# Acoustic Glossary

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

Sound Pressure (p) is the difference between the pressure caused by a sound wave and the ambient pressure of the media the sound wave is passing through.

Sound Pressure (p) is a field quantity, also known as a root-power quantity and is not a 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 Relationships:

Sound pressure = sound intensity ÷ particle velocity
Sound pressure = (sound intensity × acoustic impedance)
Sound Pressure = particle velocity × acoustic impedance

Sound Pressure SI unit is the pascal, 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, discussed below, neatly solves this problem.

Sound Pressure Level (SPL) formula = 20 log (p/po) dB, where p is the sound pressure in pascals and po is the reference sound pressure of 0.00002 pascals in air, equivalent to the threshold of hearing at 1KHz = 0dB (decibels). The SI units of sound pressure levels are pascals, expressed in decibels, see the table below.
 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

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.

See also our sound pressure level calculations article

### 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, 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. Also known as the time-average sound pressure level.

Equivalent continuous sound pressure level should not be confused with
Equivalent continuous sound level, to compare these two different definitions directly click here

Impact Sound Pressure Level under sound insulation

Instantaneous Sound Pressures Levels, fluctuate over time, so a single sample tells us very little. However monitoring the levels over a period of time, also known as time averaged, accumulates very useful information. For example the effective sound pressureequivalent sound pressure levelLeqLmaxpeak 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.

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

Reference Sound Pressure Level (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.

Sound Pressure Levels and Distance from the Source. Under free-field conditions i.e. no reflecting surfaces, the sound pressure level reduces by 6 dB every time the distance from the source is doubled. This is because, left to it's own devices, sound will radiate uniformly in all directions (spherically) and consequently the sound pressure is 'spread' over larger and larger areas as the distance from the source increases.

For example if the sound pressure level at 1 metre from the source is 2 pascals (100 dB) then the sound pressure level at 2 metres will be 1 pascal (94 dB) and at 4 metres distance the SPL will be 0.5 pascals (88 dB). Often referred to as the inverse square law, but 'strictly speaking' complies with inverse proportionality and the inverse distance law

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 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 equivalent continuous sound pressure level and should be confused with the equivalent continuous sound level. To compare these definitions directly click here

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

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