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2.0 TERMINOLOGY

This chapter presents pertinent terminology used throughout the Handbook and CD-ROM.

A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

A

A-Weighting

The weighting network used to account for changes in level sensitivity as a function of frequency. The A-weighting network de-emphasizes the high (6.3 kHz and above) and low (below 1 kHz) frequencies, and emphasizes the frequencies between 1 kHz and 6.3 kHz, in an effort to simulate the relative response of the human ear. See also frequency weighting.

Acoustic Energy

Commonly referred to as the mean-square sound-pressure ratio, sound energy, or just plain energy, acoustic energy is the square of the ratio of the mean-square sound pressure (often referred to as frequency weighted), and the reference mean-square sound pressure of 20 µPa, the threshold of human hearing. It is arithmetically equivalent to 10(SPL/10), where SPL is the sound pressure level, expressed in decibels.

Ambient Noise

All-encompassing sound that is associated with a given environment, usually a composite of sounds from many sources near and far.

Amplitude

A non-negative scalar measure of a wave’s magnitude of oscillation.

B

Background Noise

All-encompassing sound of a given environment without the sound source of interest.

Bridge

A structure which provides a roadway or walkway for the passage of vehicles and pedestrians across an obstruction, gap, water course, or facility and which is typically greater than 3 m (10') in span.

C

Community Noise Equivalent Level (CNEL, denoted by the symbol, L_den)

A 24-hour time-averaged L_AE adjusted for average-day sound source operations. In the case of highway traffic 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 L_den noise descriptor is used primarily in the State of California. L_den is computed as follows*.

L_den = L_AE + 10*log10(N_day + 3*N_eve + 10*N_night) - 49.4 (dB)
where:

L_AE = Sound exposure level in dB ;
N_day = Number of vehicle pass-bys between 0700 and 1900 hours, local time;
N_eve = Number of vehicle pass-bys between 1900 and 2200 hours, local time;
N_night = 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*log₁₀(86,400 seconds per day) = 49.4 dB.

* (In accordance with the technical definition, a 5-dB penalty is added to evening operations when computing the Lden noise metric. The 5-dB penalty, expressed in terms of a weighting factor, is equivalent to 3.16 not 3. However, in Title 21 Subchapter 6, 5001 of California state law, a factor of 3 is used. Since the State of California is the primary user of the Lden metric, the computation of the metric is consistent with state law, rather than the traditional technical definition.)

D

Day-Night Average Sound Level (DNL, denoted by the symbol, L_dn)

A 24-hour time-averaged L_AE, adjusted for average-day sound source operations. In the case of highway traffic 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. L_dn is computed as follows:

L_dn = L_AE + 10*log₁₀(N_day + 10*N_night) - 49.4 (dB)
where:

L_AE = Sound exposure level in dB;
N_day = Number of vehicle pass-bys between 0700 and 1900 hours, local time;
N_night = Number of vehicle pass-bys between 1900 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*log₁₀(86,400 seconds per day) = 49.4 dB.

Decibel (dB)

A unit of measure of sound level. The number of decibels is calculated as ten times the base-10 logarithm of the square of the ratio of the mean-square sound pressure (often referred to as frequency weighted), and the reference mean-square sound pressure of 20 µPa, the threshold of human hearing. A-weighted decibels are expressed as dBA or dB(A).

Diffracted Wave

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

Degradation

The reduction of noise barrier effectiveness at receivers due to conditions such as multiple reflections of the noise between parallel barriers, noise leaks in a barrier, etc.

Divergence

The spreading of sound waves from a source in a free field environment. In the case of highway traffic 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. Cylindrical divergence is that which would occur for sound emanating from a line source, or many point sources sufficiently close to behave as a line source, e.g., a continuous stream of roadway traffic.

E

Energy

See Acoustic Energy.

Equivalent Sound Level (L_eq, denoted by the symbol, L_AeqT)

Ten times the base-10 logarithm of the square of the ratio of time-mean-square, instantaneous A-weighted sound pressure, during a stated time interval, T (where T=t₂-t₁), divided by the squared reference sound pressure of 20 µPa, the threshold of human hearing, e.g., L_eq(h), denoted by the symbol, L_Aeq1H, represents the hourly equivalent sound level. L_AeqT is related to L_AE by the following equation:

L_AeqT = L_AE - 10*log₁₀(t₂-t₁) (dB)
where:

L_AE = Sound exposure level in dB .

Existing Level

The measured or calculated existing noise level at a given location.

Exponential Time-Averaging

A method of stabilizing instrumentation response to signals with changing amplitude over time using a low-pass filter with a known, electrical time constant. The time constant is defined as the time required for the output level to reach 63.4 percent of the input, assuming a step-function input. Also, the output level will typically reach 100 percent of an input-step function after approximately five time constants.

F

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. For a line source, 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.

Frequency

The number of cycles of repetition per second or the number of wavelengths that have passed by a stationary point in one second.

Frequency Weighting

A method used to account for changes in sensitivity as a function of frequency. Three standard weighting networks, A, B, and C, are used to account for different responses to sound pressure levels. Note: The absence of frequency weighting is referred to as "flat" response. See also A-weighting.

Fresnel Number

A dimensionless value used in predicting the attenuation provided by a noise barrier positioned between a source and a receiver.

G

Grade

The slope of the roadway, or roadway segment (expressed in percent). For example, a roadway that is 400 m in length and its end is 20 m higher in elevation relative to its start, has a 5-percent grade, i.e., 20/400 * 100 percent.

Ground Effect

The change in sound level, either positive or negative, due to intervening ground between source and receiver. Ground effect 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 (e.g., grass) 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 30.5 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.

H

Hard Ground

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

I

Insertion Loss (IL)

The sound level at a given receiver before the construction of a barrier minus the sound level at the same receiver after the construction of the barrier. The construction of a noise barrier usually results in a partial loss of soft-ground attenuation. This is due to the barrier forcing the sound to take a higher path relative to the ground plane. Therefore, barrier IL is the net effect of barrier diffraction, combined with this partial loss of soft-ground attenuation.

J

K

L

Line-of-Sight

Refers to the direct path from the source to receiver without any intervening objects or topography.

Line Source

Multiple point sources moving in one direction, e.g., a continuous stream of roadway traffic, radiating sound cylindrically. Note: Sound levels measured from a line source decrease at a rate of 3 dB per doubling of distance.

M

Maximum Sound Level (MXFA or MXSA, denoted by the symbol, L_AFmx or L_ASmx)

The maximum, A-Weighted sound level associated with a given event (see Figure2.1). Fast-scale response (L_AFmx) and slow-scale response (L_ASmx) 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.

Note: Fast response is typically used for measuring individual highway vehicle pass-bys. Slow response is recommended for the measurement of long-term impact due to highway traffic noise, where impulsive noises are not dominant, and is also used for measurements of sound source levels which vary slowly as a function of time, such as aircraft.

N

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" and "sound" are used interchangeably in this document.

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 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 at 250, 500, 1000, and 2000 Hz, rounded to the nearest multiple of 0.05.

Noise Reduction Goal

The amount of noise reduction that is desired. This value should be defined by a respective State Highway Agency and should typically be in the range of 5 to 10 dBA. Noise barriers must provide at least a 5 dBA reduction in highway traffic noise levels in order to provide noticeable and effective attenuation. A noise barrier should be designed to achieve the greatest reduction possible, but in no instance less than 5 dBA.

Normal Incidence (Sound)

(Also referred to as 0-degrees incidence) Occurs when sound waves impinge at an angle perpendicular, or normal, to the microphone diaphragm.

O

P

Panel

The panel component of a noise barrier is that portion which, when joined together, produces a solid wall. In most cases, the panels span the distance between supports, typically posts.

Parallel Barrier

The condition where two noise barriers flank a roadway, i.e., one on each side.

Parapet

Parapets are low walls, or railings or a combination of both which are located along the outside edges of bridge decks. They are designed to prevent vehicles from running off the sides of the bridge.

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.

Posts

Posts are typically considered as vertical supports for the noise barrier panels.

Prestressed Concrete

Reinforced concrete in which internal stresses and deformations are initially introduced, of such magnitude and distribution that the subsequent stresses and deformations resulting from dead and live loads are counteracted to a desired degree.

Q

R

Random Incidence (Sound)

Occurs when sound waves impinge at random angles to the microphone diaphragm. Such waves are common in a diffuse sound field.

Retaining Walls

A wall built to hold back earth or water.

Responsible Organization

Government transportation agency, emergency response unit, fire department, police department, etc.

Right-of-Way (ROW)

The entire strip or area of land adjacent to a highway used for highway purposes.

S

Sabine Absorption Coefficient (α_Sab)

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.

Shoulder

The part of a roadway contiguous to the traffic lanes for accommodating stopped vehicles, bikeways, and cleared snow.

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 acoustically hard ground

Sound Absorption Coefficient (SAC)

(See also Sabine Absorption Coefficient )

The ratio of the sound energy, as a function of frequency, absorbed by a surface, and the sound energy incident upon that surface.

Sound Energy

See Acoustic energy.

Sound Exposure Level (SEL, denoted by the symbol, L_AE)

Over a stated time interval, T (where T=t₂-t₁), ten times the base-10 logarithm of the ratio of a given time integral of squared instantaneous A-weighted sound pressure, and the product of the reference sound pressure of 20 µPa, the threshold of human hearing, and the reference duration of 1 sec. The time interval, T, 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 2.1)

In addition, L_AE is related to L_AeqT by the following equation:

L_AE = L_AeqT + 10*log₁₀(t₂-t₁) (dB)
where:

L_AeqT = Equivalent sound level in dB

Figure 2.1. Graphical representation of L_AE

Sound Pressure

The root-mean-square of the instantaneous sound pressures during a specified time interval in a stated frequency band.

Sound Pressure Level (SPL)

Ten times the base-10 logarithm of the square of the ratio of the mean-square sound pressure, in a stated frequency band (often weighted), and the reference mean-square sound pressure of 20 µPa, the threshold of human hearing.

SPL = 10*log₁₀(p² / p_ref²) (dB )
where:

p = mean-square sound pressure; and
p_ref = reference mean-square sound pressure of 20 µPa.

Sound Transmission Class (STC)

A single-number rating used to compare the sound insulation properties of barriers. STC is derived by fitting a reference rating curve to the sound transmission loss (TL) values measured for the 16 contiguous one-third octave frequency bands with nominal mid-band frequencies of 125 Hz to 4000 Hz inclusive, by a standard method. The reference rating curve is fitted to the 16 measured TL values such that the sum of deficiencies (TL values less than the reference rating curve), does not exceed 32 dB, and no single deficiency is greater than 8 dB. The STC value is the numerical value of the reference contour at 500 Hz.

Spectrum

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

Structures

Includes retaining walls, bridges, culverts, and concrete drainage channels.

T

Ten-Percentile Exceeded Sound Level (L₁₀)

The sound level exceeded 10 percent of a specific time period. For example, from a 50-sample measurement period, the fifth (10% of 50 samples) highest sound level is the 10-percentile exceeded sound level. Other similar descriptors include L50 (the sound level exceeded 50 percent of a specific time period), L90 (the sound level exceeded 90 percent of a specific time period), etc.

Transmission Loss (TL)

The loss in sound energy, expressed in decibels, as sound passes through a barrier or a wall. Measurements to determine a barrier's TL should be made in accordance with ASTM Recommended Practice E413-87.

U

Utilities

Transmission and distribution lines, pipes, cables and other associated equipment used for public services including, but not limited to, electric transmission and distribution, lighting, heating, gas, oil, water, sewage, cablevision, data communications, and telephone.

V

W

Wavelength

The perpendicular distance between two wave fronts in which the displacements have a difference in phase of one complete period.