explainers
How Decibels Add: 60 + 60 dB Is About 63 dB
Two independent 60 dB sound sources combine to about 63 dB at the same point, not 120 dB. OSHA gives the logarithmic addition rule for sound levels and a worked 90 + 90 = 93 dB example. Primary source.
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| Item | Value or question | Why it matters |
|---|---|---|
| Headline example | 60 + 60 dB Is About 63 dB | Read with source context |
| Measurement | Keep metric and duration | Makes comparisons possible |
| Next check | Use the same position | Reduces avoidable variation |
The idea behind the number
The arithmetic is based on adding underlying sound energy, then converting back to logarithmic units. For independent equal-energy sources, doubling the energy adds approximately 3.01 dB. The practical shortcut is 3 dB for two equal sources. If one source is much quieter than the other, it contributes less to the combined reading.
The number in the headline is an example tied to the linked source, not a universal conversion for every situation. When you see a sound figure, ask what quantity it describes, how long the observation lasted and where the microphone was. The source page and its units belong with the figure. This habit prevents a clear-looking chart from quietly combining an average, a brief maximum and a peak into one misleading ladder. Keep the cited how decibels add example attached to its original scope.
A concrete example
The same rule helps explain why switching off one of two identical fans may lower a room by around 3 dB rather than half its displayed number. Real fans may not be identical and a room can reflect sound, so a before-and-after measurement is still needed. The formula is 10 log10 of the sum of 10 raised to each level divided by ten.
Use the example to frame a question you can test rather than to label a person or room. A measurement at one point is about sound at that point. If you move the phone, change its case or turn a second source on, write down the change. The before-and-after difference can be meaningful even when the absolute value still needs calibration. A small before-and-after check makes how decibels add easier to understand.
How to read a result
Never add two readings from different times and call the result a combined level. They must represent sources acting together at the same observation point, with compatible weighting and statistic. An A-weighted average and a C-weighted peak cannot be mixed in the calculation.
Keep level, weighting, statistic and period together. “dBA, Leq, two minutes at the desk” carries more information than “90 dB.” Read the first as an observation of a defined interval; the second could be a maximum, a pressure peak, a headphone playback estimate or a digital value without reference calibration. If the source does not specify one of these, do not invent it. Document the selected setting when explaining how decibels add.
Make the next check useful
Use the dB calculator to explore 60 + 60, 60 + 70 and 85 + 85 examples. Then compare a real room with one source on and both sources on from the same position. The gap between model and observation can reveal other background sources or room effects.
Start with a simple baseline, make the check at a normal listening position and save a note about conditions. dB Pulse can keep active sessions and show context from its free sound-reference library; Pro adds detailed analysis and reports. It stops metering when the app leaves the foreground. If the result could affect a legal, occupational or medical decision, use the equipment and procedure required for that decision. Choose the listening position before repeating the how decibels add check.
Common mistakes to avoid
Do not infer physical accuracy from a display that has several decimal places. A phone or browser can show a stable number while its microphone sensitivity or software processing differs from another device. For absolute sound-pressure work, compare beside a trusted meter with matching settings. For relative changes, keep the same phone and position. The how decibels add example works best with compatible measurements.
Do not judge exposure from level alone. NIOSH’s occupational model uses 85 dBA over an eight-hour workday and halves the recommended duration with each 3 dB rise. That is a work-exposure framework, not an automatic personal safety guarantee for every sound. Record duration and other loud activities before using the noise-dose calculator. Check the method before using how decibels add in a decision.
A detail that changes the answer
You can try the formula without memorizing it. Convert each level back to an energy-like quantity, add those quantities, then convert back to dB. A 70 dB source contributes ten times the intensity of a 60 dB source at the same point, so adding the 60 dB source to it changes the combined result by less than 3 dB. The three-decibel shortcut works only for equal levels. The calculator can help you see the pattern, but a room measurement still depends on how the sources interact with walls and the microphone position.
Sources
Phone and browser readings are estimates. For formal decisions, use the specified equipment and procedure; see the accuracy notes.
Try the next check
Decibel calculator can help with this question. See the matching dB Pulse feature.