SPL Addition
Adding independent sources (different signals):
SPL_total = 10 × log₁₀(10^(SPL₁/10) + 10^(SPL₂/10) + ...)
Simplified — equal level sources:
SPL_total = SPL_single + 10 × log₁₀(N)
Where N = number of identical sources
| Sources | SPL addition |
|---|---|
| 2 | +3 dB |
| 4 | +6 dB |
| 8 | +9 dB |
| 10 | +10 dB |
| 100 | +20 dB |
Adding coherent sources (same signal, same phase):
SPL_total = SPL_single + 20 × log₁₀(N)
| Sources | SPL addition (coherent) |
|---|---|
| 2 | +6 dB |
| 4 | +12 dB |
Worked example — two subwoofers:
Two identical 12" subwoofers, each producing 100 dB at 1W/1m:
Incoherent (different signals — stereo): 100 + 3 = 103 dB Coherent (same mono signal, summed): 100 + 6 = 106 dB
This is why SPL competition uses mono signal and carefully phased multiple drivers — coherent addition gives maximum output.