Am I Hearing Things?
March 18, 2010 11:19 AM   Subscribe

Why do my headphones whine when going through security sensors?

Whenever I am listening to music (through my standard-issue Apple earbuds) and I walk across the threshold of the anti-theft security devices (they look basically like this) I hear a brief high-pitched whine. It doesn't happen when I am not wearing headphones, or my iPod is off. I am not very well-versed in sound, acoustics, radio waves, or the like so I am very curious as to what is causing the sound. I am assuming something emanating from the security towers is interfering with the iPod - any ideas on what?
posted by hepta to Technology (4 answers total)
Those sensors work by sending out a powerful radio wave. If a security tag passes through it, it is activated and sends a signal back. (Some of them work by simply distorting the radio wave, which the barrier detects.)

The signal is strong enough to interfere with the amplifier stage of your iPod, causing a sound you can hear. It's the same phenomenon which causes GSM cellphones to make funny buzzing noises when you put them near audio devices.

If you wrap your iPod in metal foil, it will probably stop.
posted by Mwongozi at 11:24 AM on March 18, 2010

The "gates" are actually transmitters, usually putting out a very low frequency (about 60khz) radio signal which induces a current in the anti-theft sensor tags. That same radio signal is also inducing a current somewhere in your iPod, which is being rectified (converted) into an audio-frequency signal that you hear in your earphones.
posted by deadmessenger at 11:27 AM on March 18, 2010

[warning: I forget most of my physics]

Judging by the fact that those sensors can have problematic effects on pacemakers (at least according to the warning on the ones at my library), these sensors have the effect of interfering with electrical current. Likely, they can modify the current that's going through them.

So the likely explanation is that the sensors create a magnetic field in the area between, and attempt to detect when a tag goes through them - I'm assuming that the tag isn't itself powerful enough to warp the field (and thus detection would notice anything magnetic), so rather it probably sets up some form of resonance that the sensors notice?

Something along those lines, at least. They use a magnetic field to somehow notice things going throw. Magnetic fields fuck with current. Your speakers make sound by reacting to the alternating current transmitted along the headphone wire, and so inducing a different current makes a different (in this case whine-y) sound.

Now that I look it up, security sensors and headphones imply that I was close enough for government work.
posted by Lemurrhea at 11:30 AM on March 18, 2010

I always wondered your question too, and thought it was from the magnetic field generated by the sensors (like Lemurrhea describes). To understand what happens, you need to understand how speakers work. All speakers have a permanent magnet inside which moves an electromagnetic voice coil, depending on the electric current coming from your ipod/CD player/whatever. The coil is attached to the speaker cone, which then moves in an out depending on the polarity of the coil. The movement of the cone vibrates the air, creating sound waves.

So I guess when you walk through the security magnetic field, it effs up with the polarity of the coil, causing the cones in your headphones to vibrate in a way that generates that high-pitched noise.

(I think everyone above has touched upon the issue, but please correct me if my explanation is wrong. I'm only guessing what is happening...)
posted by hellomina at 5:23 PM on March 18, 2010

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