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Measurement of Highway-Related Noise

2. Terminology

This section presents pertinent terminology used throughout the document. These terms are highlighted with boldface type when they first appear in subsequent sections. Note: Definitions are generally consistent with those of the American National Standards Institute (ANSI) and References 5 through 8.

A-WEIGHTING: A frequency weighting network used to account for changes in sensitivity as a function of frequency (See Section 3.1.3.4.2).

ABSORPTION COEFFICIENT: See Sound absorption coefficient.

ACOUSTIC ENERGY: Commonly referred to as sound energy, or just plain energy, acoustic energy is arithmetically equivalent to 10[Sound Pressure level (SPL)/10], where SPL is expressed in decibels re 20 µPa.

AMBIENT NOISE: All-encompassing sound that is associated with a given environment, excluding the analysis system's electrical noise and the sound source of interest.

ARTIFICIAL NOISE SOURCE: An acoustical source that is controlled in position and calibrated as to output power, spectral content, and directivity.

AUDIOMETRY: The measurement of human hearing acuity.

ANTI-ALIAS FILTER: A low-pass filter applied to the input signal of a digital system prior to the digitization process. This filter, unique to digital systems, ensures that spurious signals (alias signals) resulting from the digitization process are not contributing components of the sampled signal. An anti-alias filter must be included in all digital systems, prior to the analog-to-digital conversion.

BACKGROUND NOISE: All-encompassing sound of a given environment that includes ambient, as well as analysis system noise, excluding the sound source of interest.

COMMUNITY-NOISE EXPOSURE LEVEL (CNEL, denoted by the symbol Lden): A 24-hour time-averaged LAE (see definition), adjusted for average-day sound source operations. In the case of highway noise, a single operation is equivalent to a single vehicle pass-by. The adjustment includes a 5-dB penalty for vehicle pass-bys occurring between 1900 and 2200 hours, local time, and a 10-dB penalty for those occurring between 2200 and 0700 hours, local time. The Lden noise descriptor is used primarily in the state of California. Lden is computed as follows:

Lden = LAE + 10 x log10(Nday + 3 x Neve + 10 x Nnight) - 49.4           (dB)

where:

LAE = Sound exposure level in dB (See definition);

Nday = Number of vehicle pass-bys between 0700 and 1900 hours, local time;

Neve = Number of vehicle pass-bys between 1900 and 2200 hours, local time;

Nnight = Number of vehicle pass-bys between 2200 and 0700 hours, local time; and

49.4 = A normalization constant which spreads the acoustic energy associated with highway vehicle pass-bys over a 24-hour period, i.e., 10 x log10(86,400 seconds per day) = 49.4 dB.

CONTAMINATION: (See Noise Contamination).

DAY-NIGHT AVERAGE SOUND LEVEL (DNL, denoted by the symbol Ldn): A 24-hour time-averaged LAE (See definition), adjusted for average-day sound source operations. In the case of highway noise, a single operation is equivalent to a single vehicle pass-by. The adjustment includes a 10-dB penalty for vehicle pass-bys occurring between 2200 and 0700 hours, local time. Ldn is computed as follows:


Ldn = LAE + 10 x log10(Nday + Neve + 10 x Nnight) - 49.4           (dB)

where:

LAE = Sound exposure level in dB (See definition);

Nday = Number of vehicle pass-bys between 0700 and 1900 hours, local time;

Neve = Number of vehicle pass-bys between 1900 and 2200 hours, local time;

Nnight = Number of vehicle pass-bys between 2200 and 0700 hours, local time; and

49.4 = A normalization constant which spreads the acoustic energy associated with highway vehicle pass-bys over a 24-hour period, i.e., 10 x log10(86,400 seconds per day) = 49.4 dB.

DECIBEL (dB): A unit of level which denotes the ratio between two quantities that are proportional to power; the number of decibels is 10 times the base 10 logarithm of this ratio. For the purpose of this document, the reference level is 20 µPa, or the threshold of human hearing.

DIFFRACTED WAVE: A sound wave whose front has been changed in direction by an obstacle in the propagation medium, typically air for the purposes of this document.

DIVERGENCE: The spreading of sound waves from a source in a free field environment. In the case of highway noise, two types of divergence are common, spherical and cylindrical. Spherical divergence is that which would occur for sound emanating from a point source, e.g., a single vehicle pass-by. It is independent of frequency, and is computed using a 20 x log10(d1/d2) relationship. For example, if the sound level from a point source at 15 m was 90 dB, at 30 m it would be 84 dB due to divergence, i.e., 90 + 20 x log10(15/30). Cylindrical divergence is that which would occur for sound emanating from a line source, e.g., a single vehicle pass-by. It is independent of frequency, and is computed using a 10 x log10(d1/d2) relationship. For example, if the sound level from a point source at 15 m was 90 dB, at 30 m it would be 87 dB due to divergence, i.e., 90 + 10 x log10(15/30).

DOPPLER EFFECT: The change in the observed frequency of a wave in a transmission system caused by a time rate of change in the effective length of the path of travel between the source and the point of observation.

DYNAMIC RANGE: The difference between the highest input sound pressure level achievable without exceeding a specified non-linearity or distortion of the output signal, for a specified frequency range, and the lowest input sound pressure level for which the level linearity is within specified tolerances.

EQUIVALENT SOUND LEVEL (TEQ, denoted by the symbol LAeqT): Ten times the base-10 logarithm of the ratio of time-mean-squared instantaneous A-weighted sound pressure, during a stated time interval, T (where t=t2-t1), to the square of the standard reference sound pressure. For the purpose of this document, the reference sound pressure is 20µPa, or the threshold of human hearing. LAeqT is related to LAE by the following equation:

LAeqT = LAE - 10 x log10(t2-t1)           (dB)

where:

LAE = Sound exposure level in dB (See definition).

EXCHANGE RATE: The amount a sound level is increased or decreased to preserve a certain noise exposure when the exposure duration is doubled or halved. Typically, for transportation-related noise, an exchange rate of 3 dB is used; for occupational noise exposure, 5 dB is used.

FAR-FIELD: That portion of a point source's sound field in which the sound pressure level (due to this sound source) decreases by 6 dB per doubling of distance from the source, i.e., spherical divergence; or if the sound source is linear, then the far-field is the portion of the sound field in which the sound pressure level decreases by 3 dB per doubling of distance.

FREE FIELD: A sound field whose boundaries exert a negligible influence on the sound waves. In a free-field environment, sound Spreads spherically from a source and decreases in level at a rate of 6 dB per doubling of distance from a point source, and at a rate of 3 dB per doubling of distance from a line source.

GROUND ATTENUATION: The change in sound level, either positive or negative, due to intervening ground between source and receiver. Ground attenuation is a relatively complex acoustic phenomenon, which is a function of ground characteristics, source-to-receiver geometry, and the spectral characteristics of the source. A commonly used rule-of-thumb for propagation over soft ground (i.e., grass, terrain) is that ground effects will account for about 1.5 db per doubling of distance. However, this relationship is quite empirical and tends to break down for distances greater than about 30 to 61 m (100 to 200 ft).

GROUND IMPEDANCE: A complex function of frequency relating the sound transmission characteristics of a ground surface type. Measurements to determine ground impedance must be made in accordance with the ANSI Standard for measuring ground impedance scheduled for publication in the second half of 1996.(50)

HARD GROUND: Any highly reflective surface in which the phase of the sound energy is essentially preserved upon reflection; examples includes water, asphalt and concrete.

INSERTION LOSS (IL): The difference in levels before and after installation of a barrier, where the source, terrain, ground, and atmospheric conditions have been judged as equivalent.


LAE: See Sound exposure level.

LAeq: See Equivalent sound level.

LAFmx and LASmx: See Maximum sound level.

Lden: See Community-noise exposure level.

Ldn: See Day-night average sound level.

L90: A statistical descriptor describing the sound level exceeded 90 percent of a measurement period.


LINE SOURCE: Multiple point sources moving in one direction radiating sound cylindrically. Note: Sound levels measured from a line source decrease at a rate of 3 dB per doubling of distance.

LOWER BOUND TO INSERTION LOSS: The value reported for insertion loss when background levels are not measured or are too high to determine the full attenuation potential of the barrier.

MAXIMUM SOUND LEVEL (MXFA or MXSA, denoted by the symbol LAFmx or LASmx, respectively): The maximum, A-weighted sound level associated with a given event (See Figure 1). Fast-scale response (LAFmx) and slow-scale response (LASmx,) characteristics effectively damp a signal as if it were to pass through a low-pass filter with a time constant of 125 and 1000 milliseconds, respectively. See Section 3.1.3.4.4 for a more detailed discussion of exponential time-averaging.

NEAR FIELD: The sound field (between the source and the far field). The near field exists under optimal conditions at distances less than four times the largest sound source dimension.

NOISE: Any unwanted sound.

NOISE BARRIER: The structure, or structure together with other material, that potentially alters the noise at a site from a BEFORE condition to an AFTER condition.

NOISE CONTAMINATION: Any noise event, other than that which is intended for measurement. Contamination typically occurs when the background noise is within 10 dB of the noise produced by the source intended for measurement.*

NOISE DOSE: A measure of the noise exposure to which a person is subjected in the workplace. For the purposes of this document, the workplace is any highway-related environment.

NOISE REDUCTION COEFFICIENT (NRC): A single-number rating of the sound absorption properties of a material; it is the arithmetic mean of the Sabine absorption coefficients (See below) at 250, 500, 1000, and 2000 Hz, rounded to the nearest multiple of 0.05.

PINK NOISE: A random signal for which the spectrum density, i.e., narrow-band signal, varies as the inverse of frequency. In other words, one-third octave-band spectral analysis of pink noise yields a flat response across all frequency bands.

POINT SOURCE: Source that radiates sound spherically. Note: Sound levels measured from a point source decrease at a rate of 6 dB per doubling of distance.

SABINE ABSORPTION COEFFICIENTSab): Absorption coefficient obtained in a reverberation room by measuring the time rate of decay of the sound energy density with and without a patch of the sound-absorbing material under test laid on the floor. These measurements are performed in accordance with the American Society of Testing and Materials (ASTM) Standard C 423-90a.

SOFT GROUND: Any highly absorptive surface in which the phase of the sound energy is changed upon reflection; examples include terrain covered with dense vegetation or freshly fallen snow. (Note: at grazing angles greater than 20 degrees, which can commonly occur at short ranges, or in the case of elevated sources, soft ground becomes a good reflector and can be considered hard ground).*

SOUND ABSORPTION COEFFICIENT (α): (See also Sabine Absorption Coefficient) The ratio of the sound energy, as a function of frequency, absorbed by a surface, to the sound energy incident upon that surface.

SOUND EXPOSURE LEVEL (SEL, denoted by the symbol LAE): Ten times the logarithm to the base 10 of the ratio of a given time integral of squared instantaneous A-weighted sound pressure to the squared reference sound pressure of 20 µPa, the threshold of human hearing. The time interval must be long enough to include a majority of the sound source's acoustic energy. As a minimum, this interval should encompass the 10 dB down points (See Figure 1).


An X-Y graph with Time in seconds on the X-axis and Sound Pressure Level in decibels on the Y-axis. The graph depicts a 10-decibel rise and fall between the times t1 and t2, where the maximum A-weighted sound level is the peak of the curve and the sound exposure level is the shaded area underneath the curve.

Figure 1. Graphical representation of LAE and LAFmx noise descriptors.


In addition, LAE is related to LAeqT by the following equation:

LAE = LAeqT + 10 x log10(t2-t1)           (dB)

where LAeqT = Equivalent sound level in db (See definition).

SOUND PRESSURE LEVEL (SPL): Ten times the logarithm to the base 10 of the ratio of the time-mean-squared pressure of a sound, in a stated frequency band, to the square of the reference sound pressure of 20 µPa, the threshold of human hearing.

SOUND TRANSMISSION CLASS (STC): A single-number rating used to compare the sound insulation properties of barriers.

SPECTRUM: A signal's resolution expressed in component frequencies or fractional octave bands.


Updated: 07/06/2011
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