• Apr 27, 2006
New Scientist magazine reports that researchers at the University of Tennessee and Batelle Institute have applied for patents on a system that detects the speed of a passing vehicle based on its exhaust note.

The system uses microphones to capture the sound of a passing car, and then filters out all but the sound made by the engine. By measuring the doppler shift of the engine sound as it moves past the speed trap, the system can calculate the vehicle's speed.

While the sound detector is unobtrusive and completely passive (therefore undetectable by a conventional radar/laser detector), identifying speeders will require something a little less stealthy, like a traffic camera. Of course, other early detection systems are possible, like the subtle warning in the accompanying picture.

Hmmm. Sounds like better muffler technology is in order.

[Source: New Scientist]


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  • 19 Comments
      • 8 Years Ago
      What about hybrids? When operating on the electric motors do they produce any noise from the tailpipes?
      • 8 Years Ago
      just learn how to rev the throtte as you pass... itll counteract the doppler effect (apparent frequency decreases as outbound velocity goes up, so, just rev it to boost the freq!)... if its listening to exhaust note.
      • 8 Years Ago
      This is crap, what about performance exhausts? Am I going to get a $500 ticket because they think i'm going a bazillion miles an hour? doubtful, there are plenty of noises that will obstruct such a device, especially in the ny metro area
      • 8 Years Ago
      Perhaps you could put a highly directional speaker in the back on your car. You could then produce a warble tone at or around the frequencies they are using. This would mask the Doppler effect of your engine.
      • 6 Years Ago
      I bought a green laser pointer form http://www.highlasers.com.
      Pretty good. Awesome!
      • 8 Years Ago
      There are some real idiots here that either don't understand science, or don't read beyond the headline. This doesn't measure the pitch or loudness of the exaust, but the doppler shift, which basicly is the change of pitch over time.

      As for trying to fool it by accellerating at the right time, I'm not sure if that would work or not. Depends how sophisticated the software is, I guess.
      • 8 Years Ago
      even if you have a completely silent drivetrain, the car still makes noises. like tires on the road surface and the air as it parts around the car. you could just as easilly filter on those sounds and measure the doppler shift in them.

      solution, 115dB freight train airhorn. audio recording systems can't filter a heck of a whole lot when they are saturated by 115dB of shorts soiling sound. "sorry officer, i went fot the cruise control and hit the horn"
      • 8 Years Ago
      This is nothing new. I looked at this 9 yrs ago as a way of verifying that certain racing teams weren't violating a rev limiter rule. Not only could you pick out the RPM but also the speed of the car...provided your measurement wasn't contaminated by too much background noise. It's amazing what you can learn about a car based on its raidated noise.
      • 8 Years Ago
      Engine noise? Won't this be inaccurate if the driver is accelerating or decelerating? The noise is only constant if the engine is at constant revs.
      • 8 Years Ago
      what about tuner cars. most exhaust notes are higher than normal cars?
      • 6 Years Ago
      I bought a green laser pointer form http://www.highlasers.com.
      Pretty good. Awesome!
      • 8 Years Ago
      explain to me how they can target YOU specifically on a highway unless there was no other car nearby.

      also, another reason to get a hybrid which can shut down the internal combustion engine at speed (or by user input).
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