One of the most common questions is, “If device A is 65 dB and device B is 68 dB, what’s the total?” The answer depends on logarithms, not simple addition. This post demystifies the process with rules of thumb and a method you can use in under a minute.
Why logarithms?
Sound levels represent ratios; the scale compresses a huge range into manageable numbers. That’s why two equal sources add only about 3 dB and why a much quieter source barely moves the needle.
The method
- Convert each level Lᵢ to power Pᵢ = 10^(Lᵢ/10).
- Sum the powers: P_total = ΣPᵢ.
- Convert back: L_total = 10·log₁₀(P_total).
Rules of thumb
- Equal levels: +3 dB.
- Within 1–2 dB: ~+2–2.5 dB.
- Within 3–5 dB: ~+1–1.5 dB.
- 10 dB difference: < +0.5 dB (negligible).
Scenarios
Two fans at 70 dB: 73 dB total. 68 dB + 65 dB: Convert → 6.3M + 3.2M = 9.5M → 69.8 dB. 60 dB + 50 dB: 1M + 0.1M = 1.1M → 60.4 dB.
Adding distance
If you also change distance, adjust each source first: in free field, doubling distance is ~−6 dB. Then combine. Example: a 90 dB mower at 1 m and an 80 dB trimmer at 1 m; listener at 4 m. Adjust: mower 90→78 dB, trimmer 80→68 dB; combine ≈ 78.4 dB.
What about A vs C weighting?
If sources have different spectra (one boomy, one bright), A‑weighted values may combine differently from C‑weighted ones. Combine values that use the same weighting to keep assumptions consistent.
Documentation matters
When you report totals, state weighting, time response, distance, and sources. That way, a colleague (or your future self) can reproduce or build on the result.