p140w07_ct_26 - Physics 140: Winter 2007 Lecture #25 April...

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Unformatted text preview: Physics 140: Winter 2007 Lecture #25 April 17, 2007 Dave Winn acquetball Striking a Wall Copyright: Loren M. Winters Mt. Etna Andrew Davidhazy Final exam information Final exam: Friday night April 20, 7:30- 9:30 PM Bring a calculator and 4 3x5 notecards (or 1 8.5x11 sheet of paper) 25% of exam is from chapter 15 &16, rest is cumulative from 1- 14. 25 questions total The mystery of musical sound Why are some sounds just noise, and others musical? Lets look at some sounds and see Three sounds: Tuning fork Crow Call Guitar string Musical sounds are not just sine waves (too simple) They are not just a random oscillations (noise is too complex) They are a sum of many sine waves, well separated in frequency Musical instruments: resonant cavities All an instrument does is to pick amplify a set of well separated frequencies Usually multiples of a fundamental frequency Higher multiples called overtones Why separated frequencies? What happens if we play two frequencies close together? They add together, producing interference Oscillating amplitudes are produced Add putty to slightly change the frequency of one tuning fork BF = f 1 f 2 Close in frequency: Beat frequency is LOW Sometimes out of phase and destructive Sometimes in phase and constructive Farther apart in frequency: beat frequency is HIGHER Two violins try to play a note which has a frequency of 2000 Hz. When they do a prominent beating is heard with a frequency of 5 Hz. If the first violinist is playing a 2000 Hz note, what is the frequency the second violinist is playing? 1: 2005 Hz 2: 2010 Hz 3: 1995 Hz 4: 1990 Hz 5: Impossible to determine with this information Second musicians frequency could be either high or low and would produce the same beat frequency. Beats and musical sound The beats produced by close sounds are unpleasant Musical sounds contain only well separated frequencies Standing waves produced in resonant cavities provide this Building up and breaking down complicated sounds Musical sounds are sums of sine waves with separated frequencies They produce these sounds because of standing wave resonances Fourier analysis...
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This note was uploaded on 04/04/2008 for the course PHYSICS 140 taught by Professor Evrard during the Fall '07 term at University of Michigan.

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p140w07_ct_26 - Physics 140: Winter 2007 Lecture #25 April...

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