**Sound Pressure** is the difference between the pressure caused by a sound wave and the ambient pressure of the medium it is passing through. **Sound Pressure** is measured in Pascals, symbol Pa

**Sound Pressure** is a sound field quantity, NOT a sound energy or sound power quantity.

**Sound Pressure Definition** (IEC 801-21-20) RMS (root mean square) of instantaneous sound pressures over a given time interval, unless specified otherwise

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

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 |

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

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

10 dB = a factor of 3 in sound pressure

20 dB = a factor of 10 in sound pressure

**Sound Pressure Level Definition** (IEC 801-22-07) 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

● Note 1 : unless otherwise specified, the **reference sound pressure** is 20 µP 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.

See also :
Standard Reference Levels.

Sound Levels and Integrating Sound Level Meters.

LA • LAF • LAFmax • LAFmin • LCpeak etc. and the IEC Definition of Level

**Band Sound Pressure Level Definition** (IEC 801-22-12) level of the **sound pressure** produced within a specified frequency band

● Note : the band may be specified by its lower and upper cut-off frequencies, or by its geometric centre frequency and bandwidth. The width of the band may be indicated by a modifier such as one octave band (sound pressure) level, one-half octave band level, one-third octave band level.

**Effective Sound Pressure Level** is the RMS (root mean square) of the instantaneous sound pressure over a given period of time.

**Equivalent Continuous Sound Pressure Level Definition** (IEC 801-22-11) 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.

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

**Instantaneous Sound Pressure Definition** (IEC 801-21-19) at a point in a medium, difference between the pressure existing at the instant considered and the static pressure

**Peak Frequency-weighted Sound Pressure Level Definition** (IEC 801-22-15) greatest instantaneous value of a standard frequency weighted **sound pressure level**, within a stated time interval

● Note : if frequency weighting is not specified, the A-frequency weighting is understood.

**Sound Pressure Gradient** is the change in **sound pressure** with distance.

**Weighted Sound Pressure Level Definition** (IEC 801-22-14) logarithm of the ratio of a given **sound pressure** to the **reference sound pressure** of 20 μPa, the **sound pressure** being obtained with a standard frequency weighting and with a standard exponential time weighting. Sound level in decibels is twenty times the logarithm to the base ten of that ratio

● Note 1 : standard frequency weightings A and B used in IEC 61400-11 are given in IEC 60651 (1979) : Sound level meters.

● Note 2 : time and frequency weightings employed should be specified, but if not stated explicitly, Fast (F) exponential time weighting and A-frequency weighting are understood