# Sound Pressure : Definitions, Terms, Units and Measurement

**Sound Pressure** is the difference between the pressure produced by a sound wave and the ambient pressure of the medium it is passing through.

**Sound Pressure** is measured in
Pascals - symbol Pa, units
Newton per square metre; 1 Pa = 1 N/m^{2} = 1 J/m^{3} = 10^{-5} bar = 1 kg/(m·s^{2})

**Sound Pressure** is a
Sound Field quantity and not a
Sound Energy or
Sound Power quantity.

**Sound Pressure Level IEC Definition : ** logarithm of the ratio of a given **sound pressure** to the **reference sound pressure level** in decibels is 20 times the logarithm to the base ten of the ratio. So **Sound Pressure Level : SPL = 20 log (p/po) dB**

♦ Note 1 unless otherwise specified, the reference sound pressure is 20 µPа for airborne sound and 1 µPa for sound in media other than air.

♦ Note 2 unless otherwise specified, the sound pressures are understood to be expressed in root-mean-square values.

## Because of the immense range of human hearing, see the table below, the absolute sound pressure values are not practical for everyday use. So the Logarithmic **Sound Pressure Level** dB descriptor is a neat solution.

Some typical Sound Pressure Levels in air
Sources at 1 m | Sound Pressure | SPL re 20 µPa * |

Rifle | 200 Pa | 140 dB |

Threshold of pain | 20 Pa | 120 dB |

Pneumatic hammer | 2 Pa | 100 dB |

6 dB = double the Pa | 1 Pa | 94 dB |

Street traffic | 0.2 Pa | 80 dB |

Talking | 0.02 Pa | 60 dB |

Library | 0.002 Pa | 40 dB |

TV Studio | 0.0002 Pa | 20 dB |

Threshold of hearing | 0.00002 Pa | 0 dB |

* **Reference Sound Pressure po in air = 2 x 10**^{-5} Pa = 20 µPa = 0 dB

The **Reference Sound Pressure** was chosen conventionally to correspond to the quietest sound at 1000 Hz that the human ear can detect i.e. the Threshold of Hearing

SPL uses the 20 lg equation so, as a rule of thumb:

6 dB = a factor of 2 in sound pressure

10 dB = a factor of 3 in sound pressure

20 dB = a factor of 10 in sound pressure

see other Standard Reference Levels.

see other Sound Levels and Integrating Sound Level Meters.

see also L_{A}, L_{AF}, L_{AFmax}, L_{AFmin}, L_{Cpeak} etc..

**Effective Sound Pressure** more commonly known as the RMS - Root Mean Square of the Instantaneous Sound Pressure over a given period of time.

**Equivalent Continuous Sound Pressure Level** under Leq - Equivalent Continuous Sound Level

**Instantaneous Sound Pressure : ***p* is the change in ambient pressure caused by a sound wave at an instant in time.

**Instantaneous Sound Pressure** IEC Definition at a point in a medium, difference between the pressure existing at the instant considered and the static pressure.

**Peak Sound Pressure** IEC Definition greatest absolute instantaneous sound pressure during a given time interval.
Symbol : Lpeak or Lpk not to be confused with Lmax

**Reference Sound Pressure** IEC Definition sound pressure, conventionally chosen, equal to 20 µPa for gases and to 1 µPa for liquids and solids

**Sound Pressures** measured or 'averaged' over time are known as
RMS (root mean square) or
Integrating-averaging Sound Levels or
Leq, Equivalent Levels or
Statistical, L10, L90, etc., Levels

**Time Average Sound Pressure Level : LAT** : the equivalent steady level over a given period of time that contains the same amount of noise energy as the actual fluctuating level.

**Time-average Sound Pressure Level** IEC Definition logarithm of the ratio of a given root-mean-square sound pressure, during a stated time interval, to the reference sound pressure. Average sound pressure level in decibels is 20 times the logarithm to the base ten of that ratio

♦ Note : unless otherwise specified, the reference sound pressure for airborne sound is 20 µPa.

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