Travis AFB Sustainability Study Report Final Background Report | Page 213
and either cause sound to travel further and be louder or reduce the
distance traveled and the level at which the sound can be heard.
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 focus 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.
Technical Background
Due to the technical nature of this resource topic and its importance to the
TSS process, this section provides a discussion of the characteristics of sound
and the modeling process used to evaluate noise impacts.
Characteristics of Sound
It is important to understand that there is no single perfect way of
measuring sound, due to variations used by different entities when
conducting sound studies or sound modeling. Sound is characterized by
various parameters that include the oscillation rate of sound waves
(frequency), the speed of propagation, and the pressure level or energy
content (amplitude). The sound pressure level has become the most
common descriptor used to characterize the loudness of an ambient sound
level. The decibel (dB) scale is used to quantify sound intensity. Because
sound pressure can vary by over one trillion times within the range of
human hearing, a logarithmic loudness scale, i.e., the dB scale, is used to
present sound intensity levels in a convenient format.
The human ear is not equally sensitive to all frequencies within the entire
spectrum, so noise measurements are weighted more heavily within those
frequencies of maximum human sensitivity in a process called “A‐weighting”
written as dBA. The human ear can detect changes in sound levels of
approximately 3‐dBA under normal conditions. Changes of 1 to 3‐dBA are
typically noticeable under controlled conditions, while changes of less than
1dBA are only discernible under controlled, extremely quiet conditions.
A change of 5‐dBA is typically noticeable to the average person in an
outdoor environment. Figure 5.18‐1 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, how fast the noise fluctuates, and the amount of variation.
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
TSS Background Report
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