Acoustic Glossary


 

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

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

Because of the immense range of human hearing the absolute sound pressure values are not practical for everyday use, however the Sound Pressure Level dB descriptor is a neat solution, see the table below.


Sound Pressure Level in decibels is 20 x the logarithm of the ratio of the sound pressure to the reference sound pressure of 20 micro Pascals.
 
Sound Pressure Level : SPL : Lp = 20 log (p/po) dB
 
Reference Sound Pressure Level : po = 2 x 10-5 Pa = 20 µPa ≡ 0 dB in air
Some typical Sound Pressure Levels in air
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
 

The Reference Sound Pressure Level was chosen conventionally to correspond to the quietest sound at 1000 Hz that the human ear can detect i.e. the threshold of hearing

SPL is a sound field quality 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 level 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 : Standard Reference Levels.
Sound Levels and Integrating Sound Level Meters.
LA • LAF • LAFmax • LAFmin • LCpeak etc. and the IEC Definition of Level


Absolute Sound Pressure Level is the sound pressure level in Pascals, 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 Level is the RMS (root mean square) of the instantaneous sound pressure 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 IEC 810-22-16 Equivalent Continuous Sound Level Definition

Instantaneous Sound Pressure : p is the change in ambient pressure caused by the sound pressure at an instant in time.

Instantaneous Sound Pressure Definition (IEC 801-21-19) at a point in a medium, 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.


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

Reference Sound Pressure Definition (IEC 801-21-20) sound pressure, conventionally chosen, equal to 20 µPa for gases and to 1 µPa for liquids and solids

Sound Pressure Gradient is the change in sound pressure with distance.


Time-average Sound Pressure Level also known as Equivalent Continuous Sound Pressure Level listed above

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 and B used in IEC 61400-11 are given in IEC 60651 (1979) : 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 the Sound Level
 

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