What is Sound Pressure?

What is Sound Pressure?

What is Sound Pressure?

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
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 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:
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
Sound pressure levels in decibels from some typical 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
Virtual silence, Barely audible
Audio-metric test room
Rustling leaves
Noticeably Quit - Voice, soft whisper
Quiet whisper (4 ft, 1 m) 30
Quiet room
Bird call
Quiet street
Quiet office
Whispered speech
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
Loud singing
Car driven at 105 km/h, 65 mph
Washing machine
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)
Noisy factory
Newspaper press
Subway (inside) 94
Very Loud
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)
Power saw
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)
Pneumatic chipper
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
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

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