How exactly do speakers make a noise when a mobile phone, that’s about to receive a text message, is right next to it?
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When a phone is emitting radio energy, part of it is absorbed by surrounding objects inducing a voltage into them. Multiple conventional cellphones divide the channel among themselves by taking turns transmitting. The speaker effectively gets an irregular square wave signal added to its output, alternating between low when the phone is idle, receiving or waiting, and high when sending data. The speaker is not fast enough to actually respond to the modulated data. The interference would be much less noticeable if the transmission was continuous.
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