The resultant sound pressure level of two sound sources, each having a sound pressure level of 50 dB, is

When two equal sound sources each produce L dB, combined SPL = L + 10*log10(2) = L + 3 dB. General: L_combined = 10*log10(10^(L1/10) + 10^(L2/10)). Adding +3 dB per doubling of identical sources is the standard noise control rule of thumb. For two sources with different levels, the combined level is dominated by the louder source. Two 50 dB sources combine to 53 dB.

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