Lewis L. Spence
The unit used for the measurement of sound intensity level is called decibel, dBA sometimes dB for a change in decibel levels. A sound which doubles in perceived loudness, means that there is a change of about10 decibel. This implies that a sound with an intensity change of 20 dB has increased its loudness by a multiple of four. Similarly, a drop of 10 dB in intensity of the sound means that the loudness has dropped to half its original level. If a sound source 100 feet away has an intensity of 90 dB and you were to move to a distance 200 feet from the source, the intensity of the sound would drop by 6 dB (if the distance from the source is doubled, there is a drop of 6 dB). This is true for the behavior of sound outdoors.
Source Level (in dBA)
Faintest audible sound 0
Whisper 20
Quiet residence 30
Soft stereo in residence 40
Cafeteria 80
Cafeteria kitchen 90
Loud crowd noise 100
Accelerating motorcycle 110
Hard rock band 120
Jet engine (75 feet away) 140
