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when car alarms go off in a thunderstorm,

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soccerballtux said:
Do you think it's because of the noise, or because of the EM disturbance?
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I'd guess it's the vibration from the thunder on alarms that are set to be too sensitive. In my neighborhood, the cars triggered by thunderstorms are the same ones that go off when someone with a really loud subwoofer drives by.
 
I guess you can tell by seeing whether or not the car alarms go off a little after the flash... or a little after the thunder.
 
Yup, once in awhile someone will light off an M80 firecracker nearby & the shockwave enables some of the parked car alarms. (Im sure that's part of the strategy.)
 
There is a car in my parking lot that goes off when it rains... at least it did until the third of fourth noise complaint.

I'd wager most alarms are simply accelerometers that go off to various intensities of touch. Some set far too sensitive.

The EM from a storm is not going to affect any modern (not much old stuff either) electronics unless the lightning actually strikes it.
 
Here is how shock sensors work:

It is a little vacuum tube with 2 very light gauge wires in parallel. When something shakes the tube (or whatever the tube is attached too, quite often the dashboard of the alarmed vehicle) - those light gauge wires vibrate too - often touching. There is logic on the alarm to only "pre-alarm" which is to not go off half-cocked, just let a little yelp or two out. The idea is to scare off the kids who bumped into your car without going into full blown alarm panic.

Often thunder is very violent and contains a lot of deep bass (well below hearing threshold, which is why you often feel thunder). This trips the shock sensor for an extended period, leading to all out alarm.

Also some alarms have glass break sensors, which are mics tuned to a particular frequency. Thunder often overloads them.

I used to set off alarms with my 89 firebird - its exhaust was pretty much cosmetic.
 
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