FOR teenagers, it is a sound as irritating as their mother’s nagging: a high-pitched shriek that refuses to go away. For adults, the sound is . . . silence.

It is called the Mosquito and it is designed to deter young people from loitering outside shops. It emits a piercing, high frequency sound that is audible only, in 90 per cent of cases, to people under the age of 20. While teenagers are forced to run for cover, most adults remain oblivious.

Nearly 100 stores and some local councils have placed orders for the device, which went into production on Wednesday. A retail chain also plans to introduce it in its corner shops.

The Times tested a prototype at a Hallmark newsagent’s shop in Wembley, northwest London, where staff said that they were powerless to deter congregating youths.

“The worst time is when school finishes, between 2pm and 3.30,” Gunapal Murugan, who lives above the shop, said. “They make trouble, throwing vegetables from (the shop) next door. Also, they throw bottles of water. Sometimes they come and fight.

“You can’t do anything. They’re under age. If we say anything, they swear back.”

At first Mr Murugan was sceptical about the Mosquito, which he could not hear — and he was surprised by its £500 price — but when some young customers came to the shop he watched their reactions. The group of students from Wembley High Technology College all agreed that the sound was disquieting.

“It’s like a dog whistle but I’m not a dog,” a bemused Shaker Zaman, 17, said.

Sunil Pankhania, also 17, said: “I can feel it — it hurts. It’s like your ears are blocked. It’s hurting my teeth. I’ve got sensitive teeth.”

Not everyone was convinced, however. Ekta Shah, 17, said: “It might work on some people. It’s not annoying enough, but it is annoying. If someone wants to make trouble, they’re going to do it anyway. It’s not going to stop them.”

By the time the teenagers had left, Mr Murugan was a convert and said that he would speak to the manager about the device. “I think it would make a difference,” he said.

The first retailer to try the system was Robert Gough, who owns a Spar shop in Barry, South Wales. He said that gangs had now disappeared from his shop. “Either someone has come along and wiped them off the face of the earth, or it’s working,” he said.

The Mosquito was invented by Howard Stapleton, the managing director of Compound Security Systems in Merthyr Tydfil, Mid Glamorgan, who designed it for his local store. He said he came up with the idea after reading how hearing levels changed with age and had spent “a couple of hundred hours” researching the subject.

NIGEL HAWKES SIXTYSOMETHING

OLDER people hear things less acutely, as most over the age of 60 can testify; so I could not hear what the fuss was about when the Mosquito was set off in the office. Others seemed irritated by it, though, which was encouraging.

The condition has a name: presbycusis. It involves the progressive loss of hearing, beginning with high-frequency sounds such as speech. The problem is especially acute in crowded environments, so it is sometimes referred to as cocktail party hearing loss. Sufferers become adept at lip-reading, or lose interest in cocktail parties: either is a perfectly sensible adaptation.

Age-related hearing loss tends to run in families. About a quarter of people aged 65 to 75 suffer from it, and 70 to 80 per cent of those over the age of 75.

SAM LISTER TWENTYSOMETHING

YOU learn to adapt to most loud noises in a sprawling, open-plan office of 100, occasionally intemperate, people, but this particular piercing screech is not one of them.

When the pulse — a cross between fingernails down a blackboard and a dental laser — kicked in, my first thought was a computer malfunction. Then a brain malfunction. Then a heart malfunction.

I turned to my older and wiser colleagues for help. They looked back in bemusement. I worriedly inquired if this was some sort of computer system failure, or a fire alarm short-circuit, perhaps. The waves of thin, high-pitched noise were unpleasant in the extreme.

They shook their heads, perplexed. With looks of part sympathy, part eye-rolling weariness, they gently ignored me and returned to their work.

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