Low Frequency Noise Detector
Hunt down that low, droning rumble. This tool zooms your microphone in on the sub-100 Hz band — a low-frequency spectrum, automatic 50/60 Hz mains-hum detection, a band breakdown (infrasound, deep sub-bass, low hum), and a reference for “the Hum” phenomenon — so you can see what frequency a complaint actually sits at.
⚠ Read this first: your mic almost certainly can’t hear the very low end. Consumer laptop and phone microphones roll off steeply below ~50–100 Hz and essentially cannot capture true infrasound (<20 Hz) at all. Anything shown below ~50 Hz is mic-limited and unreliable. This is an uncalibrated, approximate estimate from a consumer mic — not a calibrated sound-level meter and not legal or complaint evidence. And a rumble you feel but the tool doesn’t show may be structure-borne vibration, not airborne sound. Nothing is recorded or uploaded.
Microphone
Microphone access is requested only when you press Start. Audio is analyzed live and never recorded or sent anywhere.
Mains frequency
Europe / Asia / Africa / Australia use 50 Hz mains; North America and parts of South America/Japan use 60 Hz. Auto picks whichever shows a stronger hum peak.
SPL calibration
Low-band spectrum (0–120 Hz)
Band breakdown
Relative band energy (dBFS) — not affected by SPL calibration.
Mains-hum check
How It Works
When you press Start, the tool opens your microphone with automatic gain, noise suppression and echo cancellation switched off (those would distort any noise measurement), runs a large FFT, and throws away everything above ~120 Hz. The result is a zoomed low-frequency spectrum: the horizontal axis covers roughly 0–120 Hz so a faint 47 Hz drone that would be one invisible pixel on a full-range analyzer becomes a clear, labelled peak. It marks the dominant low-frequency component and breaks the energy into three bands — infrasound (<20 Hz), deep sub-bass (20–50 Hz) and low hum (50–100 Hz).
It also runs a dedicated mains-hum check. Electrical equipment, transformers, dimmers and motors radiate a tone at the power-line frequency — 50 Hz in most of the world, 60 Hz in North America — plus harmonics. Within the zoomed band (up to ~120 Hz) that means a sharp peak at 50 and 100 Hz for 50 Hz mains, or 60 and 120 Hz for 60 Hz mains. The tool looks for sharp peaks at those frequencies and tells you whether your rumble is most likely electrical hum versus broadband mechanical noise. Frequency content like this — what frequency, the harmonic spacing, the spectral shape — is calibration-independent, so it is genuinely meaningful even though the absolute loudness number is not.
Levels are an uncalibrated estimate
A browser cannot know your microphone’s real-world sensitivity, so the level it computes is dBFS (decibels relative to digital full scale, always ≤ 0), not true environmental dB SPL. If you have a real sound-level meter or a calibrated phone app, read the current level while the tool is running, type it into the calibration box, and press Set — the tool stores an offset (offset = your reading − the current dBFS) and shows an estimated SPL afterwards. That offset is saved in your browser under the shared key fd-noise-cal, so calibrating here also calibrates every other Noise Analysis tool. Until you calibrate, levels stay in honest dBFS. Even calibrated, this remains an approximation, not certified data.
Vibration vs. airborne sound, and the mic’s low-end limit
This is the part to take seriously. Small consumer microphones have a steep low-frequency roll-off: they lose sensitivity rapidly below ~50–100 Hz and are effectively deaf to true infrasound below 20 Hz. So if you can feel a deep rumble in your chest or through the floor but this tool shows little or nothing below 50 Hz, that does not mean the rumble isn’t real — it may be structure-borne vibration travelling through the building, or genuine infrasound your mic simply can’t register. Airborne low-frequency sound that the tool can see (roughly 50–100 Hz) is the reliable zone. For the band below that, treat any reading as a weak hint at best, and consider a contact/vibration sensor or a professional survey.