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inexplainlikeimfive
GSM phones (AT&T and TMO in the U.S.A) use a type of communication where each phone is allowed to talk for a short period of time. (like old mainfraim computers, time sharing) In this brief amount of time the phone sends out large amounts of radio waves. When your phone’s transmit time is over the next phone gets to transmit. (Your phones get to transmit about every 4.3 milliseconds) This is how a cell phone tower is able to talk to multiple phones at once.
Your speakers most likely have an amplifier in them and one or more of the short copper wires on the circuit board are acting as an antenna. This antenna is picking up the short, but powerful, transmissions from your phone. (Happens at about 217Hz)
Source: I design speaker systems for iPhones and we make sure (so does Apple) that you can’t hear the noise.
for those older than 5 check out this link for more in depth sources: (http://www.google.com/search?gcx=w&ix=c2&sourceid=chrome&ie=UTF-8&q=TDMA+217+Hz)
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