Noise control
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 material science applications of constructing sound baffles or using sound-absorbing liners for interior spaces. Industrial noise control is a subset of interior architectural control of noise, with emphasis on 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.
Standards, recommendations, and guidelines
Organizations each have their own standards, recommendations/guidelines, and directives for what levels of noise workers are permitted to be around before noise controls must be put into place.Occupational Safety and Health Administration (OSHA)
's requirements state that when workers are exposed to noise levels above 90 A-weighted decibels in 8-hour time-weighted averages, administrative controls and/or new engineering controls must be implemented in the workplace. OSHA also requires that impulse noises and impact noises must be controlled to prevent these noises reaching past 140 dB peak sound pressure levels.Mine Safety and Health Organization (MSHA)
requires that administrative and/or engineering controls must be implemented in the workplace when miners are exposed to levels above 90 dBA TWA. If noise levels exceed 115 dBA, miners are required to wear hearing protection. MSHA, therefore, requires that noise levels be reduced below 115 dB TWA. Measuring noise levels for noise control decision making must integrate all noises from 90 dBA to 140 dBA.Federal Railroad Administration (FRA)
The FRA recommends that worker exposure to noise should be reduced when their noise exposure exceeds 90 dBA for an 8-hour TWA. Noise measurements must integrate all noises, including intermittent, continuous, impact, and impulse noises of 80 dBA to 140 dBA.U.S. Department of Defense
The Department of Defense suggests that noise levels be controlled primarily through engineering controls. The DoD requires that all steady-state noises be reduced to levels below 85 dBA and that impulse noises be reduced below 140 dB peak SPL. TWA exposures are not considered for the DoD's requirements.European Parliament and Council Directive
The European Parliament and Council Directive require noise levels to be reduced or eliminated using administrative and engineering controls. This directive requires lower exposure action levels of 80 dBA for 8 hours with 135 dB peak SPL, along with upper exposure action levels of 85 dBA for 8 hours with 137 peak dBSPL. Exposure limits are 87 dBA for 8 hours with peak levels of 140 peak dBSPL.Approaches to noise control
An effective model for noise control is the source, path, and receiver model by Bolt and Ingard. Hazardous noise can be controlled by reducing the noise output at its source, minimizing the noise as it travels along a path to the listener, and providing equipment to the listener or receiver to attenuate the noise.Path
The principle of noise reduction through pathway modifications applies to the alteration of direct and indirect pathways for noise. Noise that travels across reflective surfaces, such as smooth floors, can be hazardous. Pathway alterations include physical materials, such as foam, absorb sound and walls to provide a sound barrier that modifies existing systems that decrease hazardous noise. Sound dampening enclosures for loud equipment and isolation chambers from which workers can remotely control equipment can also be designed. These methods prevent sound from traveling along a path to the worker or other listeners.Receiver
In the industrial or commercial setting, workers must comply with the appropriate Hearing conservation program. Administrative controls, such as the restriction of personnel in noisy areas, prevents unnecessary noise exposure. Personal protective equipment such as foam ear plugs or ear muffs to attenuate sound provide a last line of defense for the listener.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
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 kilometers per hour. Above that range, sound emissions double with every 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 kilometers per hour. Quieter pavements are porous with a negative surface texture and use small to medium-sized aggregates; the loudest pavements have transversely-grooved surfaces, positive surface textures, and 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 applied to existing or planned surface transportation projects. They are one of the most effective actions taken in retrofitting existing roadways 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 airports at which the technology was applied early on 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 the US. Residential land development is discouraged within high impact areas identified by the forecast.