Environmental Engineering Reference
In-Depth Information
DID YOU KNOW?
•
Adding together two identical noise sources will increase the total
sound-power level by 3 dB (10 log
2
).
•
Sound power
and
sound-power level
are often used to specify the
noise of sound emitted from technical equipment such as fans, pumps,
or other machines.
•
Sound measured with sensors or microphones is
sound pressure
.
•
SPL = sound production level.
•
Decibel (dB) = unit of sound production level (logarithmic scale).
•
dBA = A-weighted scale decibel (levels weighted according to sound
frequency).
•
L
eq
= equivalent continuous sound level.
•
SEL = sound equivalent level integrated per 1 second.
•
Hertz (Hz) = cycles per second (measure of sound frequency).
•
1 kilohertz (kHz) = 1000 hertz.
•
ROW = right of way (pertaining to highway noise)
Sound Pressure
Units used to describe sound pressures are
1 µbar = 1 dyne/cm
2
= 0.1 N/cm
2
= 0.1 Pa
(2.3)
Sound Pressure Level
SPL (decibels) = 10 log(
p
2
/
p
0
)
(2.4)
where
p
= Measured rms sound pressure (N/m
2
, µba rs).
*
p
0
= Reference rms sound pressure (20 µPa, N/m
2
, µba rs).
Speed of Sound
c
=
f
1
(2.5)
Wavelength
λ =
c
/
f
(2.6)
Octave Band Frequencies
Upper frequency band =
f
2
= 2
f
1
(2.7)
where
f
2
= Upper frequency band.
f
1
= Lower frequency band.
*
The root-mean-square (rms) value of a changing quantity, such as sound pressure, is the square root of
the mean of the squares of the instantaneous values of the quantity.
