MadSci Network: Physics

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

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 
far apart your ears are. 


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