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Noise Barrier Design Handbook

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 References 8 through 12.

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
The maximum value of a sinusoidal quantity measured from peak to peak.
Anchor
A bolt, stud or reinforcing bar embedded in concrete.
Artificial Noise Source
An acoustical source that is controlled in position and calibrated as to output power, spectral content, and directivity.

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B

Backer Rod
Flexible foam polyethylene rope manufactured in various diameters. Suitable for use as seals and joint fillers.
Backfill
The envelope of engineered soil, excluding the bedding, placed around the footing or foundation in a controlled manner.
Background Noise
All-encompassing sound of a given environment without the sound source of interest.
Bay
The area between two posts in a noise barrier system. This area can contain full height or stacked panels, cast in place panels, or other types of assembled panels components.
Bedding
The prepared portion of the engineered soil on which the footing or foundation is placed.
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.

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C

Caisson
A wood, metal or concrete casing sunk or constructed below ground or water level.
Check (Wood)
A separation of the wood along the grain, the greater part of which occurs across the rings of annual growth.
Coating Holiday
Unwanted breaks or interruptions in coating integrity.
Community Noise Equivalent Level (CNEL, denoted by the symbol, Lden)
A 24-hour time-averaged LAE 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 Lden noise descriptor is used primarily in the State of California. Lden is computed as follows:(note 1)Lden = LAE + 10*log10(Nday + 3*Neve + 10*Nnight) - 49.4 (dB)

where:

LAE = Sound exposure level in dB ;
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*log10(86,400 seconds per day) = 49.4 dB.
Compaction
The process of soil densification, at a specified moisture content, due to application of loads through rolling, kneading, tamping, rodding, or vibratory actions of mechanical or manual equipment.
Concrete Cover
The least distance between the surface of the reinforcing bar, strands or post tensioning ducts and the surface of the concrete.
Continuous Footing
A reinforced concrete beam running the entire length of the noise barrier panels and are set on, or just below the groundline. These are designed to support and evenly distribute the dead load of the entire length of each noise barrier panel. Posts and appropriated footings are still required to support the lateral loading of the panels.

drawing of footing with brick wall

Figure 1. Example of Continuous Footing

Contour
Graphical plot consisting of a smooth curve, statistically regressed through points of equal level.
Crack (Wood)
A separation of the wood cells across the grain (this may be due to internal strains resulting from unequal longitudinal shrinkage or to external forces)
Creep
Time-dependent deformation of a material under sustained load.
Culvert
A structure which is designed to provide an opening under a roadway, railway or side entrance for the passage of surface water, livestock or pedestrians

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D

Day-Night Average Sound Level (DNL, denoted by the symbol, Ldn)
A 24-hour time-averaged LAE, 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. Ldn is computed as follows:
Ldn = LAE + 10*log10(Nday + 10*Nnight) - 49.4 (dB)

where:

LAE = Sound exposure level in dB;
Nday = Number of vehicle pass-bys between 0700 and 1900 hours, local time;
Nnight = 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.
Dead Load
The weight of all material supported by the structure and not subject to movement.
Decay (rot)
The disintegration of the wood substance, due to the action of wood-destroying fungi
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.
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 increase in noise levels at receivers due to conditions such as reflections from a single barrier, multiple reflections of the noise between parallel barriers, noise leaks in a barrier, etc.
Delamination
A fracture plane below the surface of concrete or other material, typically parallel with the surface.
Direct Embedment (Buried) Support
A support, the lower end of which is placed in an excavated hole which is then backfilled with soil, concrete or other material.
Distress
Excessive cracking or deformation
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.

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E

Energy
See Acoustic energy.
Engineered Soil
Placed soil of known geotechnical properties.
Equivalent Sound Level (TEQ, denoted by the symbol, LAeqT)
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., 1HEQ, denoted by the symbol, LAeq1H, represents the hourly equivalent sound level. LAeqT is related to LAE by the following equation:

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

where LAE = Sound exposure level in dB .

Existing Level
The measured or calculated existing noise level at a given location.
Expansion Coefficient
The rate at which specific materials expand and contract as a result of changes in temperature.
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.

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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.
Flashing
A metal, plastic or fabric sheeting placed so as to cover unavoidable gaps between noise barrier component, and to prevent water from entering cavities, gaps, joints or cracks in a noise barrier wall system which may cause premature deterioration of the noise barrier components.
Fluting
Fluting is a texture produced on panels by incorporating ribbed form liners into the molds.
Foundation
That part of a structure or bridge substructure that transfers loads to the soil, rock, or engineered soil.
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 value used in predicting the attenuation provided by a noise barrier positioned between a source and a receiver.
Frost Heave
A seasonal upthrust of the ground or pavement caused by the formation of ice layers or lenses in frost susceptible soil.

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G

Gabion (or Stone Crib)
A wire mesh basket filled with stones or broken concrete which forms part of a larger unit used for slope sability, erosion control, or other purposes such as noise barrier walls.
Girts
The metal beams attached between the posts forming a framework to support the anchoring of metal panels to the structure.
Glue-Laminated Wood/Timber
A structural wood component produced by glueing together a number of laminations having their grain essentially parallel.
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, scheduled for publication in 1999.Ref.13

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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.

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I

Ice Credation
The build-up of an ice layer on the exposed surfaces of a body due to freezing rain or in-cloud icing.
Impact Criterion Level
The level defined by a State Highway Agency (SHA). It should be at least 1 dB(A) less than FHWA's appropriate Noise Abatement Criterion.
Incising
The process of inserting and withdrawing a series of closely-spaced knife points into the surface of wood prior to pressure preservative treatment. This promotes deeper penetrations of the treatment.
Insect Damage
The damage caused by boring insects or insect larvae, typically associated with wood products
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.

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J

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K

Kerf
Small incisions cut into the surface of wood products to allow deeper penetration of the preservative solutions during the pressure treating process.
Knot
That portion of a branch or limb that has become incorporated in the body of a tree.

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L

LAE
See Sound exposure level.
LAeqT
See Equivalent sound level.
Lden
See Community noise equivalent level.
Ldn
See Day-night average sound level.
L10
See Ten-percentile exceeded sound level.
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.
Live Load
A load imposed by vehicles, pedestrians, equipment or components subject of movement, other than collision load.
Load Factor
A factor applied to loads to take into account variability of loads, lack of precision in analysis for load effects, and reduced probability of loads from different sources acting simultaneously.
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.

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M

Mandrels
The center pin used in sheet metal rivets to expand and anchor the rivet body in place
Masonry
A general term applied to structures of stone, concrete or brick.
Maximum Sound Level (MXFA or MXSA, denoted by the symbol, LAFmx or LASmx)
The maximum, A-weighted sound level associated with a given event (see Figure3). 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.

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.

Monolithic
One or more parts constructed as one

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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 5-10 dB(A). Noise barriers must provide at least a 5-dB(A) 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 dB(A).
Non-Destructive Testing
The in situ determination of the physical properties of a component without impairment of, or removal of any material, excluding the necessary surface preparation for testing.
Normal Incident (Sound)
(Also referred to as 0-degrees incidence) Sound waves that strike a receiver at an angle perpendicular, or normal, to the angle of incidence.

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O

Oil Canning
A moderate deformation or buckling of sheet material, particularly common with flat sheet metal surfaces. Typically caused by uneven stresses at the fastening points. This terminology also refers to the popping sound made when pressure is applied to the deformed sheet forcing the deformation in the opposite direction.

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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.
Perturbation
The height increment that a noise barrier's input height is increased (perturbed up) or decreased (perturbed down) during the barrier design process.
Pile
A relatively slender deep foundation unit, wholly or partly embedded in the ground, installed by driving, drilling, augering, jetting or otherwise, and which derives its capacity from the surrounding soil and /or from the soil or rock strata below its tip.
Point Loads
Point loads are generated when loads are unevenly distributed and are concentrated on a very small area.
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.
Precast Members
Concrete elements cast in a location, other than their final position
Pressure Preservative Treated Wood
Wood impregnated under pressure with a chemical formulation which is toxic to fungi, insects, borers, and other wood destroying organisms
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
Proctor Density
The optimum unit weight of a soil determined in accordance with ASTM Standard D698ref.14

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Q

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R

Random Incident (Sound)
Sound waves that strike a receiver randomly from all angles of incidence. Such waves are common in a diffuse sound field.
REMEL
Reference Energy Mean Emission Level.
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 used for highway purposes

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S

Sabine Absorption Coefficient (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.ref.15
Shear Slip Circle
Defines the arc at which the soil will typically fail behind a fill or retained fill section
Shoulder
The part of a roadway contiguous to the traffic lanes for accommodating stopped vehicles, bickeways, and cleared snow.
Slope Hinge Point
The point at which the highway's sholder slope intersects the top of the highway embankment for a highway located on fill.

drawing of a lane with a shoulder and slope

Figure 2. Slope Hinge Point

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, LAE)
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 3)

chart comparing sound level to time in seconds

Figure 3. Graphical representation of LAE

In addition, LAE is related to LAeqT by the following equation:LAE = LAeqT + 10*log10(t2-t1) (dB)

where:

LAeqT = Equivalent sound level in dB

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/pref2) (dB )

where:

p = mean-square sound pressure; and
pref = 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.
Spalling
Separation and removal of a portion of the surface concrete.
Spectrum
A signal's resolution expressed in component frequencies or fractional octave bands.
Split
A lengthwise separation of the wood due to the tearing apart of the wood cells which usually extends from surface to surface.
Spread Footings
Large, horizontal, reinforced concrete slabs (see Figure 4), which transfer structure loads directly to the underlying soil, rock, or engineered soil. They are typically placed or poured just below the ground line. Noise barrier posts are either attached to or embedded into the center of these slabs. These footings support the lateral and dead loads for the noise barrier system.

Drawing of a spread footing foundation

Figure 4. Example of spread footing

Structures
Includes retaining walls, bridges, culverts, concrete drainage channels.
Subdrain
A pipe, perforated or non-perforated, which is placed in locations for the purpose of collecting sub-surface water and conveying it to a proper outlet.
Swale
A shallow ditch with gently sloping sides.
Sweep
The deviation of a piece of lumber from straightness.

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T

Ten-Percentile Exceeded Sound Level
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.
Through Check (Wood)
A check which extends from surface to surface of the wood and usually through the center of the pith.
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.ref.16 TL is determined as follows:TL=101og 10 [10(SPLs/10)/10(SRr/10)) (dB)

where: SPLs is the sound pressure level (see Section 3.1) on the source side of the barrier; and

SPLr is the sound pressure level on the receiver side of the barrier.

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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.

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V

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W

Water Table
The upper surface of the zone of saturation of the soil where the groundwater is not confined by an overlying impermeable formation.
Wavelength
The perpendicular distance between two wave fronts in which the displacements have a difference in phase of one complete period.
Weathering Steel
Is a type of finish on steel panels and structural members which allows the surface to rust at a controlled rate. This type of surface treatment is purported to be self protecting and normally does not require any other type of coating for protection.
Weatherometer Testing
Is a test procedure used to determine the effects that salt spray, salt fog, ultraviolet light and severe temperature changes have on a specific type of material or coating
Wind Classes
Near-ground wind effects are generally separated into three wind classes: upwind, calm, and downwind. When sound is propagating during upwind conditions, sound waves tend to refract upward away from the ground, which may result in a decrease in sound levels at a receiver. When sound is propagating during downwind conditions, sound waves tend to refract downward towards the ground, which may result in an increase in sound levels at a receiver.

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X

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Y

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Z

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Updated: 07/07/2011
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