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[Phys-l] Tuning fork to drive a speaker



I was playing around with speakers for some class demos and come up with
a new (to me, anyway) idea.

The old idea ...
If you connect a battery directly to a speaker, it will either move in
or out depending on the polarity of the battery. Pretty clearly, you
can relate this to an attraction of repulsion of the electromagnet on
the cone interacting with the permanent magnet in the base.

If you repeatedly touch the wire to the battery, you can make the
speaker vibrate. But it is hard to touch the wire to the battery more
than a few times per second.

The New Idea! ...
Enter the tuning fork. The students can clearly see the tuning fork is
vibrating (especially if you dunk it in water). Well I touched the
vibrating tuning fork to the battery. One of the wires was brought up
close to the tuning fork, making intermittent contact with each
vibration. The speaker immediately resounded with the pitch of the
tuning fork. (At least until the vibrations died down too much in a few
seconds.) The intermittent connection to the tuning fork is, I think,
much more "visceral" than a function generator for a square wave input
to the speaker.


Depending on the situation, you could use this to demonstrate something
about sound or something about electromagnets and speakers.

Tim Folkerts