The Inverse Square Law ProblemIn an open area, sound drops off according to the inverse square law. In an auditorium where the front seats are 6 meters (20 ft) from the sound source and the back seats are 60 m (200 ft) from the sound source, the sound intensity would drop by a factor of 100 ( = 20 decibels ) between the front seats and the back seats if it followed this pattern. This is an unacceptable loss which is prevented partially by reverberation.
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Inverse Square Law ExampleIf a sound level of 90 decibels is produced at the front row of an auditorium and the back row is 10 times as far away, then the sound intensity will fall by a factor of 100 if the inverse square law applies. A factor of 100 is a drop of 20 decibels to a level of 70 decibels on the back row. 90 dB is a loud sound which would be assigned a dynamic level ff , but 70 dB is a medium soft sound, dynamic level mp.
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Inverse Square Law by RatioExpressed as a ratio, the inverse square law takes the form You can explore numerically to confirm that doubling the distance drops the intensity by about 6 dB and that 10 times the distance drops the intensity by 20 dB.
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