People who hear it have spent decades searching outdoors for the machine responsible: a ventilation unit, a heat pump, distant traffic, a factory over the hill. A study of 28 of them in Germany suggests the search was in the wrong place. The sound, for most, appears to originate inside the auditory system itself.
The phenomenon known as The Hum – a persistent low-frequency drone or buzz audible to an estimated 2 to 4 percent of people, and inaudible to others in the same room – surfaced publicly in Bristol in the mid-1970s, when the local newspaper began collecting letters from residents asking what they were hearing. Reports later spread to Plymouth, Southampton, Swansea and London, then in the 1990s to Taos, New Mexico and Kokomo, Indiana, and since to Canada, Australia, New Zealand, South Africa and cities across Europe. Explanations offered over the years have ranged from acoustic pollution and ocean waves to the CIA and aliens.
A team led by Markus Drexl, a professor at the Norwegian University of Science and Technology, tested the two hypotheses that could actually be measured. The results were published in PLOS One in March 2026.
Two explanations, both ruled out
The first was that Hum perceivers simply hear better than the rest of us at low frequencies, picking up real sound waves that travel long distances and are notoriously hard to trace to a source. The vast majority of participants showed no such advantage; only two had above-average sensitivity in parts of the low range.
The second was that they were hearing their own ears. The cochlea generates faint sounds as a by-product of amplifying incoming noise – oto-acoustic emissions, typically between about 500 and 5,000 hertz, which can be picked up with a microphone in the ear canal and are occasionally perceived as tinnitus. The team tested for them at low frequencies. They were not there.
That leaves a phantom sound generated within the auditory system and not measurable from outside: subjective tinnitus, in an unusually low register. It fits the pattern sufferers describe, in which the noise is first blamed on something external and only later recognised as travelling with them from place to place.
Drexl flags one limit. Standard hearing tests are not built to detect very narrow variations in threshold, so it remains conceivable that some people are unusually sensitive within a sliver of frequency – between 50 and 51 hertz, say.
The finding does not switch the sound off. But it reframes what is often a genuinely disabling experience – one that disrupts sleep and can make people physically unwell – as a recognised hearing condition with established clinical pathways, rather than an unexplained anomaly to be hunted outdoors indefinitely.