CH16 Longitudinal waves, speed of sound, intensity, decibels

# CH16 Longitudinal waves, speed of sound, intensity, decibels

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Unformatted text preview: Standing sound waves (Resonance) Standing sound waves are responsible for the production of musical tones by wind instruments. Prime examples are organ pipes. There are two kinds: 1.) open at both ends (open pipe) 2.) open at one end only (stopped pipe) A pipe stopped at both ends is not considered. It is hard to excite the normal modes, and to get the vibrations out of the pipe. PHYS 153 09W 1 Open pipe A pipe open at both ends has displacement anti- nodes, or pressure nodes, at the ends. For standing sound waves, use displacement, rather than pressure, diagrams. 1 Fig. 16.17 (a) fundamental The lowest frequency (or longest wavelength) standing wave that can be fitted into the pipe is as shown. 2 λ = L L v v f 2 1 = = λ (b) 2 nd harmonic λ = L 1 2 2 f L v v f = = = λ PHYS 153 09W 2 (c) 3 rd harmonic 2 3 λ = L 1 3 3 2 3 f L v v f = = = λ In general, L nv nf f n 2 1 = = (n=1,2,3…) open pipe Stopped pipe (a) Fundamental The lowest frequency, or longest wavelength, standing wave that can be fitted into the pipe is shown. 4 λ = L L v v f 4 1 = = λ PHYS 153 09W 3 (b) 3 rd harmonic 4 3 λ = L 1 3 3 4 3 f L v f = = Fig. 16.18 (c) 5 th harmonic 4 5 λ = L 1 5 5 4 5 f L f = = λ Problem In Oct. 1991, Sacramento city council adopted a law to reduce the allowed sound intensity level of the much despised leaf blowers from a level of 95 dB to 70 dB by May 1992. With the new law, what is the ratio of the new allowed intensity to the previously allowed intensity?...
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## This note was uploaded on 01/27/2011 for the course PHYS 153 taught by Professor Measday during the Spring '08 term at UBC.

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CH16 Longitudinal waves, speed of sound, intensity, decibels

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