A deep, persistent hum. No visible source. No neighbor who hears it. For decades, the phenomenon known as "The Hum" has baffled scientists and haunted the sleep of millions. Now a team of Norwegian researchers may have cracked the case.
Markus Drexl, a professor at the Norwegian University of Science and Technology (NTNU), led a study of 28 people in Germany who reported hearing an unexplained buzzing or humming sound. Their findings offer the most thorough scientific explanation yet.
What They Tested — and Ruled Out
The researchers examined two main hypotheses. First, that The Hum might be an externally produced low-frequency sound that could be physically measured — from industrial equipment, traffic, or natural sources. Second, that the ear itself might be generating detectable sounds known as oto-acoustic emissions.
Neither fit most participants.
Only two of the 28 had better-than-average hearing at certain low frequencies. And spontaneous oto-acoustic emissions — faint sounds the inner ear naturally produces as a by-product of amplification — did not explain the phenomenon either.
The Most Likely Culprit: Low-Frequency Tinnitus
"Then there are people who hear something that cannot be measured objectively. We believe people in this category have a form of low-frequency tinnitus," Drexl said.
Tinnitus is typically described as ringing in the ears — a sound perceived inside the head or ear with no external source. In low-frequency tinnitus, that phantom sound manifests as a hum or buzz rather than a ring. Drexl's team concluded this is likely the explanation for the majority of cases.
A small minority may genuinely have unusually sensitive low-frequency hearing, picking up ambient noise others cannot detect. But for most, the sound originates within their own auditory system.
Why It's So Hard to Pin Down
Low-frequency sound waves have long wavelengths, making their source nearly impossible to localize. A fan in a distant factory can sound like it's coming from the next room — or inside one's head. The confusion between genuine external noise and internally generated tinnitus has fueled decades of mystery.
Drexl noted that standard hearing tests are not designed to detect the microstructures in hearing thresholds that might explain some cases.
The Hum in History
The phenomenon first gained widespread attention in Bristol, England, in the 1970s. Reports later emerged in Taos, New Mexico, Kokomo, Indiana, and cities across Canada, Australia, New Zealand and Europe. The World Hum Map and Database Project, founded in 2012 by Canadian teacher Glen MacPherson, now collects reports from around the globe.
Estimates suggest 2 to 4 percent of the world's population experiences The Hum. For some it is merely irritating. For others, the low-frequency sound causes physical discomfort, disrupts sleep, and can feel like a vibration passing through the body.
What's Next
The study, published July 19, highlights how little scientists still know about low-frequency hearing. As concerns grow about noise pollution from wind turbines, heat pumps and industrial infrastructure, understanding how humans perceive low-frequency sound has never been more relevant.
