In the latest installment of SocketHop, Minister of Miracles, Marcus the Magn(et)ificent Levine, answers why your headphone-covered ears ring when you walk through the Butler detectors?
Don’t let the tranquil, seldom broken silence of the reading rooms fool you—Butler is a very loud place. Not just Butler, but practically everywhere in the modern world is saturated with the silent noise of electromagnetic radiation. As normally functioning human beings, we usually only come into contact with visible and infrared light. Still, innumerable other frequencies of electromagnetic radiation beyond our perception, from radio to ultraviolet, permeate the air.
While you’re strolling through the Butler book theft detectors lookin’ fly with your doughnut sized headphones, you may experience a strange sharp ringing noise. The 3M security gates operate using magnetic EAS (Electronic Article Surveillance) technology, and the way the system works can directly interfere with your musical experience, and perhaps drive you to question your sanity.
Each of Butler’s books in circulation has a small strip of “amorphous metal” hidden under its cover. While the book is sitting on the shelf, this metal strip is in a demagnetized state so that it produces certain harmonic oscillations, or vibrations of the metal’s electrons. Checking out a book magnetizes the strip of metal, and organizes all of the charged particles in the metal strip so that they no longer produce harmonics.
In order to detect when books tagged in this manner enter and exit the library, traffic must flow through gates, each of which emits a magnetic field of a particular frequency (between 10 and 1000 Hz). When a demagnetized book (not checked out) passes through the gates, the harmonics interfere with the magnetic field and an alarm sounds. This infringes on your listening enjoyment because your headphones turn that same magnetic field into sound, and since the range of frequencies produced by the gates is within the range of human hearing, you are able to perceive that sound as a mildly annoying screech.
Most headphones consist of three parts: a length of wire, a permanent magnet, and a diaphragm. To produce sound, the headphones transmit a rapidly fluctuating electric signal down the wire to a coil wrapped around the permanent magnet. As the signal fluctuates with the music, the coil moves through the magnetic field created by the permanent magnet, which in turn moves the attached diaphragm back and forth. This moves the air in front of the diaphragm, creating sound. When you pass through a Butler detector, its magnetic field performs the same role as your iPod, and sends an electrical signal down the wire to the coil where it is turned into sound. Unfortunately there is no way around this other than wrapping your headphones entirely in tin foil, which could put a serious crimp in your style. Alternatively, this may launch a fashion phenomenon that sweeps the globe.
Mozart effect via Wikimedia Commons