13B Sound waves

13B Sound waves - 4/11/2011 Speed of Sound Physics 1C...

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4/11/2011 1 Physics 1C Lecture 13B "I did never know so full a voice issue from so empty a heart: but the saying is true 'The empty vessel makes the greatest sound'." --William Shakespeare Speed of Sound Example values (m/s): Description of a Sound Wave A sound wave may be considered either a displacement wave or a pressure wave The displacement of a small element is: s ( x,t ) = s max sin ( kx w t ). Here k is the wavenumber, is the angular frequency of the piston. The variation D P in the pressure of the gas about its equilibrium value is also sinusoidal: D P = D P max cos ( kx – t ) The pressure wave is 90 o out of phase with the displacement wave. Doppler Effect Why does an ambulance siren moving towards you sound different than when it is moving away from you? It is wave property of sound. Let’s say there is a stationary source that is emitting sound at a certain frequency. Observers on either side of the source measure the same frequency (also λ is the same for both). s O1 O2 f v sound Doppler Effect But what if the source is moving to the left?: O1 O2 s v s Observer 1 believes that the wavelength has decreased (compared to before): f 1 v sound 1 Observer 2 believes that the wavelength has increased (compared to before): increased decreased f 2 v sound 2 decreased increased Doppler Effect We find that for a source moving : where f o is the frequency the observer detects, f s is the frequency the source emits, v s is the velocity of the source, v sound is the velocity of the sound wave. Use “–” if the source is moving toward the observer and use
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This note was uploaded on 02/11/2012 for the course PHYS 1C 1C taught by Professor Wethien during the Spring '11 term at UCSD.

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13B Sound waves - 4/11/2011 Speed of Sound Physics 1C...

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