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This presentation has been developed to provide a
general understanding of highway traffic noise. This
is the first of a three-part program.
Part I explains what noise is, how noise is
measured, how noise is perceived, how noise changes
with distance, and how mobile sources affect noise.
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!
Measuring Noise
Noise is measured in
decibels
(dB). Decibels are established on a
logarithmic
scale .
On this scale, a 10 dB increase in noise
represents a doubling in noise to the human ear.
For example, 60 dB is perceived to be twice as loud
as 50 dB.
Noise is composed of a range of frequencies,
measured in
hertz.
The average human with normal hearing can hear
sounds with
frequencies
ranging from 20 to 20,000 hertz.
Since humans are not equally
sensitive to all frequencies, noise meters use the A-weighted
scale to filter sound similar to the
human ear. Without A-weighting, noise monitors
would respond to noise events people cannot hear,
such as a dog whistle. A-weighted noise levels
are reported using the units of
dBA.
Highway noise is evaluated using the A-weighted
scale as it:
-
Approximates the ear’s sensitivity to sound;
-
Matches surveys studying human annoyance to
noise;
-
Has been used since the early 1930s; and
-
Is used as the basic unit of noise by most
governmental agencies.
Generally, noise ranges from 0 dBA, the
approximate threshold of hearing, to around 120
dBA, which approaches the point at which people
feel pain from sound.
Figure 1
illustrates common indoor and outdoor sound
levels.
Noise levels are random and vary with time.
To describe noise, a single number value is used to
represent the time-varying noise. This single
number value is the
equivalent sound level or Leq.
The Leq describes the overall noise level for a
specified period of time. The graph below represents
the time-varying data and the overall Leq for a
50-second monitoring period. For this period,
the approximate Leq is 72 dBA. 
Addition of Noise Levels
Because noise is measured on a logarithmic scale,
70 dBA plus 70 dBA does not equal 140 dBA.
Two sources of equal noise added together result
in an increase of 3 dBA. That is, 70 dBA
plus 70 dBA yields a total noise level of 73
dBA. Therefore, doubling traffic volumes
will increase the noise level by 3 dBA.
Perception of
Noise Changes
-
A 3 dBA change in noise levels is not
typically perceived by persons with average
hearing.
-
Some people can detect a change in noise
levels between 3 dBA and 5 dBA.
-
Changes greater than 5 dBA are readily
perceived by people with average hearing.
How Noise Changes
with Distance
Highway noise is generated by a line of vehicles
closely spaced. This gives a listener the
perception of a
line source rather than a single,
identifiable point
source.
As distance increases from the highway, noise is
reduced. Generally, every time the
distance doubles, the noise level will decline 3
dBA when it travels over hard surfaces (e.g.
asphalt). Over soft surfaces (e.g. grass),
the noise level will decline 4.5 dBA for every
doubling of distance.
For example, assume traffic produces a noise
level of 75 dBA measured 50 feet from the highway:
Soft Site
If grass is the predominant cover, then at 200
ft, the noise level will be 9 dBA lower, or 66 dBA.
Hard Site
If asphalt is the predominant cover, the
resulting noise level at 200 ft will be 6 dBA lower,
or 69 dBA.
How Mobile Sources Affect Noise
| At Low Speeds |
Engine
Gear Box and Transmission
Exhaust |
| At High Speeds |
Tire/Road Noise
Aerodynamics of vehicle |
Traffic noise levels at the source depend on
three main factors:
-
Traffic Volume
-
A doubling of the traffic volume (e.g. from
1,000 to 2,000 vehicles per hour)
increases the sound level by 3 dBA*
-
Traffic Speed
-
As speed increases, noise levels increase.
-
Number of Trucks within the Traffic.
-
One truck traveling at 55 miles per hour
generates as much noise as 28 cars
traveling at the same speed.
*Source: FHWA, Highway Traffic Noise, Fact Sheet,
http://www.fhwa.dot.gov/environment/htnoise.htm
This completes the first part
of a 3 part series on Highway Noise.
Part II includes
information on noise analysis, noise monitoring and
computer noise modeling. Part
III
includes information on noise abatement evaluations |
