### 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.

root mean square of the instantaneous sound pressures over a given time interval, unless specified otherwise

**Because of the immense range of human hearing, ***Sound Pressure* values in Pascals are not practical for everyday use, see the full range in the table below and the *Sound Pressure Level* decibel solution.

### Sound Pressure Levels

in decibels are 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 = 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 |

**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 given period of time, long enough to include cyclical noise level fluctuations. The suffix *eff* is often used in formulae, for example **Peff **

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.

Also known as the **Time-average Sound Pressure Level**

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

### Instantaneous Sound Pressures

usually fluctuates 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.

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

Also known as the **Peak Sound Level**

### Peak Sound Pressure

the maximum instantaneous sound pressure during a measurement period or noise event. Essential information in the assessment of noise induced hearing loss. Symbol *Lpk or Lpeak*, see also Peak Hold, True Peak and Lmax

### 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

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

### Reference Sound Pressure Definition IEC 801-21-22,

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

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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