What is Noise?
SOUND – A vibratory disturbance capable
of being detected by the ear.
NOISE – Unwanted
sound that may interfere with normal activities.
NOISE IS AN UNINVITED GUEST!
Noise is measured in
are established on a logarithmic scale.
A 10 dB
increase represents a doubling in noise to the human
ear. For example, 60 dB
is perceived to be twice as loud as 50 dB.
The unit of measure is dB(A).
not hear all noise frequencies equally.
A-weighted scale indicates sound is filtered similar
to the human ear, which reduces the strength of very
low and very high frequencies. Without A-weighting,
a noise monitor would respond to noise events people
cannot hear, such as a dog whistle.
below shows common outdoor and indoor sound
levels with associated
The Leq is used to describe the period as an
overall noise level. The graph below represents the
time-varying data for a 50-second monitoring period.
For this period, the approximate Leq is 72 dB(A).
PERCEPTION OF NOISE CHANGES
- Changes less than 3 dB(A) are not typically
perceived by a human listener with average hearing.
- Changes from 3 to 5 dB(A) will be perceived by
humans with sensitive ears.
- Changes greater
than 5 dB(A) are readily perceived by humans with
NOISE REDUCTION WITH DISTANCE
Highway noise is generated by a line of
vehicles closely spaced. This gives a listener the
perception of a line noise source rather than a
single, identifiable point of noise. As distance
increases from the highway, noise is reduced.
Generally, every time the distance doubles, the
noise level will decline 3 dB(A) when it travels
over hard surfaces.
Over soft surfaces, the noise level will
decline 4.5 dB(A) for every doubling of distance.
For example, assume traffic produces a noise level
of 75 dB(A) measured 50 feet from the highway:
Soft Site - If grass is the predominant
cover, then at 200 ft, the noise level will be 9
dB(A) lower, or 66 dB(A).
Hard Site - If asphalt is the predominant
cover, the resulting noise level at 200 ft. will
be 6 dB(A) lower, or 69 dB(A).
NOISE FROM MOBILE SOURCES
- Gear Box and Transmission
- Tire/Road Noise
- Aerodynamics of vehicle
TRAFFIC NOISE LEVELS AT THE SOURCE
Highway noise generation is dependent on three
main factors, including:
Number of trucks in the traffic
Each of these varies at any given moment.
HOW TRAFFIC VOLUMES AFFECT NOISE
Doubling the traffic volume (e.g. from 1,000 to
2,000 vehicles per hour) will increase the sound
level by 3 dB(A).
FREQUENTLY ASKED QUESTIONS
- At what level will hearing damage occur?
Generally, 120 dB(A) is recognized as the
threshold of pain and considered a dangerous
noise level. Noise levels less than 120dB(A) can
damage hearing if the listener is exposed to the
noise for an extended time period. Noise levels
less than 90 dB(A) are generally not recognized
as noise levels that can cause hearing damage.
Typically, traffic noise levels in areas of
frequent human use do not approach these noise
levels. A 90 dB(A) traffic noise level would
occur if a person stood 10 to 20 feet from a
roadway carrying approximately 1,000 trucks per
hour. It is unlikely that residents would be
exposed to this level of noise, and therefore it
is unlikely residents experience hearing damage
due to traffic noise.
- What is
Leq is the equivalent steady-state sound level
which in a stated period of time contains the
same acoustic energy as a time-varying sound
level during the same period.
- How loud is 67 dB(A)?
A sound level of 67 dB(A) is associated with
normal speech at 3 ft. Other examples are shown
in the "Common Sound Levels" chart within this
Additional information regarding noise issues is
available in our "Highway
Traffic Noise Assessment Manual".