ItalianoEnglish
It opens measuring. No start button, no settings to visit first: the number is already there, with the word that goes with it and the last minute drawn underneath.
It opens measuring: the level, and the word that translates it. 
A room is judged by its noise over a while, not at an instant. 
The average is the energy one: the earlier noise still counts.
What it does
- The level in dB, and the word that translates it. Calm, Busy, Loud, Harmful over time: 62 dB means nothing to anyone who does not do this for a living.
- Minimum, maximum, average and peak since you last started again.
- The last minute under the number, with the 85 dB line always visible: a room is judged by its noise over a while, not at an instant.
- Fast and Slow, the two time weightings of real meters — 125 ms and 1 s.
- Above 85 dB the phone buzzes and the screen turns red: that is the level where prolonged exposure damages hearing.
- In English and Italian, following the phone's language.
The average is the energy average, and that is not a detail
Decibel computes Leq, the energy average, not the arithmetic mean of the decibels. The difference shows the day somebody slams a door: decibels are a logarithmic scale, and adding them like ordinary numbers means cancelling that door by averaging it with the silence around it. The energy average will not allow that — which is why it is the one the noise exposure standards use.
It is an estimate, and that has to be said
A phone microphone is not a calibrated instrument, and no app can make it one: it differs between models, compresses loud sounds and has a noise floor of its own. If you have a real meter you can compare and move the calibration, and from then on the scale is yours. What the app will not do is fake a precision the sensor does not have.
Your data stays yours
Decibel does not record: the sound is analysed in the phone's memory and thrown away, and no file is ever written. No account, no location, no server of ours. Three permissions — the microphone to hear the room, Internet for the ads, vibration for the 85 dB warning.
How it is paid for
A banner at the bottom, away from the controls, and an occasional full-screen ad only after a finished measurement. No subscription, no feature held hostage.