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Wave Boundaries and Superposition

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Noise Cancellation – Does it work? What would waves have to do with noise cancellation? Some companies sell very expensive devices that claim to cancel unwanted noise. Do you think they work?

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Wave’s Today we are going to discuss two topics: What happens when a wave crosses a point where the medium changes? What happens when more than one wave is in the same medium at the same time?

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Wave Boundaries A boundary is the point where two different mediums meet. A boundary is where two different springs meet, a spring and a rope, or it can be where air meets the earth. It can also be where air of one temperature meets air of a different temperature.

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Wave Boundaries When a wave crosses a boundary what characteristics of a wave change? Frequency? Wavelength? Speed? Amplitude?

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Wave Boundaries Boundary: Low Density string on left and a high density string on right. What changes as the wave moves through the boundary? What else happens at the boundary?

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Wave Boundaries Boundary: High Density string on left and a low density string on right. What changes as the wave moves through the boundary? What’s different from the previous example?

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Wave Boundaries What would you expect to happen at a fixed boundary?

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Wave Boundaries Boundary: Fixed boundary Is it what you expected?

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Wave Boundaries Boundary: Soft boundary Is it what you expected?

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Wave Boundaries Review: What wave property stays constant as the wave moves through a boundary? What happens to the energy in the original wave? What determines whether the reflected wave is erect or inverted?

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Wave Boundary Demonstrations

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Wave Superposition What happens when more than one wave is in the same medium at the same time? Each wave acts on the medium independently The displacement of medium (for mechanical waves) is the algebraic sum of all the waves acting on the medium. (This is called the principle of superposition) The resultant of two or more waves is called interference.

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Wave Superposition As I suspect you have already guessed – there are several different types of interference. This animation shows what we call constructive interference. Note what happens to the amplitude.

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Wave Superposition This animation shows both constructive and destructive interference. What happens to the resulting waveform when destructive interference takes place?

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Wave Superposition Here we have two sine waves that are the same frequency, same wavelength and are traveling in opposite directions. Note the appearance of the resultant. What is different about this one from the last one?

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Wave Superposition Do you see the difference? What is unique about the black dots on the bottom animation?

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Wave Superposition We call the points that do not move nodes. We call the points that has the largest displacement antinodes. The resultant wave is called a Standing Wave.

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Noise Cancellation – Does it work? What would waves have to do with noise cancellation? Some companies sell very expensive devices that claim to cancel unwanted noise. Do you think they work?

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References http://www.gmi.edu/~drussell/Demos/refl ect/reflect.html http://www.gmi.edu/~drussell/Demos/refl ect/reflect.html http://www.gmi.edu/~drussell/Demos/refl ect/reflect.html http://www.gmi.edu/~drussell/Demos/refl ect/reflect.html www.bose.com www.bose.com www.bose.com www.aa.com www.aa.com www.aa.com http://www.kettering.edu/~drussell/Demo s/superposition/superposition.html http://www.kettering.edu/~drussell/Demo s/superposition/superposition.html http://www.kettering.edu/~drussell/Demo s/superposition/superposition.html http://www.kettering.edu/~drussell/Demo s/superposition/superposition.html

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Lecture Outline Chapter 13 College Physics, 7 th Edition Wilson / Buffa / Lou © 2010 Pearson Education, Inc.

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