Acoustic Glossary


 

Sound Pressure and related terms, definitions, units etc.,


Sound Pressure

is the difference between the pressure caused by a sound wave and the ambient pressure of the medium the sound wave is passing through.

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

Sound pressure is measured in pascals, 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, detailed below, neatly solves this problem.

Sound Pressure Level (SPL or Lp)

in decibels is 20 x the logarithm of the effective sound pressure divided by the reference sound pressure (20 μPa).

Sound Pressure Level = 20 log (p/po) dB, where p is the sound pressure in pascals and po is the reference sound pressure of 0.00002 pascals = 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 = twice or half the pressure 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.

Related Terms - listed alphabetically

Average Sound Pressure see the 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.
Effective Sound Pressure is 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.

Equivalent Continuous Sound Pressure Level is also known as the Time-average sound pressure level and should Not be confused with the Equivalent continuous sound level definition IEC 801-22-16.
To compare these definitions directly click here


Impact Sound Pressure Level under Sound Insulation

Instantaneous Sound Pressures

fluctuate over time, so a single sample tells us very little. However monitoring the levels over a period of time, 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. See also • Peak Hold, True Peak and Peak-to-Peak.

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 values.


Reference Sound Pressure

po = 20 μPa ≡ 0dB, 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.

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.

Time-average Sound Pressure Level under equivalent continuous sound pressure level

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

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