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Author Topic: Motion dectectors and acoustics  (Read 3268 times)

Tomas Danko

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Motion dectectors and acoustics
« on: June 26, 2007, 08:43:18 AM »

I saw an episode of Mythbusters on Discovery where they tried to beat a motion detector.

They said it worked by sending out high frequency waves that bounced around the entire room and then back. The sum of it all became the "fingerprint", and it should remain the same at all times.

If someone should enter the room, the "fingerprint" would obviously change and set off the alarm.

First they wanted to pad the person entering the room with thick fabric. But I mean, any body inside a room will act as a form of absorber resulting in some loss of reflecting waves to the detector. This would of course alter the "fingerprint".

It didn't work. But I'll tell you what worked:

Holding up a white bed sheet in front of you while walking through the room.


I don't get it. How could this work, theoretically speaking? I'm thinking the explanation above how the detector works must be all wrong, because physics sure works.
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jimmyjazz

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Re: Motion dectectors and acoustics
« Reply #1 on: June 26, 2007, 05:16:24 PM »

I thought they were IR (infrared) detectors?  Are you implying they work on audio-band (or near-audio-band) wavelengths?
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Tomas Danko

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Re: Motion dectectors and acoustics
« Reply #2 on: June 26, 2007, 05:36:38 PM »

jimmyjazz wrote on Tue, 26 June 2007 22:16

I thought they were IR (infrared) detectors?  Are you implying they work on audio-band (or near-audio-band) wavelengths?


They mentioned sonic, I think. They did another test with a different infracamera, but that's a whole different issue.

Still, how would the physics differ then it's sending off wavelenghts that bounce around the room and hit the detector again? If so, I'd like to know what's going on.
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"T(Z)= (n1+n2*Z^-1+n2*Z^-2)/(1+d1*z^-1+d2*z^-2)" - Mr. Dan Lavry
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Jessica A. Engle

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Re: Motion dectectors and acoustics
« Reply #3 on: June 26, 2007, 05:56:28 PM »

We had a motion detector in my old house.  It worked with a laser (is synonamous with infrared?).

The reason I know this is because the security guys who installed it had such a hard time knowing where to put it (as my parents were too cheap to buy more than one):

index.php/fa/5502/0/

There wasn't really a great way to cover both the front and side door, so this was the compromise they came up with.  

Nothing outside it's line of vision was detectable, and it didn't descriminate between a dog walking into the beam or a human.  Maybe newer systems can tell the difference?  Maybe a sheet, as a flat surface can be mistaken for a wall, but a human covered in fabric may still "look" enough like a human to set it off.  Just wonderin'.

Jessica
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Tomas Danko

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Re: Motion dectectors and acoustics
« Reply #4 on: June 26, 2007, 06:04:20 PM »

Dear Jessica,

Thank you for the blue print. May I also have the address?  Twisted Evil

We had a bunch of detectors overlapping, at my previous store. Would that require two sheets to hold up both front and back? I just don't get it.

I'm thinking a moving wall should also trigger such a detector since it's rather large don't you think? And if the detector is trying to find differences in the room, I figure any change in geometry should render a different return-pattern.

I'm puzzled.
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"T(Z)= (n1+n2*Z^-1+n2*Z^-2)/(1+d1*z^-1+d2*z^-2)" - Mr. Dan Lavry
"Shaw baa laa raaw, sidle' yaa doot in dee splaa" . Mr Shooby Taylor

Jessica A. Engle

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Re: Motion dectectors and acoustics
« Reply #5 on: June 26, 2007, 06:16:38 PM »

Tomas Danko wrote on Tue, 26 June 2007 17:04

Dear Jessica,

Thank you for the blue print. May I also have the address?  Twisted Evil




Haha!  You know what's funny, is that next to the front door (outside the range of the motion detector!) was a big window which I could easily break my way into with a wire from a hanging flower basket every time I forgot to take my house keys with me!  I was in grade school then, and I routinely broke into my own house.  Some security system, huh?  Of course, once I got inside I could find my keys and deactivate the alarm.

Sadly, tho', that house has been bulldozed to the ground.  Otherwise I'd let you have at it.

Jessica

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Tomas Danko

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Re: Motion dectectors and acoustics
« Reply #6 on: June 26, 2007, 06:27:52 PM »

Jessica A. Engle wrote on Tue, 26 June 2007 23:16

Tomas Danko wrote on Tue, 26 June 2007 17:04

Dear Jessica,

Thank you for the blue print. May I also have the address?  Twisted Evil




Haha!  You know what's funny, is that next to the front door (outside the range of the motion detector!) was a big window which I could easily break my way into with a wire from a hanging flower basket every time I forgot to take my house keys with me!  I was in grade school then, and I routinely broke into my own house.  Some security system, huh?  Of course, once I got inside I could find my keys and deactivate the alarm.

Sadly, tho', that house has been bulldozed to the ground.  Otherwise I'd let you have at it.

Jessica



Shocked

But, that's cheating!

Or should I say, sheeting? Although I guess you didn't need to wear any bed sheets to get inside.
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"T(Z)= (n1+n2*Z^-1+n2*Z^-2)/(1+d1*z^-1+d2*z^-2)" - Mr. Dan Lavry
"Shaw baa laa raaw, sidle' yaa doot in dee splaa" . Mr Shooby Taylor

franman

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Re: Motion dectectors and acoustics
« Reply #7 on: June 29, 2007, 03:44:44 PM »

Tomas and Jessica... get a room!!  Laughing

I think the 'infrared' motion detectors use light reflections not ultrasonic sounds... That's the way the ones in my office works (I'm pretty sure). You just have to be out of "site" of it to set the alarm, or you have to stay very still... I don't think it's an acoustic sensor at all, although alarm companies DO use acoustic sensors, as does your car alarm, for broken glass, etc..
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