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.
|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 hearingSPL is a sound field quality and uses the 20 log factor so, as a rule of thumb:
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.
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.
Instantaneous Sound Pressure : p is the change in ambient pressure caused by the sound pressure at an instant in time.
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.
Sound Pressure Gradient is the change in sound pressure with distance.
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