dB / Sound Level Meter
A level meter that reports dBFS honestly, because a browser cannot measure sound pressure level.
−60 dBFS ————————————— 0 dBFS (digital full scale)
How this test works
The microphone is opened with the browser's processing switched off where possible, and the RMS of each analysis frame is converted to decibels relative to digital full scale. That last phrase is the whole point. Every browser sound level meter displays a number beside the letters dB and lets you assume it means sound pressure level, and it cannot: producing dB SPL from a microphone signal needs the capsule's sensitivity, the preamp gain and the AGC state, and no browser exposes any of them for any device. So a page showing you 72 dB has invented that number. This one shows dBFS, labels it as such, and gives you the measurement that is actually valid without calibration — mark the room, mark the fan, and the difference between the two marks is real even though neither absolute figure is.
What the results mean
- dBFS
- Decibels relative to digital full scale. Zero is the loudest the converter can represent; everything else is negative.
- Peak
- The highest level in the run. Peak rather than average, because clipping is what ruins a recording and it happens at the peaks.
- Sound pressure level
- Reported as not measurable, because it is. This is the number other meters fabricate.
- Marks
- Two readings on the same input. The gap between them is an accurate measurement in decibels even without calibration.
Common problems and fixes
- I need a real dB reading for a noise complaint
- This cannot give you one and neither can any other web page. A complaint needs a calibrated meter, and phone apps calibrated against a known source are the cheap end of that.
- The level drifts when the room is steady
- Automatic gain control that the browser would not switch off. The result says when that is happening; it makes the microphone chase the level rather than report it.
- Everything reads very low
- Check the input device and the operating system's input level. dBFS is relative to the converter, so a quiet preamp gives low readings from a loud room.
Frequently asked questions
Why can no website measure decibels properly?
Because dB SPL describes pressure in the air, and a microphone signal only becomes pressure once you know the capsule's sensitivity and every gain stage between it and the browser. None of that is exposed by any browser API, for any device. What the browser has is a number relative to digital full scale, which is a different quantity.
What is dBFS?
Decibels relative to digital full scale — how close the signal is to the loudest value the converter can represent. Zero dBFS is the ceiling and everything below is negative. It says nothing about how loud the room is.
Is the comparison between two marks trustworthy?
Yes, provided the microphone, the input gain and the AGC state have not changed between them. A ratio of two signals through the same unknown chain cancels the unknowns out, which is why the difference survives having no calibration.
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