# Acoustic Glossary

## Sound Pressure Terms ...

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

Sound Pressure (p) is a field quantity, also known as a root-power quantity but is not a power quantity.

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

The sound pressure unit is the pascal (Pa), however the immense range of human 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 Level (SPL) is the sound pressure expressed 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 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.

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.

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

Sound Pressure Level (SPL) 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 level (double or half)
10 dB = a factor of 3 in sound pressure level
20 dB = a factor of 10 in sound pressure level

See also our sound pressure level calculations article

### Sound Pressure Related Terms - listed alphabetically

Band Sound Pressure Level is the sound pressure level in a specified frequency band, usually of 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.

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, but should not be confused with the equivalent continuous sound level (IEC 801-22-16)

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 - not to 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 (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.

### Sound Pressure and Distance

Sound Pressure Levels decrease by a factor of 2 each time the distance from the source is doubled, under free-field conditions. In decibels this is 20·log (2) = 6 dB, see the inverse square law.

For example if the sound pressure level at 1 metre from the source is 100 dB (2 pascals) then the sound pressure level at 2 metres will be 94 dB (1 pascal) and at 4 metres distance the SPL will be 88 dB (0.5 pascals).

See also inverse proportionality and the inverse distance law

Sound Pressure Reflection Coefficient, the effect of the reflected sound wave on the sound pressure, over the audible range.

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

Weighted Sound Pressure Levels are both frequency weighted and time weighted measurements.

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