations.
The enabling technologies for both phases of development are becoming apparent. As NASA continues to work toward the introduction of a new generation of highly fuel efficient large aircraft as early as , it is already planning wind tunnel tests of low-noise HWB aircraft Figure shows a typical HWB.
Convinced that the HWB is the only way it can meet its goals, NASA is providing funding for Boeing to study improvements to the configuration to further reduce noise and improve fuel burn. N2A has padded engines mounted above the aft fuselage. N2B has embedded engines and S-duct inlets for lower drag. Both designs incorporate hybrid laminar flow control to further reduce drag. Source: NASA If NASA can meet its targets for the next three generations of aircraft, successively quieter aircraft would enter into service by , —, and —, respectively GAO, The likelihood of meeting these targets depends on a number of factors.
Second, even if funding is available, the development of noise reduction technologies may be limited by concerns about global warming, because advances in noise reduction technologies could make it more difficult to achieve reductions in aircraft emissions of greenhouse gases.
Third, manufacturers must be willing to integrate newly developed technologies into aircraft and engine designs.
Finally, airlines must purchase new aircraft or retrofit existing aircraft with the new technologies in sufficient numbers to achieve targeted reductions in exposure to aviation noise. Although the United States recognizes that its national well-being depends on a national transportation system with a strong aviation element, there is no explicit goal to ensure the primacy of the U.
The European Aeronautics vision highlights two areas not emphasized in any U. The European Aeronautics document foresees the future in the following way:. The public sector plays an invaluable role in this success story. Crucially, they are coordinating a highly effective European framework for research cooperation, while funding programs that put the industry on more equal terms with its main rivals.
Limiting—on a yearly basis—the cumulative noise footprint in areas surrounding airports will effectively limit the capacity of the national aerospace system. Present departure and arrival procedures, which were developed when a limited range of navigational aids was available, are far from optimal from an environmental point of view. Significant investment is being made on both sides of the Atlantic to meet the demands of the market as well as the needs of the community.
The major challenge in the development of noise reduction technology for the future is that the design requirements. However, other design requirements are in direct conflict with each other, forcing engine and aircraft designers to make difficult compromises. One example of this is the requirement to increase the bypass ratio of the engine beyond the optimum to reduce fuel burn and reducing the fan speed to achieve a reduction in noise particularly at takeoff , but at the cost of increasing fuel burn and chemical emissions at cruise speed.
Even then it will take many years for current airplanes to be phased out and for the full benefits of quiet aircraft to be realized. The impact of increased number of airports and aircraft in service is likely to exceed the mitigating effects of near-term technological advances and operational improvements, and the number of people exposed to aircraft noise is likely to increase.
In addition, the sensitivity of people to noise, or at least vocal objections to it, which often depends on attitudes and socioeconomic conditions, may also increase as people become more affluent. The implication for the aviation noise research community and government agencies that must support this community is that they cannot afford to be complacent. Recommendation The National Aeronautics and Space Administration NASA should continue to fund collaborative projects by engine, airframe, and aircraft systems manufacturers.
Drawing on expert knowledge in research organizations and academic institutions, research should focus on the complex interrelationships between engine and airframe and the importance of reducing each constituent noise source to reduce the overall noise signature of aircraft. These projects should develop improved prediction tools, for example, for advanced propulsion designs; acoustic scattering and propagation models, including adequate weather and terrain models; models of the effects of interactions between engine installation and airframe configuration; and benchmark measurements necessary for the development and validation of these advanced tools.
Recommendation The Federal Aviation Administration should continue to fund the development of novel operational and air traffic management procedures to minimize noise and should work with NASA and industry to make intelligent trade-offs between competing noise mitigation and chemical pollution goals.
Noise from motor vehicles is undoubtedly the most pervasive noise in our society Bowlby, ; Sandberg, Individually, passenger cars, trucks, buses, and motorcycles emit relatively low levels of noise compared with aircraft and rail transit at equivalent distances. However, the sheer number of these vehicles in close proximity to sensitive receptors more than offsets their lower noise levels Donavan and Schumacher, Based on figures from the Environmental Protection Agency EPA , approximately four times as many people are exposed to highway noise with DNL values of more than 65 dB than are exposed to aircraft noise and almost eight times as many are exposed to highway noise than are exposed to rail noise Waitz et al.
Road traffic noise is the result of a combination of noise from several different vehicle types, each of which has its own characteristics. These include light vehicles passenger cars, pickup trucks, sport utility vehicles, and passenger-size vans , medium trucks, heavy trucks, buses, and motorcycles. Of these, light vehicles and trucks tend to dominate traffic noise. To support the modeling of traffic noise, an extensive database of vehicle pass-by noise emissions was collected in the mids by the Federal Highway Administration FHWA characterizing each vehicle type as a function of speed see Figure These data reveal a number of attributes.
At speeds of more than 20 to 30 kilometers per hour, noise emissions from trucks and light vehicles increase rapidly with speed. At highway speeds, 80 kilometers per hour and above, noise from heavy trucks is about 10 dB higher than from light vehicles; medium trucks fall somewhere in between. For this reason, the level of traffic noise is strongly influenced by the mix of cars and trucks. Power train noise includes all sources associated with vehicle propulsion and strongly depends on engine speed.
This source tends to dominate the overall noise emission at lower speeds at which speed has little effect on noise levels. At very high speeds, beyond legal speed limits in the United States, aerodynamic noise caused by flow over and under the vehicle becomes important. Noise emissions from this source are typically proportional to 60 times the logarithm of vehicle speed.
Noise levels can also be visualized using acoustic beam-forming technology, as shown for a light vehicle and heavy truck cruising at about 88 kilometers per hour in Figure Power train noise, the dominant noise source at low vehicle speeds, will be greatly reduced as new hybrid and plug-in hybrid vehicles are introduced into the fleet. This is an example of noise being reduced not by noise control engineers but by the introduction of a new technology by manufacturers.
Urban dwellers will benefit from reduced noise levels as these vehicles are introduced, but new problems are created. The sound of a vehicle in some cases serves as a warning signal, especially to children and sight-impaired persons, and consideration is being given to adding sound when vehicles are very quiet. This, however, creates opportunities for engineers interested in the product sound quality issues discussed later in this chapter.
To address noise from motor vehicles, EPA has set a noise emission limit of 80 dB for new heavy and medium trucks, buses, and motorcycles. Although there is no federal limit for light vehicles, a sufficient number of state and local jurisdictions require that emissions be limited to a level of 80 dB, effectively making this a de facto national limit. The test procedures used to obtain these levels, which are conducted at low speed and under full-throttle acceleration, essentially deal with power train noise.
Under these conditions, typically 40 percent or less of total noise emissions for light vehicles is due to tire noise Donavan et al.
As a result, these limits do little to address traffic noise under highway conditions Sandberg, In Europe a limit on tire noise has been established for moderate speeds; however, there is no equivalent regulation in the United States. Because there are no pertinent source emission regulations, road traffic noise is abated in the United States al-.
Source: Fleming et al. Sources: Adapted from Donavan ; Donavan et al. All but six states have used this method of noise abatement to some degree Polcak, For new highway projects or when the capacity of an existing highway is planned, and if federal funds are being used, federal policy allows only five types of noise abatement to be considered: traffic management, such as speed limits or vehicle restrictions; alterations of the highway alignment away from sensitive receptors; barriers in the form of sound walls or earthen berms; the creation of buffer zones along the highway; and sound insulation for some public buildings.
For practical reasons, barriers are almost always selected. Reasonable has several dimensions, one of which is the cost of the barrier compared to the benefit received by the impacted residences. To implement the federal policy, each state develops its own policies and guidelines to define other parameters, such as the level at which noise abatement will be considered and the value of each impacted residence.
If the state determines that a barrier or other form of noise mitigation is not feasible or reasonable, no abatement measures are required. Federal policy explicitly forbids the selection of pavement type for noise abatement, largely because of concerns that it will not maintain a given level of noise reduction performance over the life of the highway project. Notwithstanding federal policy, because of the initial cost of highway barriers see Chapter 7 and because they are not always feasible, both state and federal governments have an interest in investigating other possibilities for reducing road traffic noise.
FIGURE Acoustic images of typical noise source regions for light vehicles and heavy trucks obtained with acoustic beaming. Source: Adapted from Donavan and Rymer If all-terrain tires, such as those that might be used on four-wheel-drive vehicles, are included, the noise range increases by about 1 dB. Even comparing typical treaded tires to blank-tread tires produces a range of 3 dB or less, depending on the pavement. However, pass-by noise levels at 97 kilometers per hour can easily demonstrate a dB range see Figure on different pavements.
In the longest-running program, by Caltrans in California, 9 kilometers of older dense-grade asphalt concrete DGAC was overlaid with 25 millimeters of new open-grade asphalt concrete OGAC on a six-lane portion of I near Davis, California.
Initially, this produced a reduction of about 6. After 10 years, the performance. FIGURE Range in one-third octave band sound intensity levels for tires measured at 97 kilometers per hour on a dense, graded, asphalt-concrete roadway.
Source: Adapted from Donavan FIGURE One-third octave band pass-by noise levels for the same car and tires operating on different pavements at 97 kilometers per hour. In other projects, Caltrans has documented reductions of 3 to 6. As measured at five wayside noise measurement research sites, this overlay reduced traffic noise levels by 6 to 12 dB measured 15 meters from the freeway.
Using TNM predictions, this reduction was equivalent to a barrier height of 4 meters placed alongside the roadway. Two types of rakelike tining textures applied perpendicular transversely to the direction of travel were compared to tining applied longitudinally to with the direction of travel. The longitudinal tining also produced a level 5 dB lower than uniformly spaced transverse tining, which was the standard texture used by ADOT up to that time.
Following the lead of California, Arizona—and several other states—have now adopted longitudinal tining as their design standard. In an extreme example, a reduction of 10 dB was documented when Caltrans ground away an aggressively transverse bridge deck over the Sacramento River Donavan, In less extreme cases and depending on the texture of the existing PCC, reductions of 2 to 9 dB have been reported Donavan, ; Herman and Withers, The examples of quieter pavements described above are primarily quieter versions of existing pavement designs that reduce road traffic noise.
From research in Europe and more recently in the United States Rasmussen et al. For asphalt concrete AC , texture is largely determined by aggregate size; smaller sizes produce lower noise levels. Another factor is whether the texture is negative embedded in the surface or positive protruding from the surface ; negative texture is quieter. For PCC, texture is more consciously designed on the surface by tining or dragging a material over the surface before it cures. As described above, the direction of tining is important; surfaces tined longitudinally are usually quieter than those tined transversely.
Even in the longitudinal direction, however, tining can introduce some unwanted texture as the material is dug out of the surface. Surfaces that are ground or textured by dragging burlap over the uncured surface typically produce less textured surfaces and less resultant noise.
Porosity, typically associated with AC pavements, is dictated by the percentage of void area in the pavement. To be most effective in reducing noise, voids must be interconnected so that the structure provides some degree of acoustic absorption.
OGAC mix designs generally achieve higher pavement void ratios. However, OGAC constructions do not always ensure a porous pavement. Other AC pavement designs, such as DGAC and stone matrix asphalt, typically have very low void ratios and hence are only quieter if smaller aggregate sizes are used Donavan, There are also porous PCC pavements, although these are very rarely used for highways. As can be inferred from the discussion above, the quietest AC pavements measured to date fall into two categories: 1 highly porous, small aggregate OGAC pavements and 2 very fine textured, small aggregate DGAC pavements.
The latter is rarely used in highway construction, and the former is not regularly achieved in practice.
In fact, high porosity is often achieved with larger aggregate OGAC designs that tend to actually increase texture-generated noise. In Europe there are examples of double-layer porous pavements Figure , with a large aggregate lower layer and a small aggregate upper layer 2 millimeters layer over a 6 millimeters layer.
These pavements have produced the lowest noise levels measured in either the United States or Europe to date. As a next step, research is being conducted in Sweden and Japan with poroelastic surfaces that have air void ratios of 20 to 40 percent and are constructed of resilient materials, such as recycled rubber from used tires.
Source: Donavan Regulations have been promulgated to control the tire contribution, but these have been largely ineffective. Next the manufacturer considers rolling resistance and durability. The demand for quieter tires comes primarily from vehicle manufacturers. For passenger vehicles in particular, interior noise from tires is a development issue as important as other noises, such as power train and wind noise. Vehicle manufacturers work closely with tire manufacturers to tune and develop original equipment tires to meet specific targets.
However, this does not necessarily result in lower levels of exterior noise, because interior noise can often be controlled by structure-borne noise paths into the vehicle. On occasion, exterior tire noise becomes an issue for meeting the regulated pass-by noise limits determined under the ISO test procedure.
That program ended in , to be succeeded by the current Quiet Aircraft Technology program, whose near-term goal is to establish a means of reducing community noise by 50 percent by , then by to demonstrate technology to reduce community noise by 75 percent, according to Grady.
Fundamental proof-of-concept work was accomplished when OSU demonstrated that plasma actuators modify turbulence. The next step is to evaluate the concept on a simulated jet exhaust test rig to measure noise reduction benefit, then test on a full-scale engine. All Rights Reserved. Skip to main content. Aviation International News. Air Transport. Few sounds are louder than a jet aircraft at takeoff. Engines Engines. Latest Trending. Nov 12, - AM. Air bp Turns Focus toward Sustainability.
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