Page 289 - Fluid Power Engineering

Environmental Impact of W ind Projects 255

Sound Pressure Level, dBA Noise Source
140 Jet engine at 25 m
120 Rock concert
100 Jackhammer at 1 m
80 Heavy truck traffic
60 Conversational speech & TV
50 Library
40 Bedroom
30 Secluded wood
20 Whisper

Source: From A Guide to Noise Control in Minnesota, Minnesota Pollution Control
Agency, St. Paul, MN, 2008.

TABLE 12-2 dBA of Common Noise Sources

energy contained in a sound wave with respect to a reference wave
of 0 decibels. For a given frequency, the energy is proportional to the
square of the amplitude of the pressure in the pressure wave.

SE SP

Sound level in decibels = 10 log = 20 log (12-1)
10 10
RE RP
where SE, SP is the measure of sound energy, sound pressure, and RE,
RP is the reference energy, reference pressure. Reference pressure =
2 × 10 −5 Pa.
dB(A) is the A-weighted decibels for sensitivity to human ear.
Because the human ear does not have a flat spectral response, sound
pressure levels are weighted based on audible frequencies, which are
the higher harmonics of middle A (between 2 and 4 kHz). In the rest
of the document, dBA is used as the unit of measure of sound power
and sound pressure.
Sound pressure describes the effect on a receptor (a person listen-
ing) and is specified as X dBA at a distance of Y meters. Sound power,
on the other hand, describes the power of an emitter, for instance,
a wind turbine emits 105 dBA. Table 12-2 contains decibel levels of
common noise sources. Another important aspect is ambient noise
or background noise. Wind is a major source of ambient noise. In
wind projects, the relevant sound level is the level above the ambient
noise.
Since sound is a compression wave, the decibel level drops quickly
as distance from the sound source is increased. For a point source of