# Sound Pressure

## Sound Pressure is the force of sound on a surface area perpendicular to the direction of sound

### Sound Pressure (Pa)

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. 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 (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. The minimum audible level occurs between 3000 and 4000 Hz. For a normal human ear pain is experienced at sound pressures of  order 60 Pa or 6 10-4 atmospheres

It is convenient to express sound pressure with the logarithmic decibel scale related to the lowest human hearable sound - 2 10-5 Pa or 0 dB.

Sound Pressure Level can be expressed as:

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 (or 20 log (2)).

### Measuring Sound Pressure

Most Sound Level Meters measures the effective sound pressure which can be expressed as

pe = pa / 21/2                                      (2)

where

pe = measured (effective) pressure  (Pa)

pa = maximum pressure amplitude in the sound wave (Pa)

### Sound Pressure Levels

Sound pressure levels in decibels caused by some common sources:

SourceSound Pressure Level
(dB)
Threshold of Hearing
Quietest audible sound for persons with excellent hearing under laboratory conditions2) 0
Quietest audible sound for persons under normal conditions
Virtual silence
Audio-metric test room
10
Rustling leaves 20
Noticeably Quit - Voice, soft whisper
Quiet whisper (1 m) 30
Home
Quiet room
40
Moderate

Quiet street
Whispered speech

50
Loud - Unusual Background, Voice conversation 1 m
Conversation at 1 m 60
Loud - Voice conversation 0.3 m
Inside a car
Passenger Car 80 km/h (15 m)
Vacuum cleaner (3 m)
Freight Train (30 m)
70
Loud singing 75
Loud - Intolerable for Phone Use

Automobile (10 m)
Maximum sound up to 8 hour (OSHA criteria - hearing conservation program)
Pneumatic tools (15 m)
Buses, diesel trucks, motorcycles (15 m)

80
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)
Noisy factory
Newspaper press
90
Subway (inside) 94
Very Loud
Diesel truck (10 m)
Motor horns at distance of 7 m
100
Lawn mower (1 m) 107
Pneumatic riveter (1 m) 115
Threshold of Discomfort
Large aircraft (150 m over head) 110

Chainsaw (1 m)
Very noisy work - boilermakers workshop, etc.

117
Deafening, Human pain limit

Amplified Hard Rock (2 m)
Siren (30 m)
Pneumatic chipper

120
Jet plane (30 m)
Artillery Fire (3 m)
130
Upper limit for unprotected ear for impulses 140
Short exposure can cause hearing loss
Military Jet Take-off (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, A-weighted, as an 8-hr time-weighted average (85 dB(A) as an 8-hr 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

## Related Topics

• Acoustics - Room acoustics and acoustic properties - decibel A, B and C - Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation
• Noise and Attenuation - Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems

## Tag Search

• en: sound pressure level force area
• es: área vigor nivel de presión sonora
• de: Schalldruckpegel Kraftbereich

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