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Some people in Bristol say they are plagued by a mysterious low-level hum that no-one can trace. But strange inexplicable sounds are something of a nationwide problem, writes Gareth Rubin.

Over the past fortnight Bristolians have been reporting their problems with the "hum" to the local newspaper and on social media. It has been driving them to distraction, they say, and no-one can tell where on earth it's coming from.

But it's not the first time the hum has kept Bristol awake. In the 1970s hundreds of the city's residents complained to the council that a strange noise was audible at night. Most of the experts drafted in put it down to factory noise, electricity pylons or tinnitus - while some of the more imaginative suggestions included the sound from flying saucers hovering over the city or secret military activity. Eventually, it stopped as abruptly as it began, but not before it had spawned reports of equally unidentified hums in other towns across Britain.

Then last year French scientists announced that they had solved the conundrum. It was, they said, the effect of continuous waves causing the ocean floor to vibrate. "We have made a big step in explaining this mysterious signal and where it is coming from and what is the mechanism," said Fabrice Ardhuin of the Centre National de la Recherche Scientifique. But his answer doesn't immediately explain why the sound was only around for a number of years. Or why it might have returned.

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So if it's not endless waves on the ocean floor, then what are people hearing? Chris Borak, senior acoustic consultant at Ion Acoustics in Bristol says the most likely answers are disappointingly mundane. "It's probably traffic noise. When it's night and there's no background noise you can hear sounds more easily. Or it could be some sort of industrial installation nearby or air conditioning units - even a wind turbine some distance away."

If it's none of those things, it could be a medical issue for the person who hears it. "When I moved from the centre of Bristol to a much quieter area I would wake up in the middle of the night thinking I had heard a low-frequency noise but I just put it down to tinnitus. That's the most likely explanation when there is no measurable noise."

What is tinnitus?

Image copyright Science Photo Library

Tinnitus is the term for hearing sounds that come from inside your body, rather than from an outside source.

It's often described as "ringing in the ears"

Some people may hear sounds similar to music or singing, and others hear noises that beat in time with their pulse (pulsatile tinnitus)

Find out more from NHS Choices

It has been estimated that where town hums are reported, only about 1 in 50 people is able to hear them. "It's to do with the psychology of the individual," Borak says. "If you put two people in an audiology lab and test them, they will have the same sensitivity - but one will be significantly more annoyed by the noise. It's a minefield trying to assess who's going to be bothered by it."

Media playback is unsupported on your device Media caption The Bristol Hum is back, according to reports. But did it go away? In 1980, the council's environmental health team investigated.

Hums have been reported in a number of other places since Bristol first experienced it. In 2013 the residents of Hythe in Hampshire made more than 30 complaints to the local council about a mysterious sound keeping them awake. It was reported that it could be the mating call of rare fish. The Scottish Association for Marine Science - which was cited - later clarified that it hadn't heard the noise and thought fish were unlikely to be loud enough to keep people awake.

Elsewhere, Leeds had its hum keeping residents awake, and Largs in Scotland had one. In Manchester, a disconcerting wailing last year was put down to wind around the city's Beetham Tower.

But somewhere between the simple cause of wind around high towers, and alien visitation, lies another explanation. "Your brain can often fill in a sound where there isn't one," says Ze Nunes, owner of MACH Acoustics in Bristol. "You hear your mobile phone ringing, but no-one is calling you. We get called to a lot of 'fake complaints' - but it's not bogus, the person hearing the noise really does think it's there."

More from the Magazine

A village in Durham reported a strange vibrating noise in 2011 - known as "the hum". Why is it such a mystery?

Who, What, Why: Why is "the hum" such a mystery?

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