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.
Quick Reference: dB Addition Rules of Thumb
| Level Difference | Total Increase | Example |
|---|---|---|
| Equal levels (0 dB apart) | +3 dB | Two 70 dB fans → 73 dB |
| 1 dB apart | +2.5 dB | 70 + 71 dB → 73.5 dB |
| 2 dB apart | +2.1 dB | 70 + 72 dB → 74.1 dB |
| 3 dB apart | +1.8 dB | 70 + 73 dB → 74.8 dB |
| 5 dB apart | +1.2 dB | 70 + 75 dB → 76.2 dB |
| 6 dB apart | +1.0 dB | 70 + 76 dB → 77.0 dB |
| 10 dB apart | +0.4 dB | 70 + 80 dB → 80.4 dB |
| 15 dB apart | +0.1 dB | 70 + 85 dB → 85.1 dB (negligible) |
Worked Scenarios: Real Environments
| Scenario | Combined Level | Notes |
|---|---|---|
| Home office: PC (45 dB) + AC (52 dB) | 52.5 dB | AC dominates; PC adds barely 0.5 dB |
| Kitchen: dishwasher (55) + microwave (60) + vent (58) | 62.9 dB | Microwave dominates; all three = 62.9 dB |
| Workshop: table saw (100) + dust collector (85) | 100.1 dB | Dominant source rules — 15 dB gap means collector is negligible |
| Street: traffic (70) + HVAC (68) | 72.1 dB | Close levels: combine for ~72 dB total |
| Concert: PA (105) + stage wedge (95) | 105.4 dB | 10 dB gap — wedge adds only 0.4 dB to PA level |
Frequently Asked Questions
What happens when you have 10 or more sources at the same level?
Every time you double the number of equal sources, you add 3 dB. So 2 sources at 70 dB = 73 dB. 4 sources = 76 dB. 8 sources = 79 dB. 16 sources = 82 dB. The gains get smaller and smaller relative to the number of sources — you'd need 10× as many sources to add 10 dB. This is why adding a 12th identical fan to 11 existing ones barely changes the measured level.
Does the order I add sources matter?
No — sound pressure addition is commutative. Whether you add the loudest source first or last, the total is the same. The only thing that matters is the levels of all sources and whether they're coherent (related waveforms, like two identical speakers playing the same signal) or incoherent (unrelated noise sources). Most real-world scenarios involve incoherent sources, which is what the power-addition formula applies to.
What if sources aren't at the same distance?
You need to account for distance before combining. In a free field, use the inverse square law: for every doubling of distance, subtract 6 dB. So if source A is at 1 m (90 dB) and source B is at 4 m (also 90 dB at its surface), source B at your position would be roughly 90 − 12 = 78 dB (two doublings of distance). Then combine 90 dB and 78 dB: result is about 90.6 dB. The farther source barely contributes.
Why do equal sources add 3 dB and not 6 dB?
3 dB corresponds to doubling the acoustic power. Since dB is a logarithmic scale based on power ratios, two equal sources double the total power, which is +3 dB. If the sources were perfectly coherent (identical phase), they'd combine constructively and add 6 dB (pressure doubles = +6 dB). Real-world noise sources are incoherent, so power adds (not pressure), giving +3 dB.