Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

[Phys-l] Sound demos



A couple of sound demos that I do:

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

---
Michael Porter
Colonel By Secondary School
Ottawa, Ontario, Canada