Sounds Like Bad News For Speeding Motorists

Speed demon motorists watch out - the police may soon have a new weapon up their sleeves with which to trap you - and it works just by listening to sound you car makes as it speeds past a microphone. The novel trap technology, which is being developed by researchers at the University of Tennessee and Battelle Institute in Oak Ridge, relies on the doppler effect - the way a sound alters in pitch as a moving object approaches and then passes the listener - to calculate the speed of passing vehicles. And because it relies only on a passive microphone, which eavesdrops silently on a car's engine note, motorists have no chance of being able to detect it and slam the brakes on to avoid a fine. To prove that the idea works, the development team recorded the noises made by a number of moving vehicles and then calculated their speeds based on the doppler shift of the sounds made in each case. The system was right to within a few percent in 32 out of 33 of these trials. It can even work out how large an engine is, just by listening to the sounds of the pistons, and whether a vehicle is overloaded by comparing the change in road speed relative to the change in engine load as the vehicle climbs an incline.