## Sound Pressure Terms and Definitions ...

**Sound Pressure** (p) is the change in the ambient pressure of any media a sound wave passes through.

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 a field quantity, also known as a root-power quantity but is not a power quantity.

The **Sound Pressure Unit** is the pascal (Pa), however the range of our hearing, 0.00002 pascals up to 200 pascals means the pascal is not practical for everyday use - the sound pressure level in decibels, neatly solves this problem.

**Sound Pressure Relationships:**

**sound pressure** = sound intensity ÷ particle velocity

**sound pressure** = √(sound intensity × acoustic impedance)

**sound pressure** = particle velocity × acoustic impedance

**Sound Pressure Level** (SPL) is the sound pressure in decibels, to compress the wide range of pascals we hear into manageable numbers and is usually A-weighted to take into account the frequency response of our hearing mechanism.

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.

**Sound Pressure Level Formula** = 20 log (p/po) where p is the sound pressure and po is the reference sound pressure of 0.00002 pascals = 0 dB the threshold of hearing at 1 kHz.

The **Sound Pressure Level Unit** is the decibel (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 levels (double or half)

10 dB = a factor of 3 in sound pressure levels

20 dB = a factor of 10 in sound pressure levels.

The following chart demonstrates the huge range of pascals we hear and the more practical *sound pressure levels* in decibels.

Sources at 1 m | Sound Pressure | SPL re 20 μPa |

Rifle | 200 Pa | 140 dB |

Threshold of pain * | 20 Pa | 120 dB |

2 Power Mowers | 2 Pa | 100 dB |

1 Power Mower | 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 |

See also our sound pressure level calculations article

**Sound Pressure Related Terms - listed alphabetically**

**Average Sound Pressure** under effective sound pressure.

**Band Sound Pressure Level** is the sound pressure level in a standard, frequency band, usually of one octave or 1/3 octave bandwidth.

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 and also known as the *average sound pressure*.

**Equivalent Continuous Sound Pressure Level** is the time-averaged sound pressure level over a stated measurement period in decibels.

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.

**Not to be confused with the** equivalent continuous sound level.

**Impact Sound Pressure Level**under sound insulation

**Instantaneous Sound Pressure** is the sound pressure at a particular instant in time and specific point in the sound field.

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** is the maximum instantaneous sound pressure level during a measurement

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** (Lpk) is the maximum instantaneous sound pressure during a measurement period or noise event. Also known as Lpeak but should not be confused with the Lmax.

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

See also peak hold, true peak and peak-to-peak • sound exposure action and limit values.

**Reference Sound Pressure** (po) = 20 μPa ≡ 0 dB, the threshold of hearing at 1 kHz in air.

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 acoustic reference levels.

Sound Pressure Inverse Square Law

**Sound Pressure Reflection Coefficient**, when a sound wave reaches the boundary between two media, a portion of the wave is reflected and a portion is transmitted, the portions are frequency dependent.

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** is the frequency and time weighted *sound pressure level* in decibels.

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 the sound level

See also sound levels • LA • LAF • LAFmax • LAFmin • LCpeak etc., and the IEC Definition of Level