Acoustic Glossary


 

Sound Pressure and related terms, definitions ... including

Band Sound Pressure LevelEffective Sound Pressure LevelEquivalent Continuous Sound Pressure LevelInstantaneous Sound PressurePeak Frequency Weighted Sound Pressure LevelPeak Sound PressureReference Sound PressureSound PressureSound Pressure LevelSound Pressure Reflection CoefficientWeighted Sound Pressure Level


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.

Sound Pressure Definition

IEC 801-21-20, 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 are not practical for everyday use, see the range of Pascals in the table below and the sound pressure level practical solution.


Sound Pressure Level in decibels is 20 x the logarithm of the ratio of the sound pressure to the reference sound pressure of 20 micropascals.

Sound Pressure Level (SPL or Lp) = 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.


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


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


Impact Sound Pressure Level under Sound Insulation


Instantaneous Sound Pressure

(p) is the existing or measured sound pressure at a particular instant in time, see also the effective sound pressure, equivalent sound pressure and peak sound pressure on this page
 

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 (Lpk or Lpeak), the maximum instantaneous sound pressure during a measurement period or noise event. Very important in the assessment of sound exposure at work and not to be confused the Lmax. See also peak hold, true peak and other time weightings

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 Value. and Time Constants and Weightings


Reference Sound Pressure (po) was chosen to equal the threshold of hearing = 20 micropascals at 1kHz.

If you divide the measured sound pressure by the reference sound pressure, convert to logarithms and multiply the results by 10, then you get the sound pressure level in decibels (dB), i.e. more manageable numbers

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

Weighted Sound Pressure Level is also known as Sound Level


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

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