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This is a demonstration of beats. I've got two identical tuning forks on wooden boxes. The boxes are intended to amplify the sound; let's listen to both of them, and now this one. Now you can still hear it. What happened was that the vibration of this one sent this one into vibration as well. We can make that effect a little bit stronger by putting them together like this, stop this, you can still here the other one let's sound them both together. Now if you're a musician you know that if the frequencies are identical you won't hear a beat frequency. Now in this particular case there is no discernable beat frequency. Those frequencies are very nearly the same. But let's make one of them a little bit different by putting some mass on one of the tines like so. And we'll put them both towards you so you can see them. It's going to make this frequency a little bit different than this frequency just enough so we'll get a beat frequency.

So first of all let's listen to this by itself, sounds lower right? And notice it doesn't set this one into oscillation because they're no longer matched. Now let's listen to both of them together. Now you can very obviously hear a beat frequency there and if you can count the number of beats per second, then that's the difference in frequency between the two forks. I can change the beat frequency by where I position this. Put it way out on the end here. A higher beat frequency means the frequencies are further apart. I'm going to put this way down here to make the frequencies more nearly the same. Alright there's a very long beat frequency there. How about splitting the difference right about there. That's slow enough that you can count those beats. So I'm going to start this up again and I want you to have a watch handy or watch the time on the video and measure the number of beats per second. Here goes, okay now let's go back up to where we started and do it. See if you can count the number of beats here some people have the ability to count beats at a very rapid rate. One more time.

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