A mysterious humming sound has emerged as a great source of irritation for people in certain parts of the world.
Do you hear what I hear?
Are you a "lumper" or a "splitter"? Do you seek unity among apparently disparate things, or do you see significance in seemingly minor differences? The distinction means a lot to academics, and over the years has been the source of many bitter rows. Which camp is in the ascendancy varies among academic disciplines. In mathematics and physics, lumpers have the most kudos, being regarded as true believers in the underlying order that supposedly rules the cosmos. In the life sciences like biology and medicine, on the other hand, splitters tend to be seen as more rigorous and sophisticated than their "lumping" counterparts.
Most of the time, the two camps can be kept at a safe distance from one another. But some controversies force them together, with less than happy consequences. One that has reared its head recently centres on a bizarre phenomenon that plagues the lives of thousands of people around the world. It's called the Hum. Usually described as sounding like a distant idling diesel engine, the Hum has been reported at sites from Denmark to Japan, Canada to New Zealand. No one is sure when it first emerged: some reports suggest it dates back to the 1940s, when people in London and Southampton began to complain about a mysterious but pervasive noise.
Researchers started to take it seriously in the early 1970s, setting up sound meters to see if they could detect the source. And with their background in the physical sciences, they sought a "lumping" solution - a single source of noise that could explain all the cases. When they failed to track down any external source, they decided the source must lie inside the ear. They concluded that the Hum is a form of tinnitus, a recognised medical disorder that creates sounds within the human ear. The lumpers pointed out that this would explain why the Hum is only heard by some people, most of them middle-aged or elderly: tinnitus affects just a few per cent of the population, and becomes more prevalent with age.
Their grand unified theory soon ran into trouble, however. A study of people who reported hearing the Hum found that around 40 per cent did indeed have a low-frequency variety of tinnitus - but that still left the other 60 per cent unexplained. The tinnitus theory had other flaws. While the disorder is spread evenly among the population, reports of the Hum cluster around certain hot spots - like Vancouver, Canada, Copenhagen and Largs in Scotland. Tinnitus also fails to explain why some people who move away from such hot spots no longer hear the Hum.
Such awkward little facts have forced lumpers to come up with more esoteric explanations. One of the most intriguing is based on a curious phenomenon first reported in the 1950s, in which some people claimed to be able to "hear" the emissions of radar antennas. At first, the reports were dismissed as nonsense, but laboratory studies confirmed that some people - including the profoundly deaf - seem capable of hearing electromagnetic (EM) waves.
The explanation is thought to be heat generated by the EM energy inside the ear's exquisitely sensitive pressure mechanism. Despite producing temperature changes of just a few millionths of a degree, the resulting expansion and contraction is enough to create the illusion of an external sound. This strange phenomenon is at the heart of a grand unified theory for the Hum put forward by geophysicist Professor David Deming of the University of Oklahoma. He suggests the Hum may be generated by the system used by the US military to communicate with its nuclear submarines.
As conventional radio signals do not penetrate far into water, submarines rely on very low frequency (VLF) radiowaves, which penetrate further below the surface and can be picked up using long, trailing aerials while staying submerged. A key part of the US navy's submarine communication network are aircraft flying classified missions over the world's oceans. And according to Prof Deming, their transmissions may be "heard" by sensitive individuals as the Hum. As well as explaining why only some people are affected, Prof Deming's theory predicts that hot spots should be found around coastal communities - which is indeed the site of many reports of the Hum.
The submarine communication theory has its problems, however. People who can "hear" radio transmissions say that it sounds like a clicking noise, not a low rumble. In any case, it's known that some cases of the Hum have perfectly simple explanations. For example, in 1999 the small town of Kokomo, Illinois, appeared to fall victim to the Hum, with residents complaining of a constant low rumble. Investigations confirmed they were not imagining things: their homes were engulfed in waves of two types of low-frequency sound.
By monitoring the noise at different points around the town, the culprits were pinned down: an air compressor on an industrial site, and a cooling tower fan at a car manufacturing plant. Both sites underwent modifications to damp down the noise levels - and after a while reports of the Hum stopped. Last week a further twist to the Hum controversy emerged. According to Dr David Baguley, an audiologist at Addenbrooke's Hospital in Cambridge, England, the human ear can adjust its sensitivity according to circumstances, with people in stressful situations having more acute hearing. Dr Baguley suspects that some people may focus on some faint background sound, and become concerned about its cause. The resulting stress boosts the sensitivity of their ears, allowing them to pick up even fainter sounds, creating a vicious cycle.
With his background in the life sciences, Dr Baguley doesn't claim to have found the key to the whole mystery of the Hum. In true splittist style, he points out that perhaps a third of all cases may still be traced to sources like industrial plants. Even so, his theory has one big advantage: it offers the prospect of peace and quiet for at least some of those afflicted by the Hum. Dr Baguley has now won funding from the UK government to see if relaxation techniques can help sufferers break the vicious cycle that makes them overly sensitive to low-level noise.
When it comes to explaining the Hum, the lumpers appear to be losing out to the splitters. But if there is one common theme running through the various explanations now emerging, it is that the human sensory system is even more astonishing than anyone could have guessed. Robert Matthews is a visiting reader in science at Aston University, Birmingham, England