MadSci Network: Physics

Re: How can sound (radio/tv) be heard by simply turning on a fan in a room?

Date: Fri Jun 6 16:35:41 2003
Posted By: Steve Nelson, Grad student, nuclear astrophysics Ph.D. program, Nuclear Lab, Duke University
Area of science: Physics
ID: 1054329125.Ph

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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