Decibel sound levels cannot be added directly because sound powers combine, not levels

Evidence

Note that the combined sound level is only about 2 dB more than the louder of the two sounds and not 69 dB greater, which is the result that would be obtained if the SPLs were added directly in decibels. — David Howard, Jamie Angus, coustics and Psychoacoustics, 278

Categorization

• CANDY: A fun bar-trivia-style fact: two singers at 69 and 71 dB together only reach about 73 dB — doubling sources adds only 3 dB.
• DEFINE: Illustrates that dB is a logarithmic scale and can’t be added arithmetically; pressures square before summing.
• XREF: connects to: A decibel value is meaningless without an attached reference level — Almo Farina, Soundscape Ecology: Principles,…, 1943