Decibel meter
Press Start and allow the microphone to see how loud the room is. Out of the box the meter shows decibels relative to full scale, which is exact but not a real-world sound level. Calibrate it against a known meter to get an approximate dB SPL figure. Audio is analysed live and never recorded or sent.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Press Start and choose Allow when the browser asks for the microphone.
- Watch the bar and the Level figure. Peak, average and minimum update as long as the meter runs; Reset statistics starts them over.
- Press Stop to turn the microphone off completely.
How it works
On every screen frame the page takes 2048 samples from the microphone, computes their root-mean-square value and converts it to decibels: 20 × log10(RMS). That is the dBFS level. The bar covers −90 to 0 dBFS. The page asks the browser to disable echo cancellation, noise suppression and automatic gain control, because those features change the loudness before the page sees it.
Calibrating to dB SPL
dBFS says how loud the signal is inside the computer, not how loud the room is. To estimate sound pressure level, enter the reading of a real sound level meter while the sound plays. The offset is the reference value minus the current dBFS level, and it is stored in this browser under tools.decibel.
Worked example
A reference meter reads 60 dB while this tool shows −30 dBFS, so the offset is 60 − (−30) = 90. Later the tool shows −20 dBFS, and the estimate is −20 + 90 = about 70 dB SPL.
Tips
- Calibrate with a sound near the level you want to measure, since microphones are not equally accurate everywhere.
- Keep the phone still and away from your breath or wind, which show up as loud, low rumble.
Frequently asked questions
Why does it show negative numbers?
Digital audio has a maximum level, called full scale. The meter shows how far below that maximum the sound is, in dBFS, so values are zero or negative. Closer to 0 is louder, and the meter floors at −100.
Can a phone really measure dB SPL?
Only roughly. Microphones differ in sensitivity, and phones and browsers often apply automatic gain, noise suppression or filtering. This tool asks the browser to switch those off, but some devices ignore the request. After calibration, expect errors of several decibels, larger at very quiet or very loud levels.
How does calibration work?
You enter the value from a trusted sound level meter while this tool reads a steady sound. The page stores the difference between the two as an offset and adds it to every later reading. The offset is only valid for the same microphone and settings.
What is the average figure?
It is the energy average, also called Leq: the readings are averaged as sound power and then converted back to decibels, which is how sound exposure is normally averaged. It is not the plain mean of the decibel values.