How do sound levels add?
Sound pressure levels in decibels (dB) or A-weighted decibels [dB(A)] are based on a logarithmic scale (see Appendix A). They cannot be added or subtracted in the usual arithmetical way. If one machine emits a sound level of 90 dB, and a second identical machine is placed beside the first, the combined sound level is 93 dB, not 180 dB. 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 x lg ( 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.
