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Adding Decibels

When Two Decibel Levels Differ By Add the Following Number to the Higher Value
0-1 dB 3 dB
2-3 dB 2 dB
4-9 dB 1 dB
10 dB or More 0 dB
88 dB + 90 dB = 92 dB
75 dB = 81 dB = 82 dB
70 dB = 80 dB = 80 dB

When the direct sound diminishes in intensity in accordance with the inverse law the multitude of reflective sound intensities are combining to produce as increase in the reflected sound levels to a point where the reflected sound can be higher than the direct sound. A typical example of this phenomena would be a voluminous, hard surfaced gymnasium that can experience a significant build up of reflective sound intensity.

Where the direct sound and reflected sound are about equal is called the critical distance. In a typical classroom critical distance is about 12″ from the source. Beyond the critical distance the sound reduction will be less than 6 dB.