Environmental Engineering Reference
In-Depth Information
Measured Sound Power Levels
There are two important measures of the magnitude or intensity of sound that can
easily be confused. These are the sound pressure level and the sound power level.
Sound Pressure Level
Sound pressure level is a measure of the intensity of sound at a listener's location, and
as such it is a combination of the radiated acoustic power of the noise source and the
propagation of that power from the source to the listener. Thus the sound pressure level at
an observer location will depend on factors such as the distance from the source, the source
directivity, and the propagation path. The latter includes effects due to the atmosphere
(absorption, refraction) and terrain (absorption, shielding and diffraction). As explained
previously, all sound pressure levels presented in this chapter are based on root-mean-
square (RMS) values of pressure and are averaged over 30 to 180 seconds, depending on
the frequency bandwidth. Pressure levels in decibels are referenced to the threshold of
hearing, typically defined as 20 micro-Pascals, as follows:
SPL = 20 log 10 ( p / p 0 )
(7-10)
where
SPL = sound pressure level (dB or dBA)
p = RMS acoustic pressure (Pa)
p 0 = reference acoustic pressure, 2 x 10 -5 Pa
A sound pressure level can be measured with a microphone or microphone/meter at the
listener's location. Sound pressure levels can vary considerably with location and time,
because they depend not only on the generation of sound but also on its propagation.
Therefore, sound pressure level alone is not a convenient parameter for assessing and
comparing the acoustic outputs of wind turbines. Another measure is needed, and that is
sound power level.
Sound Power Level
Sound power level is a characteristic of the noise source, and as such it is independent
of the environment around the source and the location of the listener. It is expressed in
decibels and referenced to a power of 1 picowatt, as follows:
PWL = 10 log 10 ( P A / P 0 )
(7-11)
where
PWL = sound power level (dB)
P A = acoustic power of the sound source (W)
P 0 = reference acoustic power, 1x10 -12 W
The sound power level of a wind turbine is a noise characteristic of that turbine that
does not depend on either the environment in which it is located or the relative location of
a listener. For example, Søndergaard and Plovsing [2005] measured the sound power level
of a 2 MW wind turbine situated in interior Danish waters, and found that the offshore
levels were 1 dB to 3 dB above the onshore levels measured for the same model of wind
turbine located on land. This difference is consistent with expected machine-to-machine
variation when both are located on land.
While a sound power level of a wind turbine cannot be measured directly with a meter,
it can be determined from sound pressure levels measured nearby, by assuming (a) the wind
Search WWH ::




Custom Search