What Is dBFS? Decibels Relative to Full Scale, Explained

Decibels compare a signal with a reference

A decibel compares two values on a logarithmic scale: it says how much larger or smaller a value is than its reference. The letters after "dB" name the reference, so check them before you read the number.

For amplitude, the size of a signal's swing, the formula is dB = 20 × log10(ratio). A reading of −20 dBFS therefore means one tenth of the full-scale amplitude, not "twenty percent quieter than maximum", and each halving of the amplitude changes the level by about −6 dB. The logarithmic scale lets a meter show a wide range of signal sizes without needing a long string of digits.

Power ratios use a multiplier of 10. Amplitude ratios, which this meter and the table below use, take 20.

Full scale and the floating-point exception

FS stands for full scale, the nominal maximum magnitude of a digital sample representation. In fixed-point PCM and at a constrained output, 0 dBFS normally corresponds to the largest representable sample magnitude; levels below it are negative. Web Audio, though, processes 32-bit floating-point samples. As the MDN source below explains, analyser values have a nominal −1 to +1 range and can exceed it after operations such as down-mixing.

The Device Bench readout is capped: it shows 0.0 dBFS for readings above −0.05 dBFS, and the engine counts an inspected window as clipping when a sample reaches its −0.1 dBFS threshold. A clipping count tells you the stream the browser received reached that near-full-scale threshold. It is not proof of where upstream clipping occurred. Positive dBFS values inside the floating-point audio graph are not shown as extra unclipped headroom.

Level and Peak measure different things

The microphone test labels its readouts Level and Peak. Level uses RMS (root mean square) to measure average signal strength over each short analysis frame. Peak holds the highest sample peak seen since the test started or the peak was reset. It is not the highest RMS reading: a brief spike can raise Peak while Level stays much lower.

Speech produces those spikes naturally. Plosive consonants and stressed syllables can rise well above the surrounding sound, so a voice averaging −20 dBFS can have substantially higher peaks. Watch Level for the overall signal and Peak for brief excursions toward full scale. If you judge by the average alone, a recording that looked fine on the meter can still clip on every hard consonant.

dBFS is not room loudness

dB SPL uses a different reference: sound pressure in air is measured against 20 micropascals, roughly the quietest sound a human ear can detect. dBFS instead compares a digital signal with full scale. Change the input gain and the same voice at the same distance can read −40 dBFS or reach the meter's capped 0.0 display. Without a known relationship between sound pressure and that digital signal, the meter cannot tell you how loud the sound was in the room.

Perceived loudness is different again. Hearing is more sensitive to some frequencies than others, and this amplitude meter applies no loudness weighting. Broadcast and streaming programs are measured in LUFS, a loudness-weighted measurement separate from the live input level shown here.

Reading the microphone test

Start the test, grant microphone access, and read a sentence aloud at your usual speaking distance; a sentence works better than saying "test" once. Watch Level as you speak, then check the held Peak. Device Bench marks −30 to −12 dBFS as a practical working band for this browser meter's Level readout, not a universal recording or broadcast standard. The gain that gets you there depends on the microphone, operating system, processing mode, and application. If Level stays near the meter floor, move closer or raise the input gain.

Leave a gap between the held Peak and 0 dBFS. On this capped readout, that gap is your headroom: space for a laugh, cough, or raised voice before the meter reaches its clipping threshold.

To compare settings, adjust one control at a time and repeat the same phrase from the same distance.

Amplitude ratios at a glance

For an amplitude ratio a relative to full scale, the level is 20 × log10(a) dBFS. These values come straight from the formula; they are not target levels for your voice. Half the amplitude works out to −6.02 dB, and a tenth to exactly −20 dB.