For interference, one of the traditional demos is to set up two
speakers (in phase or not), and play a tone through them. Students
walk around the room and discover the nodal lines. I always found
that there was too much noise from the shuffling of feet, so now I
just hold the two speakers on outstretched arms and rotate the
interference pattern past the class. The kids hear the effect of the
nodal lines as they sweep past. If you do it slowly and get the kids
to raise their hands when they detect the node, you get another kind
of "wave"!
Beats: I do this one with an oscilloscope to show the wave form. I
get a student to whistle a tone as long as they can (we do a quick
round of auditions to see who is best at this, it's amazing to see
how many kids can't do it...). While they are doing that, I whistle
at a slightly different frequency to create the beat. The trace on
the oscilloscope looks great (I use a computer simulation that allows
me to freeze the display as well), and the sound of the higher
frequency beats (as opposed to the 1-Hz scale warbling)
sounds ...strange...
Stupid story from my youth (I tell this to the students around the
same time as the above demo) -- when I was about 7 years old, we used
to play a game where two kids would scream at each other
simultaneously. The combined shrieks produced a beat that manifested
itself as a weird, moaning sound. Hard to describe, but you get a
taste of it from the whistling demo.
(Stupid story from my adulthood -- I used to carpool with a couple of
engineers, and told them the above story. I was immediately subjected
to the sound of two grown men shrieking at each other -- in a compact
car -- trying to replicate the effect. They claimed they heard the
beat, but I wasn't so sure)
Mike
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Michael Porter
Colonel By Secondary School
Ottawa, Ontario, Canada