Noise Comparison Tool
This noise comparison tool lets you capture sample A, change something, then capture sample B with the same microphone and gain — an A vs B noise test that compares them side by side: an overlaid octave-band spectrum, the broadband dB difference, and Leq / Lmax / L90 for each with deltas. Perfect for before/after checks: a fan on vs off, a window open vs closed, or acoustic treatment installed.
ℹ The A−B difference is the trustworthy number; the absolute levels are not. A browser microphone is uncalibrated, so each capture’s Leq is a relative dBFS estimate, not a certified dB SPL and not a substitute for a Type 1/2 sound-level meter or any legal/compliance evidence. But because A and B are captured with the same mic and gain, the unknown calibration offset cancels in A−B — so the difference (and the spectral shape) is genuinely meaningful. Auto-gain and noise suppression are requested off. Nothing is recorded or uploaded.
Tip: keep the microphone in the same spot and don’t touch the gain between captures — that’s what makes the comparison valid.
| Metric | A (before) | B (after) | Δ (B−A) |
|---|---|---|---|
| Leq (average) | — | — | — |
| Lmax (loudest) | — | — | — |
| L90 (background) | — | — | — |
| Peak sample | — | — | — |
| Clips | — | — | n/a |
Optional: calibrate for an SPL estimate (shared across all noise tools)
Calibration only affects the absolute SPL estimate — the A−B difference never needs it. Read the level on a real sound-level meter (or calibrated phone app) at the same time, enter both numbers, and the offset is stored once for every noise tool on this site.
How It Works
The tool records two short snapshots of your microphone’s level and spectrum — A (your “before”) and B (your “after”) — giving you a structured way to compare noise levels under two conditions. For each capture it accumulates power over the whole window and reports Leq (the equivalent continuous level — the steady level carrying the same energy as the fluctuating sound), Lmax (the loudest moment) and L90 (the level exceeded 90% of the time, the classic statistic for the steady background under intermittent events). The Leq comparison between A and B is the headline number, but the tool also splits each capture into ten octave bands from 31.5 Hz to 16 kHz and overlays them so you can see where in the spectrum the noise changed. This noise spectrum comparison is often more useful than the single broadband number when evaluating acoustic treatment or noise-control measures, because a product may reduce one frequency region while doing little for another.
Why the A−B difference is trustworthy even without calibration
A browser cannot know your microphone’s real-world sensitivity, so every absolute number it shows is in dBFS (decibels relative to digital full scale) and carries an unknown, fixed calibration offset. The key insight: when you measure A and B with the same microphone, in the same position, at the same gain, that unknown offset is identical in both — so when you subtract (B − A) it cancels exactly. The difference in decibels, and the difference in spectral shape, are therefore genuinely meaningful: if B reads 6 dB lower than A, your change really did cut the level by about 6 dB, regardless of the fact that neither absolute figure is a certified SPL. Engineers sometimes call this kind of relative measurement an insertion loss test — the same principle used in professional acoustic panel and partition testing, just without the lab-grade equipment.
What stays uncalibrated
The single-capture numbers themselves (“A is −38 dBFS”) are not a real environmental decibel reading. You can optionally enter a calibration offset — measured once against a true sound-level meter — to display a rough SPL estimate; that offset is saved under a shared key (fd-noise-cal) so every noise tool on the site uses the same calibration. Even then, treat the SPL figure as an estimate. And remember consumer mics roll off at the frequency extremes and generally cannot capture true infrasound (below ~20 Hz) or deep sub-bass, so the lowest band is approximate. For a continuous picture of how noise level changes over time — rather than two point-in-time snapshots — the noise type identifier can also help you characterize whether the sound is steady, intermittent, or impulsive before and after your change.