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

is the difference between the pressure caused by a sound wave and the ambient pressure of the medium, the sound 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.

**However the immense range of human hearing, 20 micropascals up to 200 Pa, means the Pascal is not practical for everyday use. Fortunately the ***Sound Pressure Level* decibel descriptor, detailed below, neatly solves this problem.

## Sound Pressure Level

in decibels is 20 x the logarithm of the ratio of the sound pressure to the reference sound pressure (0dB).

Sound Pressure Level = 20 log (p/po) dB, where po = 2 x 10^{-5} Pa = 20 μPa ≡ 0 dB in air ≡ to the threshold of hearing at 1KHz

Some typical Sound Pressure and Sound Pressure Levels
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 |

Threshold of hearing | 0.00002 Pa | 0 dB |

Sound Pressure Level (SPL or Lp) 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

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.

## Absolute Sound Pressure

is measured relative to the ambient pressure of the surrounding medium. The SI unit is the Pascal, symbol Pa

## 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 RMS (root mean square) of the instantaneous sound pressure, measured over a period of time, long enough to include cyclical noise level fluctuations. The suffix *eff* is often used in formulae, for example *Peff* = effective 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. Also known as the **Time-average Sound Pressure Level**

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

**NOT to be confused with the Equivalent Continuous Sound Level Definition IEC 801-22-16**

## Instantaneous Sound Pressure

levels usually fluctuate over time, so a single sample tells us very little. However if we record the **instantaneous values** over a period of time, we gather a lot of 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

the maximum instantaneous sound pressure during a measurement period or noise event, Symbol *Lpk or Lpeak*. Provides essential information in the assessment of noise induced hearing loss, **but should NOT to be confused with **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 ≡ 0dB, the threshold of hearing at 1kHz, 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 wave and that of the incident wave.

## 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, 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

Weighted Sound Pressure Level is also known as Sound Level

See also Sound Levels •
LA • LAF • LAFmax • LAFmin • LCpeak etc. and the IEC Definition of Level

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