How the Hum Works

Hum sufferers describe a low-frequency buzz accompanied by a rumbling vibration. It's worse at night, making it almost impossible to sleep. Other physical symptoms include debilitating headaches, nausea, nosebleeds and diarrhea. In one tragic case in England, the Hum has been linked to suicide.

The Hum has baffled researchers since the 1970s, when the first widespread reports of the unexplained acoustic phenomenon cropped up in rural England. Armed with highly sensitive instruments, teams of scientists have attempted to hunt down the source of the unrelenting noise. The results? Inconclusive.

Theories abound to explain the origins of the Hum, from the plausible — power lines, cell phone towers and industrial fans — to the more, uh, imaginative. Conspiracy theorists have claimed the Hum is the work of the CIA, the military, and of course, aliens.

So what exactly is this mystifying noise, unheard by most, but an absolute torture to some? Does the Hum have an environmental origin or is it, in fact, all in their heads?

The earliest widespread reports of something resembling the Hum originated in England in the 1970s when pockets of concerned citizens across Britain reported hearing a mysterious humming, droning and buzzing sound.

The "Bristol Hum" was the first outbreak to be linked to a specific city, in this case the southern English town of Bristol. In the 1970s, a series of articles about the Hum in the U.K. newspaper the "Sunday Mirror" triggered a flood of mail from readers across Britain who had been suffering from the same maddening sound.

News of the Hum made enough noise — pun absolutely intended — to attract the first serious researchers. In a 1977 volume of the scientific journal "Applied Acoustics," two researchers explored a "particular environmental noise phenomenon which appears to be a cause of real and severe disturbance to certain people." Their best guess was low-frequency sound waves generated by distant industrial sources.

Another famous hum began plaguing the residents of the coastal Scottish town of Largs in the late 1980s. It was the same low-pitched drone, inaudible to most, but debilitating to a sensitive few. The "Largs Hum," like other hums, is loudest indoors and at night and can trigger nosebleeds, crippling headaches and chest pains in the worst cases.

In the U.S., the first large-scale outbreak of the Hum occurred in Taos, an artist's enclave in New Mexico. In the early 1990s, distressed Taos residents complained to their local and state representatives, eventually prompting an official Congressional investigation.

Joe Mullins, mechanical engineering professor emeritus of the University of New Mexico, led a team of acoustic researchers and hearing scientists from nearby Los Alamos National Laboratory and Sandia National Laboratories to hunt down the source of the Hum. The research team's results were inconclusive — people were clearly suffering from the same tormenting sound, but investigators had no idea where it was coming from, and why it only affected 2 percent of the population.

We live in a world saturated by noise. From the drone of passing traffic to the incessant "dinging" of our smartphones, we are surrounded by acoustic pollution. When researchers were dispatched to Taos, New Mexico, they tuned their specialized equipment to the lowest audible frequencies, between 8 and 80 Hz. These are the ultra-bass frequencies that register more as a rumble and a throb than a perceptible tone. However, investigators were unable to isolate a single environmental source that emitted tones at that frequency.

Researchers in Windsor, Ontario, isolated a local hum to the blast furnace of a steel plant on Zug Island in nearby Michigan. However, local industrial sources don't explain the worldwide prevalence of the Hum. What else could produce such powerful and pervasive low-frequency tones?

What about long-distance radio transmissions? There's a network of radio transmitters called LORAN (long range radio navigation) that broadcasts low-frequency signals as a form of primitive GPS. Skeptics point out that if LORAN was the culprit, then we would have more reports of the Hum closest to LORAN towers. Also, LORAN broadcasts 24/7, but Hum sufferers mostly complain of the noise at night.

A more intriguing possibility is TACAMO aircraft, military planes that employ radio frequencies in the lowest end of the spectrum to communicate with submerged submarines. The planes operate at night, and their movements are top secret. Hum hearers in Largs, Scotland have long believed that their particular hum originates from the local naval base. The TACAMO theory might also explain why many Hum sites are on the coast.

Conspiracy theorists have dreamed up all kinds of wild stories for the source of the Hum, including a large-scale mind-control scheme executed by the U.S. and U.K. governments. Others believe that regardless of its source, the hum is dangerous enough to drive people temporarily insane.

Since acoustic researchers have failed to find a single measurable environmental source for this worldwide phenomenon, it's fair to ask: Is the Hum some kind of mass auditory hallucination?

Not likely, says David Deming, a geosciences professor at the University of Oklahoma who wrote a comprehensive paper on the Hum in 2004. In previous cases of mass delusion — like the witch hunts that erupted across Europe in the 15th and 16th centuries — the perpetrators had something to gain by joining the mad throngs, namely improved social status. But the people who claim to hear the Hum have nothing to gain; some have even abandoned homes in a desperate effort to escape the noise.

What about tinnitus? As many as one in five people suffer from some degree of tinnitus — or ringing in the ears. Maybe the Hum is literally in our heads. Again, not likely. Tinnitus sufferers generally report a persistent ringing — both day and night — that registers in the highest audible frequencies, not the absolute lowest.

Another interesting theory is that Hum sufferers may simply have exceptionally sensitive hearing in the ultra-low-frequency range of 20 to 100Hz. But if no acoustic instruments have been able to record the Hum, then what is it exactly that these sensitive ears are hearing?

Dr. David Baguley, head of audiology at Addenbrooke's Hospital in England, has researched the Hum for years and blames the phenomenon on sensitive brains, not ears. Our sense of hearing is greatly heightened in times of stress, Dr. Baguley explained to BBC News in 2009. The brain cranks up the volume to detect the slightest threatening sound.

In the case of the Hum, Dr. Baguley believes people hear about the phenomenon in newspapers and around town and begin to fixate on a perceived background noise. The incessant sound becomes a source of increasing frustration, disrupting sleep, and causing additional stress, which tricks the brain into turning up the volume even further. The solution, then, is to convince the brain to back down. With his own Hum patients, Dr. Baguley has found some success with simple relaxation techniques borrowed from psychology.