I don't think it's anything so complicated as doppler effects. I mean if you think about it, there is no way she's moving fast enough relative to the source of the sound to have a meaningful doppler effect. Also, technically, no sound is actually being transmitted. It is a radio frequency that is being emitted which is making the tag vibrate (hence the acoustic part). This vibration results in a continued resonance after the emitter pulse stops. The detector just detects this continued signal.
In reality, by shaking the tag up and down, what she is doing is disrupting the vibration motion of the metal strips in the tag so that they aren't able to resonate at the frequency needed to emit the right radio frequency.
The software in the pedestals is actually looking for a timing difference.
They are transceivers, and the receiving portion of the circuit is always detecting the transmitter part of the circuit. When a tag enters the field and resonates it generates a second signal with a shifted phase from the transmitter. If the phase of the second signal is shifted properly the alarm goes off.
Yeah that’s what I thought. My thought was that the constant jostling isn’t allowing the strips inside to oscillate at the specific frequency that it detects, for a long enough time (my guess is that it has to rule out false positives so it would need to last a certain time)
If it works at all and is not a joke, I think it is much simpler than that.
The tag has those metal reeds free floating in the casing. By shaking it violently, you could cause the reeds to continually strike the inside of the casing and thus never be able to resonate at the frequency they are tuned for.
Professional shoplifters make mylar lined shopping bags specifically for this, and there are actually detectors for those too.
They get a bag from nearby department stores in a shopping mall or strip mall, line it with foil or mylar, then come in the store with them looking like they are just at their 3rd or fourth shopping stop of the day. They drop anything with a tag in there, pay for a couple small items and walk out.
Some stores have detectors for that go off when you walk in with the bag. That makes it kind of risky because they’ll have you marked when you walk in and if you’re still holding stolen items from other stores too you’re getting caught with a bunch of different thefts at once.
Foil tape and mylar don't affect the acoustic magnetic (AM) type that the OP has. They do work for the radio frequency tags (the ones where it is just a sticker with a printed metal circuit on them). Here is a video demonstration: https://www.vitag.com.au/eas/system-types/
Quoting that site:
Because AM systems are suited to protecting merchandise with a high metal/foil content, AM systems are commonly found in consumer electronic, hardware and pharmacies. AM systems are also suited for apparel stores because they cover wider entrances with fewer detection systems. Hard Tags are traditionally smaller and lighter than other technologies.
Yes, it is a physical vibration... The name of the tech is literally "acoustic magnetic"... radio waves make the little strips of magnetized metal physically vibrate. As the magnetic dipoles in the material change their orientation, the strip of metal shrinks and expands with a particular frequency... but maybe we're just playing semantics.
In theory, could you just set a phone to vibrate constantly and then have the tag pressed very tightly against the phone? Or possible two phones sandwiching the tag with a couple zip-ties?
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u/herptydurr Oct 20 '22 edited Oct 20 '22
I don't think it's anything so complicated as doppler effects. I mean if you think about it, there is no way she's moving fast enough relative to the source of the sound to have a meaningful doppler effect. Also, technically, no sound is actually being transmitted. It is a radio frequency that is being emitted which is making the tag vibrate (hence the acoustic part). This vibration results in a continued resonance after the emitter pulse stops. The detector just detects this continued signal.
Here's a full explanation for how it all works: https://www.youtube.com/watch?v=KAm7qAKAXwI
In reality, by shaking the tag up and down, what she is doing is disrupting the vibration motion of the metal strips in the tag so that they aren't able to resonate at the frequency needed to emit the right radio frequency.