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If the two tvs are at the same volume level, combined they will be only 3 decibels louder then a single unit, this is given in an open environment, though it may be a little louder because of reflection off the walls. A 3dB increase is only a slight increase in the noticed sound level. This doesn't take into account the frequency levels, as ours ears will applify or attenuate certain frequecies.
Heres the formulas for those who care:
(P1/Pref)squared = antilog(db_level/10)
same for second sound source
(Ptotal/Pref)^2 = (P1/Pref)+(P2/Pref)squared
So if you had two t.v.'s giving off an equal sound level of 75 dB each.
(P1/Prer)^2 = (P2/Pref)^2=3.16*10^7
(Ptot/Pref)^2 = 6.32*10^7
When converted back into dBs the new sound level is 78 dB.
where P1 and P2 and Ptot are the sound pressure levels in N/m^2
dB_level is the measured sound pressure level in decibels.
Hope that makes sense to everyone!
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