Sound Pressure
The Sound Pressure is the force of sound on a surface area perpendicular to
the direction of the sound.
The Sound Pressure
The Sound Pressure is the force (N) of sound on a surface
area (m2) perpendicular to the direction of the sound. The SI-units
for the Sound Pressure are N/m2 or Pa.
Sound is usually measured with microphones responding
proportionally to the sound pressure - p.
The power in a sound wave goes as the square of the pressure.
(Similarly, electrical power goes as the square of the
voltage.) The log of the square of x is just 2 log x, so this introduces a
factor of 2 when we convert to decibels for pressures. The difference in sound
pressure level between two sounds with p1 and p2 is therefore:
The Sound Pressure Level
The lowest sound pressure possible to hear is approximately
2 10-5 Pa (20 micro Pascal, 0.02 mPa), 2 ten billionths of a an
atmosphere.
It therefore convenient to express the sound pressure as a
logarithmic decibel
scale related to this lowest human hearable sound - 2 10-5 Pa, 0
dB.
The Sound Pressure Level:
Lp
= 10 log( p2 / pref2 ) = 10 log( p / pref )2
= 20 log ( p / pref
) (1)
where
Lp
= sound pressure level (dB)
p
= sound pressure (Pa)
pref
= 2 10-5 - reference sound pressure (Pa)
If the pressure is doubled, the sound pressure level is
increased with 6 dB ( 20 log (2) ).
The table below indicates the sound pressure level in
decibel caused by some common sources.
Source
|
Sound Pressure Level
(dB) |
Threshold of hearing
|
0
|
Rustling leaves
|
20
|
Quiet whisper (1 m)
|
30
|
Home
|
40
|
Quiet street
|
50
|
Conversation
|
60
|
Inside a car
|
70
|
Loud singing
|
75
|
Automobile (10 m)
|
80
|
Motorcycle (10 m)
|
88
|
Food blender (1 m)
|
90
|
Subway (inside)
|
94
|
Diesel truck (10 m)
|
100
|
Lawn mower (1 m)
|
107
|
Pneumatic riveter (1
m)
|
115
|
Chainsaw (1 m)
|
117
|
Amplified Hard Rock
(2 m)
|
120
|
Jet plane (30 m)
|
130
|
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