What dBFS Means on a Mic Level Meter

Decibels compare a signal with a reference

A decibel compares two values on a logarithmic scale. It tells you how much larger or smaller a value is than its reference, rather than giving a fixed quantity of loudness. To interpret a level in decibels, check the reference; the letters after "dB" identify it.

For amplitude, the size of a signal's swing, the formula is dB = 20 × log10(ratio). Halving the amplitude changes the level by about −6 dB; one tenth of the amplitude is exactly −20 dB. A reading of −20 dBFS therefore means one tenth of the full-scale amplitude, not "twenty percent quieter than maximum". 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 rather than 20. Here the ratio is amplitude, so the formula and table use 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. That tells you the stream received by the browser met the meter's 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 emphasized 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. An average below the clipping threshold does not rule out peaks that reach it.

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 another distinction. Hearing is more sensitive to some frequencies than others, while this amplitude meter does not apply loudness weighting. LUFS is used for loudness-weighted measurements of broadcast and streaming programs. That is a different measurement from the live input level shown here; a dBFS reading describes digital amplitude, not how loud a listener will find the sound.

Reading the microphone test

Start the test, grant microphone access, and read a sentence at your usual speaking distance. 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 you need depends on the microphone, operating system, processing mode, and application. If Level stays near the meter floor, try changing the distance or input gain. A Peak displayed at 0.0 means the stream reached the meter's near-full-scale threshold; it does not locate upstream clipping.

On this capped readout, the gap between a displayed peak and 0 dBFS is a practical headroom guide: space for a laugh, cough, or raised voice before the meter reaches its threshold. The floating-point audio graph may exceed nominal full scale internally, but the display does not count that excess as usable input headroom.

Negative readings are below the nominal reference; a 0.0 display means the meter's threshold was reached. 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 directly from the formula; they are not descriptive bands for voice levels. Halving amplitude changes the level by about −6.02 dB; dividing it by ten changes it by exactly −20 dB.