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http://www.electroacoustics.co.uk/article/anon/data.htm
Db SPL Sound Pressure Level, in (power formula) decibels. Three Db is
twice the power, ten Db is ten times the power and twice the loudness. 6
Db, 4x PWR, 20 Db, 100x PWR.
http://www.makeitlouder.com/Decibel%20Level%20Chart.txt
http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/sound/u11l2b.html
http://www.exo.net/~pauld/activities/decibelmeters/decibelsdistancetime.html
Measure the sound level at a distance of 10 cm from the end of the rod. The
quickly move the meter to a distance of 20 cm. Note the change in the sound
level. Theory predicts that the sound intensity will decrease proportional to
the inverse square of distance. So doubling the distance should decrease the
sound intensity by a factor of 4. This would be measured as a decrease in the
sound level of 6 dB. Try other distances, 1 meter,2 m, 4m, 8m etc. Plot loudness
versus distance.
http://www.physicsclassroom.com/Class/sound/u11l2b.html
The mathematical relationship between intensity and distance is sometimes
referred to as an inverse square relationship. As the intensity varies inversely
with the square of the distance from the source. So if the distance from the
source is doubled (increased by a factor of 2), then the intensity is quartered
(decreased by a factor of 4). Similarly, if the distance from the source is
quadrupled, then the intensity is decreased by a factor of 16. Applied to the
diagram at the right, the intensity at point B is one-fourth the intensity as
point A and the intensity at point C is one-sixteenth the intensity at point A.
Since the intensity-distance relationship is an inverse relationship, an
increase in one quantity corresponds to a decrease in the other quantity. And
since the intensity-distance relationship is an inverse square relationship,
whatever factor by which the distance is increased, the intensity is decreased
by a factor equal to the square of the "distance change factor." The sample data
in the table below illustrate the inverse square relationship between power and
distance.