I was halfway through my morning run when I heard it. The sound of the apocalypse. An air raid-style siren, slowly wailing up and down in pitch, was blaring over Belfast at 0830 GMT. I glanced at the sky.

But nobody else nearby seemed to be taking any notice. My wife, who had her earphones in, hadn't heard the noise at all until I pointed it out. It was the same at the supermarket on the way home. The din of the siren was audible across East Belfast, and yet, life continued as normal.

The bizarre Belfast siren was, the BBC understands, associated with a fuel storage facility in Belfast Harbour. It is a fire alarm and has to be so loud partly because of the large size of the facility in question.

This kind of siren, so strongly associated in Britain with World War Two, is actually more than a century old, and has been used for all kinds of emergencies - not just Luftwaffe bombing raids.

Other sounds and tones are sometimes deployed for outdoor alerts, but that lazily wavering siren that is so strange and chilling is still in place at multiple sites around the country - from military bases to chemical factories. Just in case.

"That wail pattern does a great job," says Evan Kerr, VP of operations at Sentry Siren, a US firm that still makes mechanical sirens. "You need to get people's attention."

He explains that the sound is produced by a fast rotating fan inside a flat cylindrical housing, the outer rim of which is perforated with slits. It's the forcing of air through these slits, at oscillating speeds, that creates the extremely loud wail in all directions.

These days, this sound can be replicated by a digital recording played through loudspeakers but Mr Kerr argues that mechanical wailing sirens, which have been manufactured since at least 1905, are proven, reliable and nearly maintenance-free. He says some sirens made by his firm, previously known as Sterling Siren Fire Alarm Company, have been in place for many decades.

In the UK, there are air raid-style sirens in a number of locations. Notably, at the Royal Navy base in Portsmouth. This siren, which is tested regularly, would be used to warn members of the public about an accident with the nuclear-powered submarines that are stationed there.

A spokesman for the Ministry of Defence says that there are also outdoor warning alarms at naval bases on the River Clyde in Scotland and Devonport in Plymouth: "The sirens are regularly maintained and monitored. There are no plans to change the emergency alarm system."

Some siren locations are, perhaps, less obvious. Take The State Hospital, a psychiatric hospital in Carstairs, Scotland. Its siren system was sounded, for example, when two murderers escaped in the 1970s.

Broadmoor used to have a siren but this was decommissioned in 2019 and, if a dangerous individual were to escape the hospital now, an alert would be sent to locals in the form of an email or text message.

Industrial sites that handle significant quantities of hazardous materials are also legally required to have outdoor warning systems. Take Hampton Water Treatment Works in West London - even it has a siren.

In Bradford, there is a siren at a large chemical plant owned by Solenis. In information sent to locals, the company explains why an accident might be harmful. A toxic cloud could form or, for example: "A release of a flammable vapour, with subsequent ignition, may result in an explosion with the potential for damage both within the Solenis site, and outside its boundaries."

Advice from the company explains that, if people hear the siren when it is not due to be tested, they should shelter in their homes, close all windows and curtains, block incoming draughts, switch on the radio and "stay calm and rest".

Source:
courtesy of BBC NEWS

by Chris Baraniuk

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