# Sound Pressure : Definitions, Terms, Units, Measurements..

**Sound Pressure** is measured in Pascals, symbol Pa

**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 a
sound field quantity and not a
sound energy or
sound power quantity.

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

**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.

● 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.

**Sound Pressure Level : SPL** : Lp = 20 log (p/po) dB

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

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 |

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** is a sound field quality and uses the 20 log factor 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 also :

Standard Reference Levels.

Sound Levels and Integrating Sound Level Meters.

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

### 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** more commonly known as the RMS (root mean square) of the instantaneous sound pressure over a given period of time.

Also known as the Leq and the **Time Average Sound Pressure Level**

### at a point in a medium, difference between the pressure existing at the instant considered and the static pressure

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

### greatest absolute instantaneous sound pressure during a given time interval.

### sound pressure, conventionally chosen, equal to 20 µPa for gases and to 1 µPa for liquids and solids

Also known as

**Equivalent Continuous Sound Pressure Level**

### 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

**Weighted Sound Pressure Level** is also known as Sound Level