## Sound Pressure and related terms, definitions, glossary, units etc ...

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

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

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

**Sound pressure**is measured in pascals, symbol Pa. However the immense range of human hearing, 0.00002 pascals up to 200 pascals, means the pascal is not practical for everyday use. Fortunately the

*sound pressure level*in decibels, detailed below, neatly solves this problem.

**Sound Pressure Level (SPL)**= 20 log (p/po) dB, where p is the

*sound pressure*in pascals and po is the reference sound pressure of 0.00002 pascals = 0 dB in air ≡ to the threshold of hearing at 1KHz

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 = twice or half the pressure | 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 |

Reference Sound Pressure | 0.00002 Pa | 0 dB |

*Sound Pressure Level* is a sound field quantity and uses the 20 log factor so, as a rule of thumb:

6 dB = a factor of 2 in sound pressure (double or half the 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 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.

Related Terms - listed alphabetically

*Average Sound Pressure*see effective sound pressure

**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**is the root-mean-square of the instantaneous sound pressure.

*Effective Sound Pressure* is also known as the *average sound pressure*.

See also • the Effective Value Definition IEC 103-02-03

**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.

*Impact Sound Pressure Level*under sound insulation

**Instantaneous Sound Pressures**fluctuate over time, so a single sample tells us very little. However monitoring the levels over a period of time, accumulates very useful information. For example the effective sound pressure • equivalent sound pressure level • Leq • Lmax • peak sound pressure etc.

**Instantaneous Sound Pressure Definition IEC 801-21-19,** at a point in a medium, is the 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. Also known as the *peak sound level*.

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

**Peak Sound Pressure (Lpk or Lpeak)**is the maximum instantaneous sound pressure during a measurement period or noise event. Provides essential information in the assessment of noise induced hearing loss, but should not be confused with the Lmax. See also • Peak Hold, True Peak and Peak-to-Peak.

**Peak Sound Pressure Definition IEC 801-21-21,** greatest absolute instantaneous sound pressure during a given time interval.

See also • sound exposure action values and limit values.

**Reference Sound Pressure po** = 20 μPa ≡ 0 dB, the threshold of hearing at 1 kHz, see our sound pressure table above

**Reference Sound Pressure Definition IEC 801-21-22,** the sound pressure conventionally chosen, equal to 20 μPa for gases and to 1 μPa for liquids and solids.

See also • other standard reference levels.

**Sound Pressure Reflection Coefficient Definition IEC 801-31-05,** at a given frequency and for a given angle of incidence, in plane waves, ratio between the sound pressure amplitude of the reflected sound wave and that of the incident sound wave.

*Time-average Sound Pressure Level*under equivalent continuous sound pressure level

**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. *Weighted sound pressure level* is also known as the *sound level*

● Note 1: standard frequency weightings A, B and C and standard exponential time weightings fast (F), slow (S) and impulse (I) are given in IEC 651 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