detect changes in sound levels of approximately three dBA under normal conditions.
Changes of one to three dBA are typically noticeable under controlled conditions, while
changes of less than one dBA are only discernible under controlled and extremely quiet
conditions.
Figure 5‐19
Sound Levels Comparison in dB
A change of five dBA is
generally noticeable to the
average person in an outdoor
environment. Figure 5‐19
summarizes typical A‐weighted
sound levels for a range of
indoor and outdoor activities.
Environmental noise fluctuates
over time. While some noise
fluctuations are minor, others
can be substantial. These
fluctuations include regular and
random patterns, changes in
how fast the noise fluctuates,
and changes in the amount of
variation between fluctuations.
Weather patterns can have a
strong effect on how far sound
travels and how loud it is.
Certain weather events can
change the consistency of the
air and either allow sounds to
travel farther and be perceived
easier (seem louder) or inhibit
wave travel and its
perceptibility. Temperature
and wind velocity are prime examples of factors that can affect sound travel. Sound
tends to travel further in cold temperatures. Specific combinations of temperature and
wind direction can create atmospheric refraction. Atmospheric refraction occurs when
atmospheric conditions bend and / or direct sound waves towards some areas and away
from others. When describing noise impacts, it is common to look at the average noise
levels over an entire average day.
Background Report
5-111