Sound Pressure and related terms, definitions, units etc.,
is the difference between the pressure caused by a sound wave and the ambient pressure of the medium, the sound 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.
However the immense range of human hearing, 20 micropascals up to 200 Pa, means the Pascal is not practical for everyday use. Fortunately the Sound Pressure Level decibel descriptor, detailed below, neatly solves this problem.
Sound Pressure Level in decibels is 20 x the logarithm of the ratio of the sound pressure to the reference sound pressure (0dB).
Sound Pressure Level = 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 = 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 |
| 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.
Absolute Sound Pressure is measured relative to the ambient pressure of the surrounding medium. The SI unit is the Pascal, 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 is the RMS (root mean square) of the instantaneous sound pressure, measured over a period of time, long enough to include cyclical noise level fluctuations. The suffix eff is often used in formulae, for example Peff = effective 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. Also known as the Time-average Sound Pressure Level
● Note: unless otherwise specified, the reference sound pressure for airborne sound is 20 μPa.
NOT to be confused with the Equivalent Continuous Sound Level Definition IEC 801-22-16
Instantaneous Sound Pressure levels usually fluctuate over time, so a single sample tells us very little. However if we record the instantaneous values over a period of time, we gather a lot of useful information. For example the effective sound pressure • equivalent sound pressure level • Leq • Lmax • peak 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 the maximum instantaneous sound pressure during a measurement period or noise event, Symbol Lpk or Lpeak. Provides essential information in the assessment of noise induced hearing loss, but should NOT to be confused with 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 1kHz, 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.
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 Levels •
LA • LAF • LAFmax • LAFmin • LCpeak etc. and the IEC Definition of Level
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