Sound Pressure
Sound Pressure is the force of sound on a surface perpendicular to the propagation of sound.
Sound Pressure (Pa)
Sound Pressure is the sound force (N) acting on the surface area (m^{2}) perpendicular to the direction of the sound.
 the SIunit for sound pressure is Pa or N/m^{2}
Sound is usually measured with microphones responding proportionally to the sound pressure. 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.
Sound Pressure Level (decibels)
The lowest sound pressure possible to hear is approximately 2 10^{5} Pa (20 micro Pascal, 0.02 mPa) or 2 ten billionths of an atmosphere. This minimum audible level occurs normally between 3000 and 4000 Hz.
For a normal human ear pain is experienced at a sound pressure of order 60 Pa or 6 10^{4} atmospheres.
Due to this range it is convenient to express sound pressure with a logarithmic decibel scale related typically to the lowest human hearable sound  2 10^{5} Pa or 0 dB.
Since the energy in the sound wave is proportional to the square of the pressure  the Sound Pressure Level in decibel can be expressed as:
L_{p} = 10 log (p^{2} / p_{ref}^{2})
= 10 log (p / p_{ref})^{2}
= 20 log (p / p_{ref}) (1)
where
L_{p} = sound pressure level (dB)
p = sound pressure (Pa)
p_{ref} = 2 10^{5}  reference sound pressure (Pa)
Doubling sound pressure (in Pa)  increases sound pressure level (in dB) with 6 dB (or 20 log (2)).
The chart below shows the sound pressure level decibel scale compared to the sound pressure Pascal scale.
Download and print Sound Pressure Level (dB) vs. Sound Pressure (Pa) Chart!
Measuring Sound Pressure
Most Sound Level Meters measures the effective sound pressure which can be expressed as
p_{e} = p_{a} / 2^{1/2} (2)
where
p_{e} = measured (effective) pressure (Pa)
p_{a} = maximum pressure amplitude in the sound wave (Pa)
Sound Pressure Levels
Sound pressure levels in decibels from some typical sources:
Source  Sound Pressure Level (dB) 

Threshold of Hearing  
Quietest audible sound for persons with excellent hearing under laboratory conditions^{2)}  0 
Quietest audible sound for persons under normal conditions  
Virtual silence, Barely audible Audiometric test room 
10 
Rustling leaves Mosquito 
20 
Noticeably Quit  Voice, soft whisper  
Quiet whisper (4 ft, 1 m)  30 
Home Quiet room Bird call 
40 
Moderate  
Quiet street Quiet office Whispered speech 
50 
Loud  Unusual Background, Voice conversation 4 ft, 1 m  
Normal conversation at 4 ft, 1 m  60 
Laughter  65 
Loud  Voice conversation 1 ft, 0.3 m 

Inside a car Passenger car 80 km/h, 50 mph (50 ft, 15 m) Vacuum cleaner (10 ft, 3 m) Freight Train (100 ft, 30 m) Background conversation restaurant 
70 
Loud singing Car driven at 105 km/h, 65 mph Washing machine 
75 
Loud  Intolerable for Phone Use  
Maximum sound up to 8 hour (OSHA criteria  hearing conservation program) Pneumatic tools (50 ft, 15 m) Buses, diesel trucks, motorcycles (50 ft, 15 m) Car wash (20 ft, 6 m) Road with busy traffic 
80 
Motorcycle (30 ft, 10 m)  88 
Food blender (4 ft, 1 m) Maximum sound up to 8 hour (OSHA^{1)} criteria  engineering or administrative noise controls) Jackhammer (50 ft, 15 m) Bulldozer (50 ft, 15 m) Noisy factory Newspaper press 
90 
Subway (inside)  94 
Very Loud  
Diesel truck (30 ft, 10 m) Motor horns at distance of 7 m 
100 
Lawn mower (4 f t, 1 m)  107 
Pneumatic riveter (4 ft, 1 m)  115 
Threshold of Discomfort  
Large aircraft (500 ft, 150 m over head) Power saw 
110 
Chainsaw (4 ft, 1 m) Very noisy work  boilermakers workshop, etc. 
117 
Deafening, Human pain limit  
Amplified Hard Rock (6 ft, 2 m) Siren (100 ft, 30 m) Pneumatic chipper Drums 
120 
Jet plane (90 ft, 30 m) Artillery Fire (10 ft, 3 m) 
130 
Upper limit for unprotected ear for impulses Threshold of pain 
140 
Short exposure can cause hearing loss  
Military Jet Takeoff (100 ft, 30 meter)  150 
Large military weapons  180 
^{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, Aweighted, as an 8hr timeweighted average (85 dB(A) as an 8hr TWA). Exposures at or above this level are hazardous.
^{2)} The reference level  10^{12}  for the decibel scale
Typical Subjective Description of Sound Pressure Level
 0  40 dB : quiet to very quiet
 60  80 dB : noisy
 100 dB : very noisy
 > 120 dB : intolerable
Averaging Sound Pressure Level from a number of Sound Pressure Level Readings
The sound pressure level from a number of sound pressure level readings can be calculated as:
L_{a} = 10 log (1/n (10^{0.1 L1} + 10^{0.1 L2} + ... + 10^{0.1 Ln}))
where
L_{a} = average sound pressure level (dB)
L_{1} .. L_{n} = sound pressure level source 1 .. n (dB)
n = number of sources