What is Sound Pressure?
When it comes down to it, your ears are pretty amazing. They allow you to hear sounds as quiet as a whisper and as loud as a jet engine. How exactly do sound waves get transmitted to your ears so that you can hear?
It starts with a vibration in the air. A whisper only vibrates the air a little bit, while a jet engine vibrates it A LOT. This vibration creates a pressure wave that passes through the air, spreading out in all directions as it goes. Sound pressure is the pressure measured within the wave relative to the surrounding air pressure. Loud sounds produce sound waves with relatively large sound pressures, while quiet sounds produce sound waves with relatively small sound pressures.
Sound pressure, like other kinds of pressure, is commonly measured in units of Pascals (Pa). The quietest sound that most people can hear has a sound pressure of 2 x 10-5 Pa, so this pressure is called the threshold of human hearing.
If your ear happens to be in the path of the sound wave, the vibrating air molecules hit your eardrum and cause it to start vibrating too. When your eardrum starts vibrating, it bumps into the tiny bones in your middle ear and makes them vibrate. The vibration passes all the way into your inner ear where the vibration of tiny hair cells sends signals to your brain, thus letting you hear the sound. The pressure of the sound wave corresponds to how loudly you hear the sound. As the sound pressure increases, the pressure on your eardrum increases as well, making the sound seem louder to you.
Sound Pressure is the force of sound on a surface area perpendicular to the direction of sound
Sound Pressure (Pa)
Sound Pressure is the sound force (N) acting on the surface area (m2) perpendicular to the direction of the sound.
- the SI-unit for sound pressure is Pa or N/m2
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:
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)
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.
Measuring Sound Pressure
Most Sound Level Meters measures the effective sound pressure which can be expressed as
pe = pa / 21/2 (2)
pe = measured (effective) pressure (Pa)
pa = maximum pressure amplitude in the sound wave (Pa)
Sound Pressure Levels
- Recommended maximum sound pressure levels in rooms with activities
|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|
|Virtual silence, Barely audible
Audio-metric test room
|Noticeably Quit - Voice, soft whisper|
|Quiet whisper (4 ft, 1 m)||30|
|Loud - Unusual Background, Voice conversation 4 ft, 1 m|
|Normal conversation at 4 ft, 1 m||60|
|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
Car driven at 105 km/h, 65 mph
|Loud - Intolerable for Phone Use|
|Maximum sound up to 8 hours (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
|Motorcycle (30 ft, 10 m)||88|
|Food blender (4 ft, 1 m)
Maximum sound up to 8 hour (OSHA1) criteria - engineering or administrative noise controls)
Jackhammer (50 ft, 15 m)
Bulldozer (50 ft, 15 m)
|Diesel truck (30 ft, 10 m)
Motor horns at distance of 7 m
|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)
|Chainsaw (4 ft, 1 m)
Very noisy work - boilermakers workshop, etc.
|Deafening, Human pain limit|
|Amplified Hard Rock (6 ft, 2 m)
Siren (100 ft, 30 m)
|Jet plane (90 ft, 30 m)
Artillery Fire (10 ft, 3 m)
|Upper limit for unprotected ear for impulses
Threshold of pain
|Short exposure can cause hearing loss|
|Military Jet Take-off (100 ft, 30 meter)||150|
|Large military weapons||180|
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