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lecture notes-Chapter12-posted

lecture notes-Chapter12-posted - Chapter 12 Sound Sound...

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1 Chapter 12: Sound Sound Waves Intensity of Sound Waves Standing Sound Waves The Doppler Effect

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2 § 12.1 Sound Waves Sound waves are longitudinal. They can be represented by either variations in pressure (gauge pressure) or by displacements of an air element.
3 The middle of a compression (rarefaction) corresponds to a pressure maximum (minimum). http://www.ngsir.netfirms.com/englishhtm/Lwave.htm

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4 Loudness of a sound is measured by the logarithm of the intensity. The threshold of hearing is at an intensity of 10 -12 W/m 2 . Sound intensity level is defined by ( 29 0 log dB 10 I I = β dB are decibels 0 I =10 -12 W/m 2
5 Example: Two sounds have levels of 80 dB and 90 dB. What is the difference in the sound intensities? ( 29 dB 90 log dB 10 0 2 = = I I β ( 29 dB 80 log dB 10 0 1 = = I I β 1 2 1 1 2 1 2 0 1 0 2 1 2 10 10 I I I I log dB 10 dB 10 I I log I I log dB 10 dB 10 I I = = = - = = - β β Subtracting:

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6 Question 3 You are playing your stereo at a sound level of 90 dB. Your roommate turns on her stereo at a sound level of 90 dB. The new sound level is A) 87 dB B) 90 dB C) 93 dB D) 180 dB
7 § 12.4 Standing Sound Waves Consider a pipe open at both ends: The ends of the pipe are open to the atmosphere. The open ends must be pressure nodes (and displacement antinodes).

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8 The distance between two adjacent antinodes is ½ λ . Each pair of antinodes must have a node in between. The fundamental mode (it has the fewest number of antinodes) will have a wavelength of 2L.
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