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About this Lesson
- Type: Video Tutorial
- Length: 4:53
- Media: Video/mp4
- Use: Watch Online & Download
- Access Period: Unrestricted
- Download: MP4 (iPod compatible)
- Size: 52 MB
- Posted: 07/01/2009
This lesson is part of the following series:
This lesson was selected from a broader, comprehensive course, Physics I. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/physics. The full course covers kinematics, dynamics, energy, momentum, the physics of extended objects, gravity, fluids, relativity, oscillatory motion, waves, and more. The course features two renowned professors: Steven Pollock, an associate professor of Physics at he University of Colorado at Boulder and Ephraim Fischbach, a professor of physics at Purdue University.
Steven Pollock earned a Bachelor of Science in physics from the Massachusetts Institute of Technology and a Ph.D. from Stanford University. Prof. Pollock wears two research hats: he studies theoretical nuclear physics, and does physics education research. Currently, his research activities focus on questions of replication and sustainability of reformed teaching techniques in (very) large introductory courses. He received an Alfred P. Sloan Research Fellowship in 1994 and a Boulder Faculty Assembly (CU campus-wide) Teaching Excellence Award in 1998. He is the author of two Teaching Company video courses: “Particle Physics for Non-Physicists: a Tour of the Microcosmos” and “The Great Ideas of Classical Physics”. Prof. Pollock regularly gives public presentations in which he brings physics alive at conferences, seminars, colloquia, and for community audiences.
Ephraim Fischbach earned a B.A. in physics from Columbia University and a Ph.D. from the University of Pennsylvania. In Thinkwell Physics I, he delivers the "Physics in Action" video lectures and demonstrates numerous laboratory techniques and real-world applications. As part of his mission to encourage an interest in physics wherever he goes, Prof. Fischbach coordinates Physics on the Road, an Outreach/Funfest program. He is the author or coauthor of more than 180 publications including a recent book, “The Search for Non-Newtonian Gravity”, and was made a Fellow of the American Physical Society in 2001. He also serves as a referee for a number of journals including “Physical Review” and “Physical Review Letters”.
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Many objects have their own characteristic natural frequencies. Take this tuning fork, for example. If I hit this tuning fork, it's going to produce a pure tone of 512 hertz; that is, 512 cycles per second. Now, to show you this graphically, what we're going to do is the following. I'm going to bring up this tuning fork next to my microphone, and my microphone is connected to this oscilloscope. This oscilloscope will show you the wave pattern of the sound produced by this tuning fork, and any other instrument that we play, on the screen. A pure tone will look like a sine wave on the screen. Let's check this one out.
I'm going to hit the tuning fork. You'll notice, incidentally, that as the tuning fork got quieter, the amplitude of the wave on the screen also got lower. So the wave on the screen is an exact picture of what the tuning fork actually sounds like.
Now, we don't have to use a tuning fork to produce a natural frequency. An ordinary rod like this, which you can find at your hardware store, will do the same thing. Now I'm not going to bang on the rod as I banged on the tuning fork. I can make this rod vibrate just by rubbing it up and down. What I've done is put some rosin on this rod, the kind of rosin a pitcher would put on his hands before pitching, which makes his hand and my hand more sticky. I'm going to rub the rod. Let's listen to what happens. Now this produces a node or a combination of nodes at a very high frequency that was so high that you couldn't discern the separate peaks on the screen.
Now, it wouldn't be much fun making an orchestra out of rods like this. For one thing, they're hard to play, and for another, they play only one note. So we designed musical instruments to make our lives easy. We designed them to be flexible and to be able to play many notes, and also to be easy to control. Let's check out how a few musical instruments actually sound.
I'm playing a C. It's 512 hertz. Let's listen and watch the oscilloscope. Let's compare what you see with the tuning fork. If you look carefully, what you notice is that the pattern of sine waves produced by the guitar when I play C at 512 hertz is very similar to the pure tone of 512 hertz produced by the tuning fork, but not exactly the same. The reason is that the guitar produces more than just one frequency. It produces harmonics or overtones of the same frequency. That's what gives the guitar its characteristic sound or timbre.
Now let's listen to this harmonica. I'm going to play the same note, C, at 512 hertz on the harmonica, and let's look at the different pattern now that you can see on the oscilloscope. Listen carefully and watch. You can hear the difference between this C at 512 hertz and the pure tone produced by the tuning fork. You can also see it clearly on the oscilloscope. You can see that there is a pattern of sine waves but there are also little ripples in the sine waves and extra little peaks and so on. Those little peaks represent other frequencies mixed in with the frequency of 512 hertz. This characteristic pattern or frequency, which is produced by the harmonica looks different from that produced by the guitar, and that pattern is part of what we call the timbre of this instrument. Timbre is what distinguishes the sound of this harmonica from that of the guitar or from that of any other instrument.
Physics in Action: Musical Instruments and Waveforms Page [1 of 1]
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