The system generates a pulsed, near ultra sonic tone at around 16KHz, close to the limit of the hearing range of people under 25 years of age. The sound, though not painful in any way, becomes highly annoying after 5 or so minutes exposure. The result is that most under 20s leave the immediate area, whilst the majority of those over the age of 30, because of natural age related hearing loss, either just hear, or, as in most cases, are totally unaware of any noise at all. The system is designed to be used for up to 20 minutes at a time. The unit incorporates a 20 minute cut-off time to avoid unnecessary annoyance. Mosquito complies with the HSE 2005 guidelines and is already widely used by police and local authorities around the UK. Effective range is at least up to 20 Metres.