The definition of dB SPL is the 20 log of the ratio between the measured sound pressure level and the reference point. This reference point is defined as 0.000002 Newton's per square meter, the threshold of hearing.
+3dB = 2x the power = 1.44x the SPL
+6dB = 4x the power = 2x the SPL
The doubling of SPL represents in four times power increase.
One acoustic watt is equivalent to 107.5 dBSPL at around a meter from an omnidirectional source.
This human sensitivity curve changes for soft and load sounds, discovered by Fletcher and Munson 1933:
The A-weighted sound level represents the human hearing and hearing damage in the possible best way. Without any other information the A-weighted sound level is the best information available for valuing noise problems.
See the A-weighted Table.
See Speech Level.
In the signal chain the reference level of " 0dB " ( = +6dBu ) also should equal an SPL listening level for radio control monitoring etc. of 92 dB(A).
For movie sound mixing the acoustical reference is defined by SMPTE to 85 dB SPL, read with a sound pressure level meter (SPL) set on C weighted/Slow scale. To achieve the acoustic level of 85 dB SPL a electric level (pink noise) of -18 dB FS is defined (SMPTE RP 200).
|Thunderclap, Air Raid Siren 1 Meter||130|
|Jet takeoff (200 ft)||120|
|Rock Concert, Discotheque||110|
|Firecrackers, Subway Train||100|
|Heavy Truck (15 Meter), City Traffic||90|
|Alarm Clock (1 Meter), Hair Dryer||80|
|Noisy Restaurant, Business Office||70|
|Air Conditioning Unit, Conversational Speech||60|
|Light Traffic (50 Meter), Average Home||50|
|Living Room, Quiet Office||40|
|Library, Soft Whisper (5 Meter)||30|
|Broadcasting Studio, Rustling Leaves||20|
|Acoustical Power (Watts)||SPL (dB)|