Sound Pressure Definition
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 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.
|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 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.
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
Also known as the peak sound level.
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.
See also other acoustic reference levels.
Sound Pressure and DistanceSound 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 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 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