reduction of an acoustic signal
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
A-Weighted levels 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
Build Condition Modeling 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.
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
Decibels (dB) Units for measuring sound.
dBA Decibels measured using the A-weighted
Level (Leq) 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.
Frequencies The number of cycles of a periodic
motion in a unit of time.
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
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
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
Line source Many single sources close together
(i.e. multiple vehicles on a
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
Noise Abatement Criteria
Noise impact thresholds for considering noise
abatement for various land uses.
Noise Sensitive Area (NSA) An area, a group, or cluster of
noise sensitive receptors, such as
residences with similar traffic
Parallel Walls Two walls constructed on the opposite sides of
Peak hourly traffic The highest hourly traffic volume
of the day.
Point source One single source (i.e. one
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.
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
Type II Projects A proposed project for noise
abatement on an existing fully
controlled-access state highway
(freeway or expressway) in an urban