Environmental Engineering Reference
In-Depth Information
3 S ound level
In order to understand how sound propagate we need to identify the nature of
sound. Sound is a series of waves and it is characterised by two properties: ampli-
tude (loudness) and frequency. Therefore there are sounds that consist of combi-
nations of low and high magnitude with low and high frequency. The human ear
can detect a very wide range of both sound levels and frequencies, but it is more
sensitive to some frequencies than others.
Sound is generated by numerous mechanisms and is always associated with
rapid small-scale pressure fl uctuations, which produce sensations in the human
ear. Sound waves are characterised in terms of their amplitude, wavelength (
l
),
frequency (
f
) and speed
c
, as follows:
(1 )
cf
=
l
The speed of sound is a function of the medium through which it travels, and it
generally travels faster in more dense mediums. Sound propagates as a wave as
shown in Fig. 7. In air it travels at a speed of 340 m/s and in water 1500 m/s. As
sound travels it transports acoustic energy with it which attenuates the further it
travels. As the sound propagates it disturbs the fl uid from its mean state.
The pressure at a position
x
is
p
=
p
0
+
p
/
p
0
<< 1.
Sound is measured in dB and the sound pressure level (SPL) is defi ned as
′
(
x
,
t
) with
p
′
−
5
( 2)
SPL
=
20 log
(
p
/ 2
×
10
) dB
′
10
rms
p
′
is the mean square level of fl uctuation [ 5 ].
As sound energy travels through the air, it creates a sound wave that exerts pres-
sure on receivers such as an ear drum or microphone and it makes our eardrums
vibrate [6]. Human whisper releases an acoustic power of 10
− 10
W and a large jet
transport at take off emits about 10 W.
The threshold of pain is between 130 and 140 dB. The threshold of hearing is
around 0 dB. The sound
power
level from a single wind turbine is usually between
90 and 105 dB(A). Figure 8 shows a few examples of sound pressure levels from
everyday life.
As described above, the decibel scale is logarithmic. A sound level measure-
ment that combines all frequencies into a single weighted reading is defi ned as a
where
rms
Figure 7: Sound propagation.
Search WWH ::
Custom Search