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 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.