dBFS vs dB SPL: Why Browser Readings Differ

What dBFS means

Decibels relative to full scale measure a digital waveform against the largest value its signal path can represent. 0 dBFS is the ceiling; ordinary microphone samples sit below it, so their values are negative. Silence is not 0 dBFS. In practice it approaches the system’s noise floor.

What sound pressure level means

dB SPL compares physical sound pressure with the standard reference pressure of 20 micropascals. A calibrated sound level meter knows how its microphone voltage corresponds to that pressure.

Why there is no universal conversion

The browser receives samples after the microphone capsule, preamplifier, analog-to-digital converter and possible automatic processing. Manufacturers use different sensitivity and gain, and browsers may not expose those values. The same acoustic sound can therefore produce different dBFS readings on two phones.

What calibration can do

A comparison against a trustworthy reference can establish a device-specific offset for the same microphone and settings. That creates an environmental estimate, not a formal instrument reading, and does not correct frequency response or changing gain. Follow the calibration guide and read the accuracy limits.

Worked example: one clap, two different numbers

Clap once from two metres while two phones record side by side. Phone A peaks near −18 dBFS and settles to a Leq near −34 dBFS across the burst; Phone B peaks near −11 dBFS with Leq near −27 dBFS for the identical sound. Neither phone is lying. Capsule sensitivity, preamp gain and hidden browser processing differ, so the same pressure lands at different digital heights. Without a reference comparison made in that spot with that microphone, there is no honest way to stamp either burst as a fixed environmental figure.

How to read the meter's two numbers side by side

Open Digital Diagnostics beside the main gauge. The raw dBFS line shows headroom below digital full scale; the headline estimate adds frequency weighting plus a nominal +100 dB offset so the figure reads in familiar environmental units. A compatible calibration profile swaps that nominal offset for a device-specific comparison against a known reference, kept to the same microphone, weighting and placement. Redo it whenever the phone, case, input or browser changes, and never carry the offset to another device.

Mistakes that mix the two scales

• Reading 0 dBFS as silence. Zero marks the digital ceiling; a quiet room sits far below it, and the idle display shows dashes until real samples arrive.
• Ranking rooms by raw dBFS across different phones. Rank only same-device runs, or calibrate each device separately first.
• Changing weighting or Fast and Slow response halfway through. A-weighting trims bass, C-weighting keeps more of it, Z-weighting stays flat, and Fast at 125 ms jumps faster than Slow at 1 s. Lock your choices before comparing.

For the full path from capsule to screen see how the meter works, for a hands-on digital check run the microphone noise-floor test, and for everyday room context try the background-noise workflow.

Measure raw dBFS noise floor → · Test background noise → · Open the meter →