At my campus' main library, we have the standard devices posted at the exits to make sure that no students are trying to steal books. These security devices are common at most stores, and I think most people in industrialized nations are familiar with them.

When passing through them with my headphones on (Stanton DJ Pro 1000 MKIIs) I hear several constant high pitched frequencies that are quite uncomfortable. They have a radius of about 2 meters, with intensity, of course, increasing as I reach the epicenter of the devices. Do these machines work with sound to determine when an unauthorized object is passing through? Is it a byproduct of the electronic nature of the device? Am I just crazy?

Any magnetic force will affect any electronic device.

That's why they recommend not taking vhs or audio tape onto subways. It could degauss them.

And don't put your speakers on top of your tv. It screws the picture.

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"WEMF is...." and I can't remember the rest because I didn't write it down.
- Blackness at WEMF 2007

Probably something to do with the...magnets...in the headphones... They pick the stuff up...and amplify it, or else maybe its just interfering with it...or

Shit I dont know, but I bet it causes cancer.

** covering my ears **

I can almost hear it now! Ow!

___________________
"WEMF is...." and I can't remember the rest because I didn't write it down.
- Blackness at WEMF 2007

Another question I have is, would it be possible to capture this sound in anyway? Im looking for something better than putting a microphone up to the earphones; would I be able to run the line straight into a laptop, perhaps?

Would I still get this effect if I had nothing playing through my headpones? If the headphones were unplugged?

If the headphones were unplugged you wouldn't hear it.

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"WEMF is...." and I can't remember the rest because I didn't write it down.
- Blackness at WEMF 2007

quote:

Originally posted by muzzybear If the headphones were unplugged you wouldn't hear it.

thats not true. not sure if anyone in here owns a nextel phone but the frequency it services at causes speakers to clip & make high frequency noises all the time. Your campus' security system might not be magnetic at all because that could damage many things that go through those sensors. It might just be an awkward radio or sub-radio frequency that is affecting them.

quote:

Originally posted by Zombie0729 thats not true. not sure if anyone in here owns a nextel phone but the frequency it services at causes speakers to clip & make high frequency noises all the time. Your campus' security system might not be magnetic at all because that could damage many things that go through those sensors. It might just be an awkward radio or sub-radio frequency that is affecting them.

I thought fer sher that he wouldn't be able to hear the frequency from his headphones if they were unplugged...

I digress... (muzzybear using Irish Graham's login, dammit!)

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Note: Very tricky to get away with saying this in the US without getting strange looks from police officers.

Last edited by Irish Graham on Jan-20-2005 at 22:49

yeah the libary at the college i used to go to had one of them detector things, i remember when i had my tape walkman, that everytime i passed through it, the tape would warp slow etc as if a dj was pitch bending it. (tapes are magnetic based? if u ran a strong magnet past a tape, it would fuck it up. great way to fuck music stores up, have magnets in your sleeves and brush the tape section LOL)


however i dont think it affects my mp3 player, as ive walk through them detector things with no change in my music. i think?. oh well

its something to do with magnetic fields and all that bullshit

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quote:

Originally posted by KilldaDJ yeah the libary at the college i used to go to had one of them detector things, i remember when i had my tape walkman, that everytime i passed through it, the tape would warp slow etc as if a dj was pitch bending it. (tapes are magnetic based? if u ran a strong magnet past a tape, it would fuck it up. great way to fuck music stores up, have magnets in your sleeves and brush the tape section LOL) however i dont think it affects my mp3 player, as ive walk through them detector things with no change in my music. i think?. oh well its something to do with magnetic fields and all that bullshit

It isn't that the music gets fucked up. Its just like interference. Imagine high pitched feedback. Now, imagine 8 different pitches of said feedback laid over the music for the 3 seconds i walk through the field. It feels like im walking through hanging foliage, but instead of physical touch, its like a beam of sound. i thought they were sound waves, but I can see it being the magnetic field.

My question now is, what is happening? Is the field causing my headphone's magnet to vibrate? or is it some other type of interference?

Wrap your head in aluminum foil. Its not a security system, they're stealing your thoughts. See, its like this, you go into the library to read a book, and when you read that book, more information is stored in your brain. on your way out they take some of the information back. They do that because if you get too smart, you'll know what they're up to. Besides, if you learned everything the first time, you wouldn't have to pay so much for college.

I'd think it would be a RF one as someone else said. They are the most likly to induce currents like that. Though I'd say it'll be inducing the current in a wire (well all but the effect will be greatest in this small signal stage, the signal to noise/interfence ratio will become greater than unity probebly here because of the small signal involved) just before the amplier in your bit of kit. Then the interferance will get amplifed and heard.

This explains how these things work

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