East Avenue Loop, PSB Docket No. _____
Exhibit PET 17.2 - A Noise Primer
April 16, 2007
A NOISE PRIMER
1. WHAT IS NOISE?
Noise is defined as “a sound of any kind, especially when loud, confused, indistinct, or
disagreeable.”1 Passing vehicles, a noisy refrigerator, or an air conditioning system are sources of
noise which may be bothersome or cause annoyance. These sounds are a part of generally accepted
everyday life, and can be measured, modeled, and, if necessary, controlled.
2. HOW IS SOUND DESCRIBED?
Sound is caused by variations in air pressure at a range of frequencies. Sound levels that are
detectable by human hearing are defined in the decibel scale, with 0 dB being the threshold of human
hearing, and 135 dB causing pain and permanent damage to the ear. Figure 1 shows the sound levels
of typical activities that generate noise. For example, urban street traffic generates 85 dBA, while
normal activities in a business office may register 55 dBA.
The decibel scale is logarithmic, which tends to weight louder noises. Therefore, it approximates the
human perception of relative loudness very well. For example, in a quiet environment, the noise of a
single car passing by would be very noticeable and cause a substantial increase in the decibel level;
while on a busy street, a single additional car would be barely noticeable.
In addition, the decibel scale can be weighted to emphasize human perceptions of annoying
frequencies. The most common of these weighting scales is the “A” weighting and this scale is used
most frequently in environmental noise analysis. Sound levels that are weighted by the “A” scale have
units of dBA. The “C” weighting, with units of dBC, is often used for high-energy sounds, such as
blasts.
To account for changes in noise over time, a weighted average sound level called the “equivalent”
sound level (Leq) is often used. The Leq averages total noise pressure, and results in weighting loud
and infrequent noises more heavily than softer and frequent noises. The Leq is also often used in
environmental noise analysis.
1 “The American Heritage Dictionary of the English Language,” Houghton Mifflin Company, 1981.
A Noise Primer
Resource Systems Group, Inc.
page 2
The day-night sound level (Ldn) is also commonly used in referencing community noise. The Ldn is
simply a time-weighted average, similar to Leq, except that the nighttime sound levels are weighted by
+10 dB.
3. HOW IS NOISE MODELED?
The decibel sound level is a logarithmic scale. One manifestation of this is that sound pressure
increases by a factor of 10 for every 20 dB increase. However, for every 10 dB increase, we perceive an
approximate doubling of loudness. Small changes in sound level, below 3 dB, are generally not
perceptible.
For a point source of noise, its sound level diminishes or attenuates by 6 dB for every doubling of
distance due to geometrical divergence. For example, if an idling truck is measured at 50 feet as 66
dBA, at 100 feet it will be heard as 60 dBA, and at 200 feet, 54 dBA. Along a road, sound is
generated as a line source. Line sources attenuate their sound at 3 dB to 4.5 dB per doubling distance,
depending on the sound reflectivity of the ground.
Other factors, such as intervening vegetation, walls, berms, buildings, and atmospheric absorption
will also further reduce the sound level reaching the listener. In each of these, higher frequencies will
attenuate faster than lower frequencies. Finally, the ground can also have an impact on sound levels.
Harder ground generally increases and softer ground generally decreases the sound level at a receiver.
Reflections off of buildings and walls can increase broadband sound levels by as much as 3 dB.
If we add two equal noise sources together, the resulting sound level will be 3 dB higher. For
example, if one machine registers 86 dBA at 50 feet, two machines would register 3 dB more, or 89
dBA at that same distance. In a similar manner, at a distance of 50 feet, four machines, all operating
at the same place and time, would register 92 dBA and eight machines would register 95 dBA.
If two sources of noise differ by 2 to 4 dB, an increase of about 2 dB will occur when they are added
together. A difference of 5 dB to 9 dB between two noise sources will increase overall sound levels
by about 1 dB. If two sources of noise differ by more than 9 dB, then the increase in the combined
decibel level will be less then 0.5 dB.
A Noise Primer
Resource Systems Group, Inc.
page 3
Figure 1: Common Sounds in Decibels
Decibels (dBA)
Near a jet engine
140
Threshold of Pain
130
Deafening
Hard Rock Band
120
Chainsaw at ear
110
Auto horn at 10 feet
100
Electric hand drill, at ear
Riding lawn mower, at ear
Very Loud
90
Shop-vac, at ear, outdoors
Inside car, windows open, 65 mph
Truck passby, 30 mph, at 50 feet
Vacuum cleaner, at ear
80
Loud
Inside car, windows closed, 65 mph
70
Car passby, 30 mph, at 50 feet
TV in quiet room
Microwave oven at 2.5 feet
60
Moderate
50
Refrigerator at 3 feet
40
Faint
Quiet rural area, no wind or traffic
30
Quiet night, rural area, no background
traffic or wind
20
Very Faint
10
Threshold of audibility at 1000 Hz
0