Definitions and terms indicated in the Glossary may vary for local regulations and standards.

A-weighted sound level

The sound level as measured on a sound level meter using a setting that emphasizes the middle frequency components similar to the frequency response of the human ear at levels typical of rural backgrounds in mid frequencies.

Ambient Noise

All noise sources that surround a location and could include 3rd party industrial noise, transportation sources, animals and nature.

Ambient Sound Level (ASL)

The sound level that is a composite of different airborne sounds both far from and near the point of measurement. The ASL does not include any energy-related industrial component and is measured without it. The ASL can be measured when the sound level in an area is not believed to be represented by basic sound levels and is measured under representative conditions. As with comprehensive sound levels, representative conditions do not constitute absolute worst-case conditions (i.e., the quietest day in this case) but conditions that portray typical conditions for the area. Also see Representative conditions.

Background Noise

The total noise from all surrounding sources in an area. This could include sounds from industry, transportation sources, animals and nature.

Bands (octave, 1/3 octave)

A series of electronic filters separate sound into discrete frequency bands, making it possible to know how sound energy is distributed as a function of frequency. Each octave band has a center frequency that is double the center frequency of the octave band preceding it. The 1/3 octave band analysis provides a finer breakdown of sound distribution as a function of frequency.

C-weighted Sound Level

The C-weighting approximates the sensitivity of human hearing at industrial noise levels (above 85 dBA). The C-weighted sound level is more sensitive to sounds at low frequencies than the A-weighted sound level, and is sometimes used to assess the low-frequency content of complex sound environments.


The procedure used for the adjustment of a sound level meter using a reference source of a known sound pressure level and frequency. Field calibration must take place before and after the sound level measurements.

Comprehensive Sound Level (CSL)

The sound level that is a composite of different airborne sounds from sources far from and near the point of measurement. The CSL includes industrial components and must be measured with them. The CSL excludes abnormal noise sources and is used to determine whether a facility is in compliance.


Between the hours of 7:00am and 10:00pm.

Daytime Adjustment

An adjustment that allows a 10 dBA increase because daytime ambient sound levels are generally 10 dBA higher than nighttime values.

dB (Decible)

A unit of measure of sound pressure that compresses a large range of numbers into a more meaningful scale. Hearing tests indicate that the lowest audible pressure is about 2 x 10-5 Pa (0 dB), while the sensation of pain is about 2 x 102 Pa (140 dB). Generally, an increase of 10 dB is perceived as twice as loud.


p = root-mean-square sound pressure (Pa)
po = reference root-mean-square-sound pressure, generally 2 x 10-5 Pa

The decibel is a linear weighting and can also be used when referring to differences in weightings.


The decibel (dB) sound pressure level filtered through the A filtering network to approximate human hearing response at low intensities. Also see dB and A-weighted sound level.

Energy Equivalent Sound Level (Leq)

The Leq is the average weighted sound level over a specified period of time. It is a single number representation of the cumulative acoustical energy measured over a time interval. The time interval used should be specified in brackets following the Leq (i.e., Leq (9) is a 9-hour Leq). If a sound level is constant over the measurement period, the Leq will equal the constant sound level. If the sound level shows a variety of constant levels for different intervals, then fi is the fraction of time the constant level Li, is present.


Far Field

The far field is the area far enough away from the noise source that noise emissions can be treated as if they come from a single point or line source, and the individual components of the noise source are not apparent as separate sources. This is typically at a distance of at least three to five times the major dimensions of the noise source.

The far field may consist of two parts – the free part and the reverberant part. In the free part, the sound pressure level obeys the inverse-square law (6 dBA loss per doubling of distance for a point source). The reverberant part exists for enclosed or semi-enclosed situations where there are many reflected sound waves from all directions. An example of a reverberant field is industrial equipment enclosed in a room.


A device separating the components of an incoming signal by its frequencies.

Industrial Noise Control

A set of strategies and products to reduce noise pollution or its impact, which emits from industry.

Linear Weighting (or Z-Weighting)

The sound level measured with the linear weighting measures the acoustic pressure without any adjustment for the sensitivity of human hearing. It is a direct measure in decibels of the variation in air pressure and is often referred to as the Sound Pressure Level. This level is sometimes called the linear weighted level or the un-weighted level, as it includes no frequency weighting beyond the tolerances and limits of the sound level meter being used for the measurements.

Low Frequency Noise

Where a clear tone is present below a frequency of 250 Hz, or when the difference between the overall C-weighted sound level and the overall A-weighted sound level exceeds 20 dB.

Near Field

The region close to the source where the inverse-square law (6 dBA loss per doubling of distance for a point source) does not apply. Usually this region is located within a few wavelengths of the noise source and is controlled by its dimensions.

Noise Impact Assessment

Identifies the expected sound level emanating from a facility as measured 15 meters from the nearest or most impacted occupied dwelling. It also identifies what the permissible sound level is and how it was calculated.

Permissible Sound Level (PSL)

The maximum sound level that a facility must not exceed 15 meters from the nearest or most impacted dwelling unit. The PSL is the sum of the BSL, daytime adjustment, Class A adjustment, and Class B adjustment.

Sound Level Meter

An instrument designed and calibrated to respond to sound and to give objective, reproducible measurements of sound pressure level. It normally has several features that would enable its frequency response and averaging times to be changed to make it suitable to simulate the response of the human ear.

Sound Monitoring Survey

The measurement and recording of sound levels and pertinent related information over a given time period.

Sound Power Level (PWL, SWL, or LW)

The decibel equivalent of the rate of energy (or power) emitted in the form of noise. The sound power level is given by


However in some older data (roughly pre-1975); the value of WO was set as 10”13W, which is no longer used. The sound power level is an inherent property of a noise source.

Sound Pressure Level (SPL or Lp)

The decibel equivalent of the pressure of sound waves at a specific location, which is measured with a microphone. Because human reaction and material behaviors vary with frequency, the SPL may be measured using frequency bands or with an overall weighting scale such as the A-weighting system. The sound pressure level depends on the noise sources, as well as the location and environment of the measurement path.


A wide range or sequence of frequencies.

Tonal Components (low frequency)

The test for the presence of tonal components consists of two parts. The first must demonstrate that the sound pressure level of any one of the slow-response, A-weighted, 1/3 octave bands between 20 and 250 Hz is 10 dBA or more than the sound pressure level of at least one of the adjacent bands within two 1/3 octave bandwidths. The second must have a minimum of a 5 dBA drop from the band containing the tone within two bandwidths on the opposite side.

The second part is that the tonal component must be a pronounced peak clearly obvious within the spectrum.

An example of tonal component determination is shown in Appendix 6.