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Noise Definitions
Ambient Noise Measurements: Ambient noise measurements are the field measurements of actual noise levels monitored at selected sampling locations, using integrating SLM's.

These noise measurements are usually conducted at noise-sensitive locations where the background noise is a combination of different noises from many sources or where the location is remote and far away from any major highway. In order to represent the Leq(h), ambient noise measurements are typically conducted for 15 or 20-minute time periods during the noisiest hour of the day. Traffic volumes and conditions, weather conditions and existing land uses are also noted at the time of the noise measurement.

Noise-sensitive Receptors: Noise-sensitive receptors can best be defined as those locations or areas where dwelling units or other fixed, developed sites of  frequent human use occur. They are usually within 1000 feet (300 m) of the highway right-of-way line and between the project termini. The FHWA has established noise impact criteria for various categories of noise sensitivity. These various land use or activity categories are listed in the FHWA's noise abatement criteria (NAC.)

Noise Abatement Criteria (NAC): The Noise Abatement Criteria (NAC) are not magical numbers. In establishing values for the NAC, the FHWA has attempted to create a balance between that which is desirable and that which is achievable. Numerous approaches were considered in establishing the NAC. However, it was speech interference that was usefully applied to the problem of highway traffic noise. Thus, it should be remembered that the NAC are based upon noise levels associated with the interference of speech.

FHWA Noise Abatement Criteria (NAC)
Hourly A-Weighted Sound Level - Decibels (dBA)
Activity Category
Leq (h)
Description of Activity Category
57 (exterior)
Lands on which serenity and quiet are of extraordinary significance and serve an important public need and where the preservation of those qualities is essential if the area is to continue to serve its intended purpose.
67 (exterior)
Picnic area, fixed recreation areas, playgrounds, active sports areas, parks, residences, motels, hotels, schools, churches, libraries and hospitals.
72 (exterior)
Cemeteries, comercial areas, industrial areas, office buildings, and other developed lands, properties or activities not included in Categories A or B above.
No Limit
Undeveloped lands, including roadside facilities and dispersed recreation.
52 (interior)
Residences, motels, hotels, public meeting rooms, schools, churches, libraries, hospitals and auditoriums.
Source: Title 23 of the Code of Federal Regulations, Part 772

Title 23 of the Code of Federal Regulation Part 772 (23 CFR 772) defines traffic noise impacts as "impacts which occur when predicted traffic noise levels approach or exceed the NAC." The term "approach" has been defined as one dBA less than the Leq(h) values listed in the NAC.

West Virginia Substantial Increase Criteria: Highway traffic noise impacts can also occur when a proposed highway project substantially increases the noise level above the existing noise environment. Title 23 of the Code of Federal Regulations Part 772 (23 CFR 772) provides State Highway Agencies with the authority to establish their own criteria for substantial noise increases. WVDOH uses a "Criteria 3" tier (step) definition in which a 16 dB or greater increase constitutes a substantial increase impact.

WVDOH Substantial Increase Criteria
Noise Level Increase Over Existing (dBA)
Subjective Descriptor
No Impact
Minor Impact
Moderate Impact
16 or greater
Substantial Impact

Decibels (dB): A decibel (dB) is the unit of measurement used to express the intensity or loudness of sound. A decibel is one-tenth of a larger unit called a bel, named after Alexander Graham Bell, the inventor of the telephone. The decibel system is a logarithmic scale of measurement. Logarithmic scales are based on powers of ten, not linear like a ruler. Under this system of measurement, changes in sound levels are hard to define. For example, if a sound of 60 dB is added to another sound of 60 dB the resulting sound is 63 dB instead of 120 dB. A sound of 0 dB is the threshold for normal human hearing. A sound of 10 dB or 1 bel transmits ten times as much energy as a sound of 0 dB. A sound of 20 dB transmits one hundred times as much energy as a 0 dB sound and ten times as much as 10 dB sound.

A-weighted levels (dBA): Sound is composed of various frequencies. The human ear does not hear all sound frequencies. Normal hearing is within the range of 20 and 20,000 vibrations per second. For highway traffic noise, an adjustment or weighting of sound frequencies is made to approximate the way that the average person hears sounds. This weighting system assigns a  weight that is related to how sensitive the human ear is to each sound frequency. Frequencies that are less sensitive to the human ear are weighted less than those for which the ear is more sensitive. The adjusted sounds are called  A-weighted levels (dBA.) It has been found that the A-scale on a SLM best approximates the frequency response of the human ear. Sound levels measured on the A-scale of a SLM are abbreviated dBA.

Leq(h): Highway traffic noise levels are always changing. As the traffic volume, type and speed of vehicles producing the noise vary, so do the noise levels. Because of these time-related variations in traffic noise on a highway, it is more convenient and practical to convert all the different noise levels for a given time period into a single representative noise level. One of the more common descriptors used to characterize the fluctuating noise levels is called the Equivalent Sound Level or Leq. The Leq sound level is the steady A-weighted sound energy which would produce the same A-weighted sound energy over the same given period of time as the specified time-varying sound. For traffic noise studies, Leq is usually evaluated over a one-hour time period and is denoted as Leq(h).
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