Sound Level Addition Calculator
Add multiple decibel levels using logarithmic combination.
Calculate combined SPL from two or more noise sources and convert between dB and sound intensity.
Why You Can’t Just Add Decibels Decibels are on a logarithmic scale: 60 dB + 60 dB ≠ 120 dB. Two equal sound sources add only 3 dB (not double) because intensity doubles. 60 dB + 60 dB = 63 dB. 70 dB + 60 dB ≈ 70.4 dB (louder one dominates).
Combining Sound Levels L_total = 10 × log₁₀(10^(L₁/10) + 10^(L₂/10) + … + 10^(Lₙ/10)) This converts each source back to intensity (relative), sums them, then converts back to dB.
Quick Rules Same level, 2 sources: L_total = L + 3 dB Same level, 4 sources: L_total = L + 6 dB Same level, 10 sources: L_total = L + 10 dB Difference ≥ 10 dB: louder source dominates (+ 0.4 dB or less) Difference 5 dB: combined = louder + 1.2 dB Difference 3 dB: combined = louder + 1.8 dB Difference 0 dB: combined = louder + 3 dB
dB Reference Values dB SPL (sound pressure level): reference 20 μPa (threshold of hearing) dB SIL (intensity): reference 10⁻¹² W/m² L_p = 20 × log₁₀(p / p_ref) — pressure ratio L_I = 10 × log₁₀(I / I_ref) — intensity ratio
Common Sound Levels (dB SPL) Threshold of hearing: 0 dB | Whisper: 30 dB Conversation: 60 dB | Traffic: 70 dB Lawnmower: 90 dB | Concert: 110 dB Jet takeoff: 130 dB | Pain threshold: 140 dB
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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