Adding sound intensities of equal level yields only a three-decibel increase

Evidence

For equal sound pressure level L1 = L2, we reach an increment 11L = 3 dB; for a difference L1 - L2 = 2 dB, we get 11L of about 2 dB; for L1 - L2 = 6 dB, the increment is only 1 dB. For differences L1 - L2 larger than 10 dB, the influence of L2 can be ignored in most of the cases. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1329

Categorization

• CANDY: Nice bar-conversation fact: two equal sounds add only 3 dB; a 10 dB-louder one swallows the quieter entirely.