The annual shareholder meeting was supposed to project competence and stability. Instead, it projected a 120 Hz hum that was audible during every pause in the CEO's speech. Not loud enough to be obvious. Just loud enough to make everyone vaguely uneasy, like background music in a horror film.

The AV team replaced the microphone. Still humming. Replaced the cable. Still humming. Replaced the mixer. The hum remained, patient and eternal. They bypassed the signal processor. They swapped the amplifier. They changed the speaker cable. At one point, someone suggested it might be the building's HVAC system, which led to a 20-minute detour involving the facilities manager and a very confused janitor.

The hum was a ground loop. The subwoofer amplifier was on a different electrical circuit than the mixer, creating a potential difference between their ground references. This difference manifested as 60 Hz mains hum — and because the sub amplifier had a full-bridge output stage, the second harmonic at 120 Hz was actually louder than the fundamental. The sub dutifully reproduced this 120 Hz tone at a level just below the noise floor of normal speech but clearly audible during pauses.

Ground loops occur when audio equipment connects to multiple electrical circuits with different ground potentials. The cure is a single circuit, proper star grounding, or an isolation transformer.

The Moral: A mysterious hum is almost always a ground loop. SonaVyx Transfer Function instantly shows the spectral spike at mains frequency and its harmonics — ending the guesswork in seconds.