Technical Background
Figure 5.18‐1.
Comparison in dB
Due to the technical nature of this resource topic and its importance to the
JLUS 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 rate of oscillation 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 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., dB scale) is used to present sound
intensity levels in a convenient format.
Since the human ear is not equally sensitive to all frequencies within the
entire spectrum, 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 1 dBA are only discernible under controlled, extremely quiet
conditions. A change of 5 dBA is typically noticeable to the general public in
an outdoor environment. Figure 5.18‐1 summarizes typical A‐weighted
sound levels for a range of indoor and outdoor activities.
Page 5‐70
Background Report
June 2017