Why Your Phone's Decibel Meter Is an Estimate, Not a Measurement

Every "decibel meter" app on a phone, browser-based or native, is doing the same fundamentally limited thing: reading raw microphone samples, computing their root-mean-square level, and converting that to a decibel-scale number. The Sound & Decibel Meter does exactly this — getUserMedia audio into a Web Audio AnalyserNode, RMS over each frame, converted with 20 * log10(rms) — and says so plainly, because the honest caveat matters more than the number.

Where the accuracy actually breaks down

A real dB SPL (sound pressure level) reading needs a calibrated microphone with a known sensitivity curve and a reference pressure. Phone and laptop microphones are tuned for voice clarity, not flat frequency response, and manufacturers don't publish calibration data per device — so there's no way to convert "how loud this mic thinks it is" into "how many actual decibels are in this room" without external calibration equipment. The tool reports a relative level: useful for noticing that a room got louder, not for citing a number to a landlord or a noise ordinance.

What it's actually good for

Comparing before/after — is the new fan quieter than the old one, did adding a rug measurably damp echo, is a room consistently loud enough to bother a video call. Relative comparisons cancel out most of the calibration problem because the same uncalibrated mic is the constant across both readings.

What it isn't for

Anything with a legal or safety threshold — workplace noise exposure limits, hearing-damage thresholds, municipal noise complaints — needs a calibrated sound level meter, full stop. Use this tool for curiosity, not compliance. If you're checking your recording setup rather than ambient noise, the Voice & Audio Recorder captures the same mic input to a downloadable file, and the Metronome & Instrument Tuner is the other half of most home-recording setups.