Noise Mitigation

Noise mitigation is a set of strategies to reduce unwanted environmental sound. The main topics of noise mitigation (alternatively known as '''noise abatement''') are: Transportation noise control, Architectural design, and Occupational Noise control. Roadway Noise and Aircraft Noise are the most pervasive sources of Environmental Noise worldwide, and remarkably little change has been effected in source control in these areas since invention of the original vehicles. The sole exception to have widespread potential impact is development of the Hybrid Vehicle .

A panoply of techniques have been developed to address interior sound levels, many of which are encouraged by local Building Code s; in the best case of project designs, planners are encouraged to work with design engineers to examine tradeoffs of roadway design and architectural design. These techniques include design of exterior walls, party walls and floor/ceiling assemblies; moreover, there are a host of specialized means for dampening reverberation from special purpose rooms such as Auditoria , Concert Hall s, dining areas and meeting rooms. Many of these techniques rely upon materials science applications of constructing Sound Baffle s or using sound absorbing liners for interior spaces. Industrial noise control is really a subset of interior architectural control of noise, with emphasis upon specific methods of sound isolation from industrial machinery and for protection of workers at their task stations.

ROADWAY NOISE MITIGATION

can operate 15 to 25 decibels more quietly than conventional autos at speeds less than 60 km/h]]

Source control in designs for Truck s in the 1970s, better sheilding of diesel stacks in the 1980s, and local vehicle regulation of unmuffled vehicles.

The most fertile area for roadway noise mitigation is in urban planning decisions, roadway design, Noise Barrier designBenz Kotzen and Colin English, ''Environmental Noise Barrier s: A Guide to Their Visual and Acoustic Design'', Spon Press, United Kingdom (1999) ISBN 0419231803, speed control, surface pavement selection and truck restrictions. Speed control is effective since the lowest sound emissions arise from vehicles moving smoothly at 30 to 60 kilometres per hour. Above that range sound emissions double with each five miles per hour of speed. At the lowest speeds, braking and (engine) acceleration noise dominates. Selection of surface pavement can make a difference of a factor of two in sound levels, for the speed regime above 30 kilometres per hour. The quietest pavement is concrete without expansion spacers; loudest pavements are chip seal and longitudinally grooved styles. Obviously surface friction and roadway safety are important considerations as well for pavement decisions.

When designing new urban freeways or arterials, there are numerous design decisions regarding alignment and roadway geometricsMyer Kutz, ''Handbook of Transportation Engineering'',. McGrw-Hill (2004) ISBN 0071391223, Use of a MTA line expansions (1970s), San Francisco Bay Area Rapid Transit System expansion (1981), Houston Light Rail system (1982), Portland, Oregon Beaverton light rail line (1983).

Noise barriers can be applicable for existing or planned surface transportation projects. They are probably the single most effective weapon in retrofitting an existing roadway, and commonly can reduce adjacent land use sound levels by ten decibels. A computer model is required to design the barrier since terrain, micro Meteorology and other locale specific factors make the endeavor a very complex undertaking. For example, a roadway in cut or strong prevailing winds can produce a setting where atmospheric sound propagation is unfavorable to any noise barrier.

AIRCRAFT NOISE ABATEMENT

Airbus A321 , on landing approach to London Heathrow Airport , showing proximity to homes.]]

As in the case of roadway noise, surprisingly little progress has been made in source quieting of Aircraft Noise , other than elimination of gratuitously loud engine designs from the 1960s and earlier. Because of its velocity and volume, jet turbine engine exhaust defies any simple means of quieting. The most promising forms of aircraft noise abatement is through land planning, flight operations restrictions and residential Soundproofing . Flight restrictions can take the form of preferred runway use, departure flight path and slope, and time of day restrictions. These tactics are sometimes controversial since they can impact aircraft safety, flying convenience and airline economics.

In 1979 the U.S. Congress authorized'' Aviation Safety And Noise Abatement Act Of 1979 (ASNAA)'', 49 U.S.C. 47501-47510 the FAA to devise technology and programs to attempt to insulate homes near airports. While this obviously does not aid the exterior environment, the program has been effective for residential and school interiors. Some of the first airports at which the technology was applied were San Francisco International Airport
''Final Report for the Aircraft funded and prepared for the city of South San Francisco, Earth Metrics Inc., Burlingame, Calif., July, 1986, Seattle-Tacoma International Airport , John Wayne International Airport and San Jose International Airport ''Pilot Noise Residential Insulation Program'', San Jose International Airport (1983) in California. The underlying technology is a Computer Model which simulates the propagation of of aircraft noise and its penetration into buildings. Variations in aircraft types, flight patterns and local meteorology can be analyzed along with benefits of alternative building Retrofit strategies such as roof upgrading, window Glazing improvement, Fireplace baffling, Caulk ing construction seams and other measures. The computer model allows cost effectiveness evaluations of a host of alternative strategies.

ARCHITECTURAL SOLUTIONS

main article: Architectural Acoustics

and isolation of Plumbing assemblies can reduce interior noise]]

Beyond the interior acoustics cited above under aircraft noise, there has been a steady trend to design quieter buildings with regard to sources within and without the structure itself. In the case of construction of new (or remodeled) (glass thickness, double pane design etc.), roof material, caulking standards, Chimney baffles, exterior door design, mail slots, Attic Ventilation ports and mounting of through the wall air conditioners.

Regarding sound generated inside the building, there are two principal types of transmission. Firstly, airborne sound travels through walls or floor/ceiling assemblies and can emanate from either human activities in adjacent living spaces or from mechanical noise within the building systems. Human activities might include voice, amplified sound systems or animal noise. Mechanical systems are of 50), which allows considerable attenuation of the sound level reaching occupants.

The second type of interior sound is called Impact Insulation Class (IIC) transmission. This effect arises not from airborne transmission, but rather from transmission of sound through the building itself. The most common perception of IIC noise is from footfall of occupants in living spaces above. This type of noise is more difficult to abate, but consideration must be given to isolating the floor assembly above or hanging the lower ceiling on resilient channel.

Both of the above transmission effects may emanate either from building occupants or from building Mechanical System s such as elevators, plumbing systems or heating, ventilating and air conditioning units. In some cases it is merely necessary to specify the best available quieting technology in selecting such building hardware. In other cases shock mounting of systems to control vibration may be in order. In the case of plumbing systems there are specific protocols developed, especially for water supply lines, to create isolation clamping of pipes within building walls. In the case of central air systems, it is important to baffle any ducts that could tranmit sound between different building areas.

Designing special purpose rooms has more exotic challenges, since these rooms may have requirements for unusual features such as Concert performance, Sound Studio recording, lecture halls. In these cases Reverberation and reflection must be analyzed in order to not only quiet the rooms but prevent echo effects from occurring. In these situations special Sound Baffle s and sound absorptive lining materials may be specified to dampen unwanted effects..

INDUSTRIAL NOISE MITIGATION

This situation classically is thought to involve primarily manufacturing settings where industrial machinery produces intense sound levelsRandall F Barron and Barron F Barron, '' Industrial Noise Control and Acoustics'', Marcel Dekker, New York (2002) ISBN 082470701X, not uncommonly in the 75 to 85 decibel range. While this circumstance is the most dramatic, there are many other office type environments where sound levels may lie in the range of 70 to 75 decibels, entirely comprised of office equipment, music, public address systems, and even exterior noise intrusion. The latter environments can also produce Noise Health Effects provided that exposures are long term.

In the case of industrial equipment, the most common techniques for noise protection of workers consist of shock mounting source equipment, creation of plexiglass or other solid barriers, and provision of ear protection equipment. In certain cases the machinery itself can be re-designed to operate in a manner less prone to produce grating, grinding, frictional or other motions that induce sound emissions.

In the case of more conventional office environments, the techniques in architectural acoustics discussed above may apply. Other solutions may involve researching the quietest models of office equipment, particularly printers and photocopy machines. One source of annoying, if not loud, sound level emissions are certain types of lighting fixtures (notably older fluorescent globes). These fixtures can be retrofitted or analyzed to see whether Over-illumination is present, a common office environment issue. If over-illumination is occurring, de-lamping or reduced light bank usage may apply.
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Noise Mitigation