clipped from: news.bbc.co.uk 
The Mosquito emits a high-pitched sound and has been designed for use in potential crime hotspots such as subways and underground car parks.
That version emitted a noise at such a high frequency that it was usually only audible to those under the age of 25.
The makers, Compound Security Systems, have come up with a solution - a Mosquito which is annoying whatever people's age.
The new Mark 4 has an additional setting which allows the user to lower the frequency enabling the sound to be heard by people of any age.
According to the company, a major chain of hotels in Canada sees them as a way to keep homeless people out of their car parks.
There is currently no specific legislation against the Mosquito.