### Re: Why do ceiling fans make a saxophone have a wavering sound?

Date: Mon Jul 27 13:56:50 2009
Posted By: Rob Fatland, Staff, Research and Education, Freelance Science Educator
Area of science: Physics
ID: 1248362100.Ph
Message:
```
Yes, I agree that you're hearing a variant of the "Leslie" effect.
However I noticed this effect when playing a synthesizer (electric piano
sound) in a small room, to the extent that I thought the synthesizer was
broken! So I do not believe the effect is confined to saxophones.

The entire acoustic space will affect how your instrument sounds, and it
may have been that the saxophone was in the right position and was
sending acoustic energy in the right direction to maximize (and make
noticeable) the chopping/wavering effect. It would be interesting to
direct a speaker upwards or have your sax player lie on his or her side
to play while the fan is on.

I think the interesting thing is the coupling of the effect to the fan
blade motion (speed). Just as you suggest the chopping may primarily be
interference between the reflection from the ceiling and the fan blades.
You could experiment with this further if your fan has a variable speed
control. Important factors would include the speed and wavelength of the
sound at a given pitch relative to the geometry of the fan/ceiling and
the speed of the fan, and the perception of tone by human hearing.

To head off into an analysis of this effect one could look at specific
examples: If a certain pitch has a wavelength of one meter and the speed
of sound is 300 meters per second, and if the fan blade is rotating at 6
revolutions per second and has four blades (24 chops) you could start to
appreciate the interference geometrically. The harmonics of that pitch
e.g. twice the frequency and so on, will run into a different type of
geometric interference and I would suggest that this will modify the
perceived tone.

One final idea for an experiment: Set up two microphones to record the
wavering sound simultaneously at two locations. While holding the note,
slowly move one of the microphones. The resulting stereo tracks might
show that the effect you hear depends on where you're standing, and how

cheers
-Rob

```

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