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My car has ultrasonic proximity sensors to help me park. I've noticed that when motorcycles whiz past me the proximity alarm goes off. I originally thought that the motorcycles were just too close, but now I have observed that that isn't the case; cars or other road users at a similar distance and speed do not set off the sensors. It doesn't seem to matter what speed I am traveling.

I am not interested in troubleshooting some aspect of the alarm. I'm just trying to understand why this occurs. What special characteristic of a motorcycle or its movement causes it to be detected by the sensors when other road users are not detected?

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    $\begingroup$ Do you happen to notice whether the motorcycles that caused this were noticeably louder than normal vehicles? $\endgroup$ – Dan Feb 17 '15 at 17:10
  • $\begingroup$ @Dan I haven't noticed per se that they are louder, but it is possible that they are. The sensor goes off and I see the motorcycle whiz past, but thinking about it I also hear the motorcycle so this might indicate they are quite loud. $\endgroup$ – nmtoken Feb 18 '15 at 11:21
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It is likely that the noise of the motorcycle is causing the sensor to activate.

This listing of the advantages and disadvantages of ultrasonic sensors lists one of the disadvantages as:

While ultrasonics exhibit good immunity to background noise, these sensors are still likely to falsely respond to some loud noises, like the hissing sound produced by air hoses and relief valves.

A loud motorcycle could fall into that same level of noise (sound pressure).

Other sensor spec sheets that I have seen have phrases like:

Many acoustic noise sources will have little to no effect on the reported range...

This would likely be why the sensor is not going off all the time, but is triggered by loud noises.

The noise from a motorcycle is about 100dB.

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