olice and councils are considering monitoring conversations in the street using high-powered microphones attached to CCTV cameras, write Steven Swinford and Nicola Smith.

The microphones can detect conversations 100 yards away and record aggressive exchanges before they become violent.

The devices are used at 300 sites in Holland and police, councils and transport officials in London have shown an interest in installing them before the 2012 Olympics.

The interest in the equipment comes amid growing concern that Britain is becoming a “surveillance society”. It was recently highlighted that there are more than 4.2m CCTV cameras, with the average person being filmed more than 300 times a day. The addition of microphones would take surveillance into uncharted territory.

The Association of Chief Police Officers has warned that a full public debate over the microphones’ impact on privacy will be needed before they can be introduced.

The equipment can pick up aggressive tones on the basis of 12 factors, including decibel level, pitch and the speed at which words are spoken. Background noise is filtered out, enabling the camera to focus on specific conversations in public places.

If the aggressive behaviour continues, police can intervene before an incident escalates. Privacy laws in Holland limit the recording of sound to short bursts. Derek van der Vorst, director of Sound Intelligence, the company that created the technology, said: “It is technically capable of being live 24 hours a day and recording 24 hours a day. It really depends on the privacy laws in a particular country.”

Last month Martin Nanninga of VCS Observation, the Dutch company marketing the technology, gave a presentation to officials from Transport for London, the Metropolitan police and the City of London police about the CCTV system. Nanninga is to return next year for further discussions.

“There was a lot of interest in our system, especially with security concerns about the Olympic Games in 2012. We told them about both our intelligent control room and the aggression detection system,” Nanninga said.

In Holland more than 300 of the cameras have been fitted in Groningen, Utrecht and Rotterdam. Locations include city centres, benefit offices, jails, and even T-Mobile shops. The sensitivity of the microphones is adjusted to suit the situation.

Police and local council officials are still assessing their impact on crime, although in an initial six-week trial in Groningen last year the cameras raised 70 genuine alarms, resulting in four arrests.

Harry Hoetjer, head of surveillance at Groningen police headquarters, recalled an incident where the camera had homed in on a gang of four men who were about to attack a passer-by. “We would not normally have detected it as there was no camera directly viewing it,” he said.

Last Friday a Sunday Times reporter visited the office of Sound Intelligence in Groningen to test the system. The reporter stood in the control centre with a view of an empty room on one of a bank of monitors. Van der Vorst entered the room, out of sight of the camera, and began making aggressive noises.

The camera swivelled to film him and an alarm went off in the control room, designed to alert police to a possible incident. “The cameras work on the principle that in an aggressive situation the pitch goes up and the words are spoken faster,” said van der Vorst. “The voice is not the normal flat tone, but vibrates. It is these subtle changes that our audio cameras can pick up on.”

Public prosecution services can use them in court as evidence. The Dutch privacy board has already given its approval to the system.

According to a spokesman for Richard Thomas, Britain’s information commissioner, sound recorded by the cameras would be treated under British law in the same way as CCTV footage. Under the commissioner’s code of practice, audio can be recorded for the detection, prevention of crime and apprehension and prosecution of offenders. It cannot be used for recording private conversations.

Graeme Gerrard, chairman of the chief police officers’ video and CCTV working group, said: “In the UK this is a new step. Clearly there is somebody or something monitoring people speaking in the street, and before we were to engage in that technology there would be a number of legal obstacles.