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Part I Noise Fundamentals

Part II Noise Analysis

Part III Noise Abatement

Frequently Asked Questions

Glossary and Acronyms

Highway Traffic Noise - Glossary and Acronyms


Attenuation   The reduction of an acoustic signal (noise).

Average Daily Traffic (ADT)   The total traffic volume during a given period divided by the number of days in that period. Current ADT volumes can be determined by continuous traffic counts or periodic counts.

A-Weighted   Adjustment or weighting of sound frequencies 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.

Barrier   A solid wall or earth berm located between the roadway and receiver location which provides noise reduction.

Benefited Receptor  The receptor of an abatement measure that receives a noise reduction of 5 dB(A) or greater.

Build Condition   Motdeling traffic volumes using the proposed roadway configuration.

CFR   Code of Federal Regulations.

Clear Zone  The unobstructed, relatively flat area provided beyond the edge of the traveled way for recovery of errant vehicles. The travel way is the portion of the roadway not including shoulders. It is desirable to provide a roadside clear of hazardous objects or conditions for a distance consistent with speed, traffic volume and geometric conditions of the site. 

Common Noise Environment (CNE)  A group of receptors within the same Activity Category that are exposed to similar noise sources and levels; traffic volumes, traffic mix and speed; and topographic features.  Generally, CNE's occur between two secondary noise sources, such as interchanges, intersections, or cross-roads.

Composite Materials  A composite material noise barrier is constructed with distinct components.  An example composite noise wall has a hollow fiberglass shell and is filled with recycled tires.

Decibels (dB)  Units for measuring sound. Decibels are logarithmic units.

dBA  Decibels measured using the A-weighted scale.

FHWA   Federal Highway Administration.

Frequencies   The number of cycles of a periodic motion in a unit of time.

Fully controlled-access state highway   A highway under IDOT jurisdiction with no at-grade intersections and no driveway access points.

Hard site   Hard ground conditions, such as asphalt or concrete, that tend to reflect noise.

Heavy trucks   Any vehicle having three or more axles and designed for the transportation of cargo.

Hertz   The unit of frequency; one Hertz has a periodic interval of one second.

Ldn (Day/Night average sound level)   Average sound exposure over a 24-hour period is often presented as a day-night average sound level (Ldn). Ldn values are calculated from hourly Leq values, with the Leq values for the nighttime period (10:00 p.m. - 7:00 a.m.) increased by 10 dB to reflect the greater disturbance potential from nighttime noises.

Leq - Equivalent Sound Level  The equivalent steady-state sound level which in a stated period of time contains the same acoustic energy as the time-varying sound level during the same time period.

Line source   Many single sources close together (i.e. multiple vehicles on a roadway).

Lmax   The maximum sound level measured over a time period.

Logarithmic  A logarithm is a short hand way to represent large numbers.  For example; log 1,000 = 3; or log 1,000,000 = 6.

Medium trucks   All vehicles having two axles and six wheels designed for the transportation of cargo.

No Build Condition   Modeling traffic volumes using the existing roadway configuration.

Noise Abatement   Measures taken to mitigate or reduce traffic noise impacts (i.e. construction of berms or noise walls, shifting roadway alignment etc.).

Noise Abatement Criteria (NAC)   Noise impact thresholds for considering noise abatement for various land uses.

Parallel Walls   Two walls constructed on the opposite sides of a roadway.

Peak hourly traffic   The highest hourly traffic volume of the day.

Point source   One single source (i.e. one vehicle).

Receptor   Any precise location selected to represent where frequent outdoor human activity occurs.

Soft site   Soft ground conditions, such as grass, that tends to absorb noise.

TNM  Traffic Noise Model.  FHWA's computer program for highway traffic noise prediction and analysis.

Type I Projects   A proposed highway project for the construction of a highway at a new location or the physical alteration of an existing highway which significantly changes either the horizontal or vertical alignment or increases the number of traffic through-lanes. In addition, the following projects are also considered Type 1:

  • Addition of a through-traffic lane(s) (including HOV lane, High-Occupancy Toll (HOT) lane, bus lane, or truck climbing lanes).
  • Addition of an auxiliary lane (except when used as a turn lane).
  • Addition/relocation of interchange lanes/ ramps to a quadrant to complete an existing partial interchange.
  • Restriping existing pavement to add a through-traffic lane or an auxiliary lane.
  • Addition of a new/substantial alteration of a weigh station, rest stop, ride-share lot or toll plaza.

Type II Projects   A proposed project for noise abatement on an existing fully controlled-access state highway (freeway or expressway) in an urban area.

Type III Projects  A Federal or Federal-aid highway project that does not meet the classifications of a Type I or a Type II project.  Type III projects do not require a noise analysis.

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