Taxi operators are being offered a sound "barrier" so intolerable to the ear that its promoter is confident it will instantly neutralise any attacker.

"I can't stand the sound of it - the longest I've lasted listening to it is four seconds," Sonic Security director Matthew Adams told the Herald, before demonstrating it yesterday to ear-piercing effect.

His device, the Inferno, was developed for the Swedish military for a crowd-control alternative to tear gas.

He has supplied about 40 units to New Zealand retailers as anti-burglar alarms and is in discussions with taxi and liquor industry representatives about their potential for protecting cabbies and sales assistants against increasingly common attacks.

The Inferno becomes intolerable to the ear by operating on four separate frequencies at once, and Mr Adams says it is about five times louder than a standard two-toned burglar alarm.

Although he says it has been cleared by Sweden's Royal Institute of Technology against causing any permanent hearing loss, Herald reporters found it impossible not to automatically clap hands over ears within milliseconds of activation.

That echoed the experience of 10 people who, in a Swedish experiment, were given keys to a box containing the equivalent of about $100 and told they could keep the cash if they could get at it while the alarm was sounding. The only one to reach the box was unable to insert the key.

"It produces wild and rapid oscillations designed to ... confuse the brain," Mr Adams said. "The four frequencies oscillate quickly, meaning the brain can't lock on to them."

He has the rights to distribute two versions of the machine, the smaller of which he believes would be suitable for installation in taxis, to offer what he says would be far greater protection than the cab cameras Transport Minister Steven Joyce wants made compulsory by this time next year.

Although taxi drivers would also find the noise intolerable, he said they would have the element of surprise.

Although the Taxi Federation favours cameras after the murders of two drivers in little more than a year, Mr Adams said he had yet to meet a cabbie who believed they would be effective enough against "the really crazy guys committing extreme crimes".

He said several taxi operators had shown an interest in acquiring the device, subject to transport licensing approval, and he was confident of supplying it for less than the estimated $1000 to $1500 cost of cab cameras.

The managing director of Papakura-based Taxis United, Grahame Webb, said he had tried the device and believed it could be highly effective in conjunction with cameras.

"It would be great because the cameras are only good after the event, whereas this other thing could prevent an event."

He confirmed giving fellow Taxi Federation executive members "a sort of a half blast" with the device at a meeting in Christchurch, before having it banished by Aviation Security Service staff to the hold of an aircraft after demonstrating it to them before his return flight to Auckland.

Federation executive director Tim Reddish believed the device would have limited appeal, as there had been attacks in which drivers had insufficient time to activate existing panic buttons.

"It might be very good in the event of attack but what we are trying to do is deter attacks."

A spokeswoman for Mr Joyce said he had received advice on a range of taxi safety measures, including cameras, screens and duress alarms and there had been "a brief discussion" of technology using high-pitched alarms in-cab. "While I would not want to pre-empt Cabinet discussions, it seems that mandating for taxi cameras in our larger centres would bring the greatest benefits and would be widely supported by the industry," she said.

ALARM BELLS

Level at which instant damage to hearing is believed to occur - 140 decibels

*Inferno sound barrier - 124-127 dB

*Rock music peak - 150 dB

*Firearms, jet engine - 140 dB

*Jackhammer - 130 dB

*Lawnmower - 90 dB

Sources: Sweden's Royal Institute of Technology, American Speech-Language Hearing Association