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How do sound levels add?
If there are two sound sources in a room - for example a radio producing an average sound level of 62.0 dB, and a television producing a sound level of 73.0 dB - then the total sound level is a logarithmic sum ie
Combined sound level = 10*log ( 10^(62/10) + 10^(73/10) ) = 73.3 dB
Note: for two different sounds, the combined level cannot be more than 3 dB above the higher of the two sound levels. However, if the sounds are phase related there can be up to a 6dB increase in SPL.
How does sound change with distance?
As you get further from a sound, it’s level will decrease and this property of sound is often used in noise control exercises. The change in sound pressure level for a point source in free field can be calculated using the equation:-
attenuation = 20*log(d2/d1)
The practical effect of this is that if you double the distance, the sound pressure level decreases by 6dB.
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