Sound Pressure is the force of sound on a surface area perpendicular to the direction of the sound
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 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)
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 |
|Threshold of Hearing|
|Quietest audible sound for persons with excellent hearing under laboratory conditions2)||0|
|Quietest audible sound for persons under normal conditions|
|Rustling leaves, quiet room||20|
|Noticeably Quit - Voice, soft whisper|
|Quiet whisper (1 m)||30|
|Loud - Unusual Background, Voice conversation 1 m|
|Loud - Voice conversation 0.3 m|
|Inside a car |
Car (15 m)
Vacuum cleaner (3 m)
Freight Train (30 m)
|Loud - Intolerable for Phone Use|
Automobile (10 m)
|Motorcycle (10 m)||88|
|Food blender (1 m) |
Maximum sound up to 8 hour (OSHA1) criteria - engineering or administrative noise controls)
Jackhammer (15 m)
Bulldozer (15 m)
|Diesel truck (10 m) |
Motor horns at distance of 7 m
|Lawn mower (1 m)||107|
|Pneumatic riveter (1 m)||115|
|Threshold of Discomfort|
|Large aircraft (150 m over head)||110|
Chainsaw (1 m)
|Deafening, Human pain limit|
|Amplified Hard Rock (2 m) |
Siren (30 m)
|Jet plane (30 m) |
Artillery Fire (3 m)
|Short exposure can cause hearing loss|
|Military Jet Take-off (30 meter)||150|
1) OSHA - Occupational Safety and Health Act - The OSHA criteria document reevaluates and reaffirms the Recommended Exposure Limit (REL) for occupational noise exposure established by the National Institute for Occupational Safety and Health (NIOSH) in 1972.
The REL is 85 dB, A-weighted, as an 8-hr time-weighted average (85 dBA as an 8-hr TWA). Exposures at or above this level are hazardous.
2) The reference level - 10-12 - for the decibel scale.