|MadSci Network: Physics|
The answer to your question appears to be very specific to your situation. It's not a general property of any fan I've ever heard of (or could find information on the web about). The first thing to do is to separate it by the physics. Is it an accoustical effect, a perception effect, or an electrical one? If your neighbors are asleep and you're picking up the evening news in your fan, that's an electrical effect. In that case, you must somehow be directly receiving the signal in your fan's motor, which is somehow converting it into sound. People used to receive AM radio in the fillings of their teeth, so it's possible. You might try a different fan, because in that case the fan itself would be generating the noise. You might also try placing some magnets near the fan to slightly alter its electrical properties and see if that interferes with such reception (these problems of noise are magically mysterious sometimes), but you'll need as strong a magnet as you can find. If the sound is being generated by a known source (the neighbors are listening to the TV, or you can hear the channels changing), then it could be an accoustical effect. Perhaps something about the airflow or vibrations is causing a change in the accoustical conductivity of your walls, ceiling, pipes, or floor. Can you move the fan around a little, or is it a ceiling fan? If it's a ceiling fan, push or pull on it a little and see if the sound changes (not hard). It's also possible that the fan noise itself increases your ability to hear the sound. There are some shrimp or crawfish whose nervous systems are optimized to perceive approaching predators' noise better in the presence of some ambient noise. The source of this effect lies in nonlinear physics and is somewhat complex, but deals with their nervous system being more easily stimulated above its excitation threshold in the presence of some small noise. Do you have kids that sneak down and turn on the TV when they hear you turn on the fan? I think it's an interesting problem, and that you should probably re-submit this question with a few details and the answers to a few questions: Is the noise always present if you turn the fan on and are very quiet? Exactly how loud would you describe it? (Perhaps relative to another sound, like crickets outside.) Does it actually go away when you turn the fan off? Can you tell if the noise is all one radio or TV station? If so, is the transmitter for that station nearby (because you might be able to block the signal with some aluminum foil)? Can you provide some detail about the fan itself, where it's located and how fast it is? Having some idea of the specifics will help someone get a better idea and give you a shorter answer, and that starts with knowing more about the fan, I think.
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