(Listen to the Teen Buzz ringtone)

An inventor from Merthyr Tydfil hopes to make his fortune by selling a mobile phone ringtone that he has never heard and never will.

Howard Stapleton made headlines last November with the launch of the Mosquito, a device that emits an annoying ultrasonic hum designed to deter teenage yobs from hanging around outside shops and public buildings. The vast majority of adults over 20 cannot hear the sound emitted.

Mr Stapleton has since sold more than 1,000 of the £500 devices through his company, Compound Security Systems, but now hopes that a spin-off product - the Mosquitotone - could be even more lucrative.

The ringtone was initially the work of an unidentified group of Welsh teenagers who decided that a high-frequency ringtone would be the ultimate in teacher-proof technology. Their product, dubbed Teen Buzz, spread like wildfire through classrooms in the UK and abroad, allowing pupils to receive surreptitious text messages without their teachers noticing.

But when Mr Stapleton's 16-year-old daughter, Isabel, came home with one of the ringtones on her phone three months ago, the inventor decided that he was missing out on an opportunity to make money out of his own invention and quickly devised the "official" Mosquito ringtone, now available via text message for £3, which he says is better.

He told Times Online: "It makes me smile. Some kid has applied science and taken my product - which is designed to keep them away from shops, and turned it into a ringtone.

"I had considered making a ringtone in January, but decided that possible misuse could cause problems in the classroom... Then I decided that if someone was going to make money out of it, I may as well."

Mr Stapleton first realised that people gradually lose the ability to hear high-frequency sound when he was just 12 and visited a factory in the Midlands with his father, Colin, then chairman of the British arm of Ever Ready batteries. The factory used ultrasonic welding techniques, using high-frequency sound to melt and fuse plastics, but the young boy had to walk straight out because of the excruciating noise - which none of the adults was even aware of.

At 39, Mr Stapleton is now too old to hear either the Mosquito deterrent system or its ringtone spin-off, but he still works with his father, now 70.

"My father is a director of Compound Security Systems and when I joked that the ringtone might be the icing on the cake, his comment was that this might actually turn out to be the cake," he said. "When I explained the revenues generated by ringtones he was quite amazed. Having four children, I'm well aware of the money they make."

Mr Stapleton is lining up the Mosquitotone for international distribution and is also working at making the tone less annoying, perhaps adding a simple melody. The 'unofficial' version of the tone reaches a frequency of 15 kilohertz while the official one reaches 17 kilohertz, making it less audible to adult ears.

An unscientifc test at Times Online suggests that it definitely works - only the callowest reporters noticed the noise.

But if Mr Stapleton has misjudged his market, he still has the original teenage deterrent device, the Mosquito, to fall back on.

That gadget has been licensed to Cooper Menvier, a leading security products manufacturer, and the Mark II version is due out soon, with added safeguards including automatic cut-off to prevent abuse and head off accusations that it infringes the human rights of teenagers.

The inventor says that the Mosquito is bound to be more efficient than a rival anti-yob sonic device being trialled in the Sydney suburb of Rockdale - loudspeakers playing Barry Manilow tunes or other "daggy" music.

"The obvious benefit of the Mosquito is that it doesn't cause noise pollution," he said. "Besides, the kids might actually get a taste for Barry Manilow or Cliff Richard. I'm yet to find a kid who likes the Mosquito."

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