Electricity in your home alternates at the mains frequency — 50 Hz in most of Europe, Africa, Asia and Oceania, and 60 Hz across the Americas and parts of Asia. Anything that vibrates in step with that current radiates sound at that frequency and at its harmonics. Transformer and motor laminations are pulled twice per cycle (the force does not care about the current’s direction), so they hum loudest at twice the mains frequency: 100 Hz on a 50 Hz grid, 120 Hz on a 60 Hz grid. Other components add a ladder of higher harmonics — 150/180, 200/240 Hz and up — and the pattern of those harmonics, sometimes called the harmonic signature, is what gives a hum its distinctive character and helps distinguish a buzzing transformer from a failing fluorescent ballast.
This tool runs a high-resolution FFT on your microphone signal and measures how far the bins at 50× and 60× their harmonics rise above the surrounding noise floor. Whichever family (50 Hz or 60 Hz) has the greater total prominence wins, telling you the mains region. It then flags which harmonics are strong and draws them as orange lines on the live spectrum so you can see the hum directly. Because this is pure frequency analysis, it is calibration-independent: the peak positions and the 50-vs-60 verdict do not depend on how loud or sensitive your mic is. If you want to see the full audio spectrum beyond just the mains-hum region, the noise frequency analyzer shows all frequencies in real time.
From the harmonic shape it offers a heuristic guess at the source: a dominant second harmonic suggests a transformer or power supply; energy spread across many upper harmonics suggests a fluorescent/LED ballast or dimmer (switch-mode electronics produce rich intermodulation and high-order harmonics that toroidal or linear transformers do not); a strong fundamental with some upper-harmonic energy suggests a motor; and a strong low-order line with weak upper harmonics, especially heard through audio gear, suggests a ground loop — note that ground-loop hum is frequently strongest at the second harmonic (100 Hz on a 50 Hz grid, 120 Hz on 60 Hz) because the heavy ground currents come from rectifier power supplies, not only at the fundamental. This is a best guess, not a measurement — sources overlap and real rooms are messy — so confirm it by turning suspects off one at a time and watching the peaks. The optional calibration only affects the loudness estimate (a shared offset used across all of our noise tools); it never changes the frequencies.
How does it tell 50 Hz hum from 60 Hz hum?
It runs a high-resolution FFT and measures how far the peaks at the 50 Hz family (50, 100, 150…) and the 60 Hz family (60, 120, 180…) rise above the surrounding noise floor. The family with the greater total prominence wins. This is pure frequency detection, so it is reliable and does not depend on calibration — though if 50 and 60 Hz are very close in strength the tool says so rather than guessing.
Why is the 100 or 120 Hz harmonic often louder than the fundamental?
Magnetic forces in transformers and motors pull the metal twice per electrical cycle, because the force does not care which way the current flows. That makes the dominant mechanical hum appear at twice the mains frequency — 100 Hz on a 50 Hz grid or 120 Hz on a 60 Hz grid — so the second harmonic is frequently the strongest line, a clue that often points at a transformer or power supply. A dominant second harmonic is not exclusive to transformers, though: ground-loop hum is also frequently strongest at 100/120 Hz, because the heavy ground currents that cause it come from rectifier power supplies rather than the 50/60 Hz fundamental.
Can I trust the “likely source” it names?
Treat it as a heuristic best guess, not a diagnosis. It is inferred only from the shape of the harmonics your microphone picks up, and many real sources overlap or sound similar. A ground loop, in particular, lives in your audio wiring and may not be in the room air at all. Confirm by switching suspected devices off one at a time and watching whether the peaks drop.
Is the loudness reading a real decibel measurement?
No. A browser microphone is uncalibrated, so the level is shown as relative dBFS, not certified dB SPL. It is useful for before/after comparisons with the same mic and setup. If you enter a reading from a real sound-level meter the tool stores a shared offset and shows an SPL estimate, but that is still an estimate — never use it as legal, complaint, or compliance evidence.
It detects nothing even though I can hear a hum — why?
Move the microphone much closer to the appliance, and keep voices, music and other sounds quiet during the reading. Make sure your browser is using this tool’s raw microphone settings (automatic gain and noise suppression are requested off, because they would mangle the hum — if your OS forces them you may need to disable microphone enhancements in system settings). Some very low rumble is true infrasound below about 20 Hz that consumer mics simply cannot capture, and a faint ground-loop hum may be inside cables rather than in the air.
Is my audio recorded or uploaded?
No. The microphone signal is analysed in real time entirely inside your browser to find the hum, and is never recorded, saved, or transmitted. The microphone connects only to an analyser node, never to your speakers, and it is released the moment you press Stop or close the tab.
How do I fix a ground loop hum in audio equipment?
A ground loop forms when two pieces of audio gear share a common ground path at different electrical potentials, creating a circulating current that induces hum — typically at 50 or 60 Hz and often loudest at the second harmonic (100/120 Hz). Common fixes: use a ground loop isolator (a passive transformer placed on the signal cable), power all interconnected gear from the same mains socket or strip, use balanced XLR connections instead of unbalanced RCA where possible, and check that all chassis are earthed at one point only. This tool can confirm the hum is there and that it is mains-related; it cannot diagnose which cable or piece of gear is the culprit, so try disconnecting cables one at a time while watching the spectrum.
Why does my LED dimmer or smart bulb produce a more complex hum than a regular transformer?
Traditional transformer hum is dominated by the second harmonic (100/120 Hz) because the magnetostrictive vibration of the core follows the squared magnetic flux. LED dimmers and SMPS-based LED drivers use rapid on-off switching — often at 20–100 kHz internally — but the switching chopper interacts with the 50/60 Hz mains envelope and can produce a wide ladder of audible harmonics and intermodulation products across 50–1000 Hz. Additionally, many dimmers use phase-cutting (TRIAC chopping), which creates abrupt waveform edges that are rich in high-frequency harmonic content. The result is a buzz that sounds harsher and more complex than a simple transformer hum, often spreading energy across many harmonics that this tool will flag as "spread across upper harmonics."
Does this tool work on a phone or tablet, or do I need a laptop?
It works on any device with a microphone and a modern browser — phone, tablet or laptop. On a phone, the built-in microphone is typically a MEMS capsule with a flat response roughly from 100 Hz to 8 kHz, which covers the 50/60 Hz region and its main harmonics well. The 50 Hz or 60 Hz fundamental itself is near the low-frequency roll-off of most phone mics, so the second harmonic (100/120 Hz) often reads more strongly than the fundamental — this is normal and does not affect the mains-region verdict. For the best low-end sensitivity, hold the phone close to the appliance and avoid a phone case that blocks the mic port.