I posted a link to this way back. In the light of recent noise legislation on both sides of the pond is it worth another look? http://www.woodynorris.com/Articles/USAToday2.htm ETA: Audio spotlight http://www.holosonics.com/technology.html The idea is to modulate an ultrasonic carrier wave with the audible frequencies required - say the audible sounds from an electric motor in the rear of the vehicle.