Noise control or noise mitigation is a set of strategies to reduce noise
pollution or to reduce the impact of that noise, whether outdoors or indoors.
Overview The main areas of noise mitigation or
abatement are: transportation noise control, architectural design, urban
planning through zoning codes, and occupational noise control. Roadway
noise and aircraft noise are the most pervasive sources of environmental
noise. Social activities may generate noise levels that consistently affect
the health of populations residing in or occupying areas, both indoor and
outdoor, near entertainment venues that feature amplified sounds and music that
present significant challenges for effective noise mitigation strategies.
Multiple techniques have been developed to address interior sound levels, many
of which are encouraged by local building codes; in the best case of
project designs, planners are encouraged to work with design engineers to examine
trade-offs of roadway design and architectural design. These techniques
include design of exterior walls, party walls, and floor and ceiling assemblies;
moreover, there are a host of specialized means for damping
reverberation from special-purpose rooms such as auditoria, concert halls,
entertainment and social venues, dining areas, audio recording rooms, and
meeting rooms. Many of these techniques rely upon
materials science applications of constructing sound baffles 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.
Sound masking is the active addition of noise to reduce the annoyance of certain
sounds; the opposite of soundproofing. Basic technologies
Sound insulation: prevent the transmission of noise by the
introduction of a mass barrier. Common materials have high-density properties
such as brick, thick glass, concrete, metal etc.
Sound absorption: a porous material which acts as a ‘noise sponge’ by
converting the sound energy into heat within the material. Common sound
absorption materials include decoupled lead-based tiles, open cell foams and
fiberglass Vibration damping: applicable for large
vibrating surfaces. The damping mechanism works by extracting the
vibration energy from the thin sheet and dissipating it as heat. A common
material is sound deadened steel. Vibration isolation: prevents
transmission of vibration energy from a source to a receiver by introducing a
flexible element or a physical break. Common vibration isolators are springs,
rubber mounts, cork etc. Roadways
Source control in roadway noise has provided little reduction in vehicle
noise, except for the development of the hybrid vehicle; nevertheless, hybrid use
will need to attain a market share of roughly fifty percent to have a major
impact on noise source reduction of city streets. Highway noise is today less
affected by motor type, since the effects in higher speed are aerodynamic
and tyre noise related. Other contributions to reduction of noise at
the source are: improved tire tread designs for trucks in the 1970s, better
shielding of diesel stacks in the 1980s, and local vehicle regulation of
unmuffled vehicles. The most fertile areas for roadway noise
mitigation are in urban planning decisions, roadway design, noise barrier
design, 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 acceleration noise dominates.
Selection of road surface pavement can make a difference of a factor of two in
sound levels, for the speed regime above 30 kilometres per hour. Quieter
pavements are porous with a negative surface texture and use medium to small
aggregates; the loudest pavements have a transversely grooved surface, and/or a
positive surface texture and use larger aggregates. 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 geometrics. Use of a computer
model to calculate sound levels has become standard practice since the early
1970s. In this way exposure of sensitive receptors to elevated sound levels can
be minimized. An analogous process exists for urban mass transit systems
and other rail transportation decisions. Early examples of urban rail systems
designed using this technology were: Boston MBTA line expansions, San
Francisco BART system expansion, Houston METRORail system, and the MAX Light Rail
system in Portland, Oregon. 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 up to
ten decibels. A computer model is required to design the barrier since
terrain, micrometeorology 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
As in the case of roadway noise, little progress has been made in quelling
aircraft noise at the source, other than elimination of loud engine designs from
the 1960s and earlier. Because of its velocity and volume, jet turbine engine
exhaust noise defies reduction by any simple means.
The most promising forms of aircraft noise abatement are 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 US Congress authorized 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, Seattle-Tacoma International Airport, John Wayne International
Airport and San Jose International Airport in California.
The underlying technology is a computer model which simulates the propagation 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, caulking construction seams and other
measures. The computer model allows cost effectiveness evaluations of a host of
alternative strategies. In Canada, Transport Canada prepares
noise exposure forecasts for each airport, using a computer model similar
to that used in US residential land development is discouraged within high
impact areas identified by the forecast. In 1998, the flight paths in all of
Scandinavia were changed as the new Oslo-Gardermoen Airport was opened.
These new paths were straighter, reducing fuel use, and disturbing fewer
people, however, vociferous protests came from people near the new paths who
had not been disturbed before, and they took legal action.
Architectural solutions Architectural acoustics noise control
practices include: interior sound reverberation reduction, inter-room
noise transfer mitigation and exterior building skin augmentation.
In the case of construction of new apartments, condominiums, hospitals, and
hotels, many states and cities have stringent building codes with
requirements of acoustical analysis, in order to protect building occupants.
With regard to exterior noise, the codes usually require measurement of the
exterior acoustic environment in order to determine the performance standard
required for exterior building skin design. The architect can work with the
acoustical scientist to arrive at the best cost-effective means of creating a
quiet interior. The most important elements of design of the building skin
are usually: glazing, perforated metal, 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 and
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, noise from amplified sound systems, or animal
noise. Mechanical systems are elevator systems, boilers, refrigeration or air
conditioning systems, generators and trash compactors. Aerodynamic sources
include fans, pneumatics, and combustion. Noise control for
aerodynamic sources include quiet air nozzles, pneumatic silencers and quiet
fan technology. Since many mechanical sounds are inherently loud, the
principal design element is to require the wall or ceiling assembly to meet
certain performance standards,, which allows considerable attenuation of the
sound level reaching occupants. The second type of interior sound is
called Impact Insulation Class 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. Low frequency noise is transferred easily
through the ground and buildings. 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 transmission effects noted above may emanate either from building
occupants or from building mechanical systems 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 transmit 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 to prevent echo effects from occurring. In these
situations special sound baffles and sound absorptive lining materials may be
specified to dampen unwanted effects. = Materials=
Acoustical wall and ceiling panels can be constructed of many different
materials and finishes. The ideal acoustical panels are those without a
face or finish material that interferes with the acoustical infill or substrate.
Fabric covered panels are one way to maximize the acoustical absorption. The
finish material is used to cover over the acoustical substrate. Mineral fiber
board, or Micore, is a commonly used acoustical substrate. Finish materials
often consist of fabric, wood or metal. Fabric can be wrapped around substrates
to create what is referred to as a "pre-fabricated panel" if laid onto a
wall, and require no modifications. Such fabrics are generally acoustically
'transparent, meaning that they do not impede a sound wave.
Prefabricated panels are limited to the size of the subas "on-site acoustical
wall panels" This is constructed by "framing" the perimeter track into
shape, infilling the acoustical substrate and then stretching and
tucking the fabric into the perimeter frame system. On-site wall panels can be
constructed to work around door frames, baseboard, or any other intrusion. Large
panels can be created on walls and ceilings with this method.
Double-glazed and thicker windows can also prevent sound transmission from the
outdoors. Industrial
This situation classically is thought to involve primarily manufacturing settings
where industrial machinery produces intense sound levels, 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 composed 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
acrylic glass 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 recent years, Buy Quiet
programs and initiatives have arisen in an effort to combat occupational noise
exposures. These programs promote the purchase of quieter tools and equipment
and encourage manufacturers to design quieter equipment.
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. Impact printers and other equipment were often fitted
with "acoustic hoods", enclosures to reduce emitted noise. One source of
annoying, if not loud, sound level emissions are lighting fixtures. 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.
Commercial Reductions in cost of technology have
allowed noise control technology to be used not only in performance facilities
and recording studios, but also in noise-sensitive small businesses such as
restaurants. Acoustically absorbent materials such as fiberglass duct liner,
wood fiber panels and recycled denim jeans serve as artwork-bearing canvasses
in environments in which aesthetics are important.
Using a combination of sound absorption materials, arrays of microphones and
speakers, and a digital processor, a restaurant operator can use a tablet
computer to selectively control noise levels at different places in the
restaurant: the microphone arrays pick up sound and send it to the digital
processor, which controls the speakers to output sound signals on command.
Urban planning Communities may use zoning codes to
isolate noisy urban activities from areas that should be protected from such
unhealthy exposures and to establish noise standards in areas that may not be
conducive to such isolation strategies. Mixed use areas present especially
difficult conflicts that require special attention to the need to protect people
from the harmful effects of noise pollution. Noise is generally one
consideration in an environmental impact statement, if applicable.
See also Acoustic quieting
Active noise control Buy Quiet
Noise reduction coefficient Noise regulation
Noise, vibration, and harshness Vibration isolation
General: Noise pollution
Health effects from noise References
External links Addressing Wind Turbine Noise
Acoustical Society of America American Institute of Architects
National Council of Acoustical Consultants
Business & Institutional Furniture Manufacturer's Association
Cambridge Sound Management 'City of Melbourne' Citysounds guide to
urban residential design Noise Abatement Society
Noise control case studies Industrial noise control case studies
Multiple sound isolation research articles for beginners
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