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The Decibel scale is not straight arithmetic progression.
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Sound power level (volume) is a base 10 logarithmic progression. 3dB = 2x or 1/2 the sound energy depending upon whether the sound is going up or down in value.
That is different than sound pressure level as that is a base 20 logarithmic progression with 6dB making a 2x or 1/2 change in sound pressure level.
The problem comes in when you try and calculate sound reduction by simply subtracting one value from another. With logarithms, addition = multiplication and subtraction = division.