A HIDDEN trap that detects how fast vehicles are going just by listening to them as they pass could catch speeding drivers unawares.
The system, being developed by the University of Tennessee and the Battelle Institute in Oak Ridge, uses microphones hidden by the roadside to measure the speed of passing vehicles. It does not emit telltale radiation, unlike radar or laser-based devices, so it cannot be picked up by dashboard detectors.
Once the microphones have detected and recorded the sound of a passing vehicle, digital filtering removes background noise to leave only the sound of the engine. Software then calculates the vehicle's speed by measuring the engine sound's Doppler shift - the change in a sound's pitch as its source moves past an observer.
The system, revealed by recently filed patents, has been developed with funding from the US Department of Energy. The microphones could easily be hidden in street signs, the patent says.
To test the concept, the researchers recorded a number of moving vehicles and then calculated their speed based on the Doppler shift. They calculated speeds to within a few per cent in 32 out of 33 experiments.
The system can also measure the engine speed in rotations per minute by detecting the pulsing sound made by the pistons. By comparing this with a library of acoustic signatures, it can estimate the size of the engine.
This information can even be used to measure the weight of vehicles, to catch overloaded trucks. Microphones installed on a slope with a known incline would record the engine as the vehicle starts to climb and use more power. Correlating the simultaneous changes in road speed and engine speed as the vehicle starts labouring up the slope, and comparing this against the size of the engine, gives a good estimation of the vehicle's weight.
From issue 2548 of New Scientist magazine, 24 April 2007, page 26
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