Understanding Urban Noise: Definition, Impact, and Mitigation
Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arises when the brain receives and perceives a sound.
Acoustic noise is any sound in the acoustic domain, either deliberate (e.g., music or speech) or unintended. In audio engineering, noise can refer to the unwanted residual electronic noise signal that gives rise to acoustic noise heard as a hiss. Environmental noise is the accumulation of all noise present in a specified environment.
Certain geographic areas or specific occupations may be at a higher risk of being exposed to constantly high levels of noise; regulation may prevent negative health outcomes. Noise regulation includes statutes or guidelines relating to sound transmission established by national, state or provincial and municipal levels of government. Environmental noise is governed by laws and standards which set maximum recommended levels of noise for specific land uses, such as residential areas, areas of outstanding natural beauty, or schools.
Measuring Sound
Sound is measured based on the amplitude and frequency of a sound wave. Amplitude measures how forceful the wave is. The energy in a sound wave is measured in decibels (dB), the measure of loudness, or intensity of a sound; this measurement describes the amplitude of a sound wave. Decibels are expressed in a logarithmic scale.
Because noise is measured using a logarithmic scale, a 3 dB(A) change in noise, a level which is generally noticeable, corresponds to a doubling in the power of the generated noise. The main instrument to measure sounds in the air is the Sound Level Meter. A-weighting is applied to a sound spectrum to represent the sound that humans are capable of hearing at each frequency. Sound pressure is thus expressed in terms of dBA. 0 dBA is the softest level that a person can hear. Normal speaking voices are around 65 dBA.
In audio, recording, and broadcast systems, audio noise refers to the residual low-level sound (four major types: hiss, rumble, crackle, and hum) that is heard in quiet periods of program. In audio engineering it can refer either to the acoustic noise from loudspeakers or to the unwanted residual electronic noise signal that gives rise to acoustic noise heard as hiss.
Measures of noise exposure integrate sound power over a period of time. The instantaneous maximum or peak values Lmax is a measure used for equipment regulation but not typically used as a measure of human exposure.
The following general relationships exist between noise levels and human perception:
- A 1 or 2 decibel increase is not perceptible to the average person.
- A 3 decibel increase is just barely perceptible to the human ear.
- A 5 decibel increase is readily perceptible to the human ear.
- A 10 decibel increase is perceived as a doubling in loudness to the average person.
Factors That Affect Sound Levels: There are many factors that affect sound levels. Sound levels are influenced by the distance and the path traveled between the source of the sound and receptor (person). There is a natural reduction of sound levels with increasing distance between the source and the receptor. The type of ground cover between the source and the receptor also affects sound levels by disrupting or deflecting the sound wave.
Highway-generated sound levels will decrease with the distance from the roadway. This reduction will vary depending upon the ground type (hard or soft). Hard ground (e.g., pavement, dirt, or water) will decrease sound levels by 3 decibels for each doubling of distance. Soft ground (e.g., grass, trees, or vegetation) will decrease sound levels by 4.5 decibels for each doubling of distance. Obstacles between the source and the receptor, such as buildings, hills, and trees, will result in additional noise reductions depending upon their size, density, and location.
Sound can best be expressed on a logarithmic basis. Consequently, sound levels cannot be added by ordinary arithmetic means. Sound levels from two equal sources will result in a total increase of 3 decibels. For example; two cars, each generating 60 decibels of sound, will result in 63 decibels of sound at a receptor, not 120 decibels.
Additionally, highway noise levels constantly vary with the number, type, and speed of the vehicles that produce the noise. When conducting measurements of highway noise, it is necessary to account for these varying noise levels. The most common way to account for the time-varying nature of sound is through a measurement known as Leq. Leq averages background sound levels with short-term transient sound levels to provide a uniform method for comparing sound levels over time.
As characterized in the figures below, noise levels from highway traffic are affected by three factors: (1) the volume of the traffic, (2) the speed of the traffic, and (3) the number of trucks in the flow of traffic. Generally, the loudness of traffic noise is increased by heavier traffic volumes, higher vehicle speeds, and greater numbers of trucks. Vehicle noise is a combination of the noise produced by the engine, exhaust, and tires.
Other conditions will also increase traffic noise levels. Usually, the noisiest hour of the day or night along a highway occurs just before or after the peak hours when the vehicle volumes, speeds, and the truck to auto ratio are in a combined optimum condition to yield the highest hourly noise level. The noisiest hour typically does not occur during the peak traffic hour because the peak hour will have the highest traffic volumes resulting in slower speeds and, therefore, lower highway sound levels. In accordance with FHWA Noise Policy and Guidance, a one-hour Leq is used for assessing highway noise impacts on different land uses.
Table 1 presents typical sound levels at distances measured from the source of the sound.
Cities have many sources of noise; many of the more significant and preventable sources arise from traffic and industry.
Health Impacts of Noise
Chronic exposure to elevated levels of community noise creates significant health risks. These health impacts of noise depend on the intensity of noise, on the duration of exposure, and the context of exposure. Noise is a biological stressor; it triggers autonomic chemical mechanisms for arousal and alertness.
Noise disturbs sleep. Noise interferes with sleep. Noise can make it difficult to fall asleep, and abrupt noises can cause awakenings, which the sleeper may not sense or recall. Even at levels below which awakening may occur, noise produces measurable physiological reactions, such as increase in heart rate and body movements and can cause disturbances of natural sleep patterns by causing shifts from deep to lighter stages. Sleep disturbance can begin at relatively modest noise levels (40 dB). At the level of noise produced by a busy road, the proportion affected by sleep disturbance becomes significant. Average nighttime noise level of 65 dB will result in self-reported disturbance of sleep in about 15% percent of the population.
Exposure to noise is associated with several negative health outcomes. Noise exposure has increasingly been identified as a public health issue, especially in an occupational setting, as demonstrated with the creation of NIOSH's Noise and Hearing Loss Prevention program. Noise has also proven to be an occupational hazard, as it is the most common work-related pollutant. Noise-induced hearing loss, when associated with noise exposure at the workplace is also called occupational hearing loss. While noise-induced hearing loss is permanent, it is also preventable.
Particularly in the workplace, regulations may exist limiting permissible exposure limit to noise. OSHA requires the use of hearing protection. But the HPD (without individual selection, training and fit testing) does not significantly reduce the risk of hearing loss.
Noise Regulations and Standards
Environmental noise is governed by laws and standards which set maximum recommended levels of noise for specific land uses, such as residential areas, areas of outstanding natural beauty, or schools. In 1972, the Noise Control Act was passed to promote a healthy living environment for all Americans, where noise does not pose a threat to human health. The National Institute for Occupational Safety and Health (NIOSH) provides recommendation on noise exposure in the workplace.
In 1972 (revised in 1998), NIOSH published a document outlining recommended standards relating to the occupational exposure to noise, with the purpose of reducing the risk of developing permanent hearing loss related to exposure at work. This publication set the recommended exposure limit (REL) of noise in an occupation setting to 85 dBA for 8 hours using a 3-dB exchange rate (every 3-dB increase in level, duration of exposure should be cut in half, i.e., 88 dBA for 4 hours, 91 dBA for 2 hours, 94 dBA for 1 hour, etc.). However, in 1973 the Occupational Safety and Health Administration (OSHA) maintained the requirement of an 8-hour average of 90 dBA.
At the national level, regulations limit the allowable sound levels produces by aircraft, railway and roadway vehicles and certain heavy equipment. For example, almost all aircraft in current operation must meet standards for runway takeoffs, landings, and sidelines, which depend on the aircraft's weight and its number of engines.
Furthermore, emissions standards do not address the problem of cumulative emissions. Enforcement of all noise laws is variable and often subjective. In addition, this legislation requires EPA to issue noise emission standards for motor vehicles used in interstate commerce (vehicles used to transport commodities across State boundaries) and requires the Federal Motor Carrier Safety Administration to enforce these noise emission standards.
The EPA has established regulations, which set emission level standards for newly manufactured medium and heavy trucks that have a gross vehicle weight rating (GVWR) of more than 10,000 pounds and are capable of operating on a highway or street.
The EPA emission level standards for existing in-use medium and heavy trucks engaged in interstate commerce are shown in Table 3. State or local governments must regulate all other in-use vehicles.
Methods Used to Reduce Highway Noise
There are several methods to reduce highway noise. Thoughtful planning and building rules and their strict enforcement can mitigate many noise conflicts. Zoning codes can limit the incompatible proximal location of residential and commercial uses. Municipalities can purchase quieter public service and transit vehicles (e.g. electric and hybrid buses). Cities can adopt building standards to require quiet interiors (e.g. Subsidize acoustical retrofits (e.g.
Noise-compatible planning encourages State and local governments to use their power to regulate land development in such a way that noise-sensitive land uses are either prohibited from being adjacent to a highway, or that the developments are planned, designed, and constructed in such a way that highway traffic noise impacts are minimized.
To promote noise compatible planning, state transportation agencies coordinate with local officials whose jurisdictions are affected by noise from proposed projects. The following information is typically furnished to the local officials:
- Estimated future noise levels for both developed and undeveloped properties in the immediate vicinity of a proposed project.
- Estimated future noise levels shown at various distances from the roadway to allow the public and local officials to understand where local communities should protect future land development from becoming incompatible with anticipated highway noise levels.
- Eligibility for Federal-aid participation for Type II projects as described in the Type II Projects section of this Web site.
Traffic Management Techniques: Controlling traffic can sometimes reduce highway traffic noise problems. Some possible ways of achieving this is are:
- Prohibiting certain vehicle types (usually trucks) from particular streets and roadways. This could be accomplished by way of traffic control devices and signing. The prohibition of trucks from a major roadway can produce up to an 8 dBA to 10 dBA noise reduction.
- Permitting certain vehicle types (again, typically trucks) to use certain streets and roads only during certain noise-sensitive periods, such as daylight hours.
- Timing traffic lights to achieve smooth traffic flow and to eliminate the need for frequent acceleration and deceleration.
- Reducing speed limits. About a 20 mile-per-hour reduction in speed is necessary for a readily perceptible decrease in noise levels.
- Separating noisier vehicles from other vehicles and placing them farther from the receivers (i.e., exclusive lane designation).
Alteration of a Roadwayâs Horizontal or Vertical Alignment: Alteration of the horizontal or vertical alignment is usually limited to new roadways or major reconstruction of existing roadways. Since noise is reduced from 3 to 4.5 dBA per doubling of distance between the source and receiver, shifting the horizontal alignment away from an affected area often is an effective method to reduce noise. Vertical alignment shifts, such as depressing the highway to form a vertical cut section, can also be quite effective since the top of cut acts as a barrier between the source and receiver. Occasionally, the roadway can be placed so that attenuation is provided by natural barriers, such as hills and wooded areas.
Acquisition of Property or Property Rights to Create Buffer Zones: Buffer zones can be created by purchasing unimproved property or property rights adjacent to an existing or proposed highway so as to preclude future highway traffic noise impacts where development has not yet occurred. An additional benefit of buffer zones is that they often improve the roadside appearance. Buffer zones can only be used in Type I projects.
Creating buffer zones is often not possible because of the large amount of land that must be purchased and because, in many cases, residences already border existing or proposed roadways.
Vegetation Planting: Vegetation, if it is high enough, wide enough, and dense enough that it cannot be seen through, can decrease highway traffic noise. A 200-foot width of dense vegetation can reduce noise by 10 decibels, which cuts in half the loudness of traffic noise. It is usually impossible, however, to plant enough vegetation along a road to achieve such reductions.
The planting of trees and shrubs, however, may provide psychological benefits, if not an actual lessening of highway traffic noise levels, and may be provided for visual, privacy, or aesthetic treatment. Vegetation planting is not a mitigation measure that is eligible for Federal-aid highway funding.
Noise Insulation in Public or Nonprofit Institutional Buildings: FHWA allows federal funds to be used for noise insulation of public use or nonprofit institutional structures, such as churches, schools, hospitals, and libraries. Insulating buildings can greatly reduce interior noise levels, especially when windows are sealed and cracks and other openings are filled. However, insulation can be costly because air conditioning is usually necessary once the windows are sealed. The installation of air conditioning has two benefits. First, windows can be kept shut in warm weather to reduce noise from exterior sources. Second, it creates a steady background noise that masks exterior and interior noises by removing rapid noise level fluctuations. Where possible, using double-paned windows and reducing window area can also be effective.
Noise Barriers: The most common measure to mitigate noise impacts is construction of noise barriers. Noise barriers are solid obstructions built between the highway and noise-sensitive receptors such as homes, schools, or churches. Effective noise barriers reduce noise levels by 7 decibels to 10 decibels, which reduces the perceived loudness of traffic noise by more than half.
Noise barriers can be formed from high vertical walls, from earth mounds (usually called earth berms), or a combination of the two. Earth berms have a natural appearance and are usually attractive. However, an earth berm can require a large amount of land to attain the desired height. For example, a 16-foot high earth berm would require 32 feet of space at the base. Noise walls require substantially less space.
For a noise barrier to be effective, it must be high enough and long enough to block the view of the road. Noise barriers are ineffective in situations where there are numerous intersecting streets or where openings for access to driveways must be provided.
Noise walls are often limited to 25 feet in height for structural and aesthetic reasons and are commonly 12 feet to 18 feet high. These regulations, titled Procedures for Abatement of Highway Traffic and Construction Noise, establish standards for abating highway traffic noise. The regulations are found in Title 23, Part 772 of the Code of Federal Regulations. Compliance with the noise regulations is a prerequisite for the granting of Federal-aid highway funds for construction or reconstruction of a highway.
Roland Barthes distinguishes between physiological noise, which is merely heard, and psychological noise, which is actively listened to. Luigi Russolo, one of the first composers of noise music, wrote the essay The Art of Noises. Felix Urban sees noise as a result of cultural circumstances. In his comparative study on sound and noise in cities, he points out that noise regulations are only one indicator of what is considered as harmful.