Why do decibel apps show different readings?
Because each app guesses differently about uncalibrated hardware. Different microphones, hidden OS processing like auto gain, different calibration offsets, A-weighting versus none, and different averaging windows can stack up to 5–10 dB of disagreement between two apps measuring the same sound.
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The five sources of disagreement
- Microphone hardware. Every phone model’s mic has its own sensitivity and frequency response. The same app on two phones can read several dB apart before software even starts.
- OS audio processing. Phones quietly apply auto gain control, noise suppression, and echo cancellation tuned for calls. An app that doesn’t explicitly disable these is measuring a signal the OS already altered. (Our browser meter disables all three.)
- Calibration offset. Converting a digital signal level (dBFS) to sound pressure level (dB SPL) requires an offset that depends on the exact microphone. Apps ship different per-model profiles — or one global guess.
- Weighting. One app shows raw dB, another shows dB(A). On bass-heavy noise those can differ by 10 dB or more by design, not by error.
- Averaging window. “Fast” (125 ms), “slow” (1 s), peak-hold, or a smoothed UI number — each responds differently to fluctuating noise, so simultaneous readouts won’t match.
How to get comparable readings
Use one app, on one device, with the same weighting and averaging every time — relative comparisons (“the bedroom is 12 dB quieter than the street”) survive calibration error even when absolute numbers drift. For absolute accuracy, calibrate against a reference: NIOSH’s research found apps with calibrated external microphones land within about ±1 dB of professional meters.
The honest baseline
No uncalibrated app — ours included — should promise instrument precision. That’s why the online meter labels every reading an estimate (±10 dB), and why apps that ship device corrections and let you fine-tune against a reference — like Decibel Shield — get closer to the truth than a generic browser reading can.