Short answer: usually within 5 to 10 dB without calibration, and within 1 to 2 dB for everyday sounds once calibrated. Here is why, and how it works.
What we measure
The meter reads the raw signal from your microphone with the browser's voice processing turned off (echo cancellation, noise suppression and automatic gain). It applies A, C or Z frequency weighting following the IEC 61672 standard, then Fast (125 ms) or Slow (1 s) time weighting, the same definitions a hardware sound level meter uses. Avg is the equivalent continuous level (Leq) since you started.
Why phones differ
Every device has a different microphone and gain, so the same room reads differently on different phones. Some phones also keep adjusting the microphone volume automatically. If that happens, the meter shows a note.
How to calibrate
Put your phone next to a trusted sound level meter, play a steady sound such as pink noise or a fan, then open Calibrate and settings and move the slider until the numbers match. The setting is saved on your device.
Limits
- Phone microphones can overload above about 100 dB, so very loud readings will be too low.
- This is not a certified instrument. For legal or workplace compliance, use a Class 1 or Class 2 meter.
How we test it
Every release is checked automatically: the filters are compared against the IEC 61672 tolerance table, and a test tone of known level is played through a virtual microphone and must read within 0.5 dB.