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Wave phenomena

Wave phenomena. Physics 12 Source: Giancoli Chapters 11 and 12. Sound waves. How are sound waves produced? Is sound a transverse or longitudinal wave?. Sound waves. Sound is caused by the vibration of an object which causes air pressure variations.

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Wave phenomena

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  1. Wave phenomena Physics 12 Source: Giancoli Chapters 11 and 12

  2. Sound waves • How are sound waves produced? • Is sound a transverse or longitudinal wave?

  3. Sound waves • Sound is caused by the vibration of an object which causes air pressure variations. • The frequency range detectable by the normal human ear is 20 Hz – 20,000 Hz

  4. Sound waves Sound wave in a tube (simulation) What factors might affect the speed of sound?

  5. Sound waves • The speed of sound is given by the expression: v = √(B/ρ) where B is the bulk modulus (how compressible the medium is) and ρis the density of the medium

  6. Sound waves • If a medium has a LOW bulk modulus, B, that means EASILY compressed. • If a medium has a HIGH bulk modulus it is DIFFICULT TO compress. Order gases, liquids, and solids in order of increasing bulk modulus. Which one transmits sound more quickly?

  7. Sound waves • If sound is traveling through air, what variables might affect the speed of sound?

  8. Sound waves • The speed of sound at room temperature (20C) and normal atmospheric pressure is 343 m/s. • How might increased temperature and pressure affect the speed of sound?

  9. Sound waves A sound wave with a frequency of 300 Hz travels through the air. • What is its wavelength? • If its frequency is increased to 600 Hz, what is its wave speed and the wavelength?

  10. Sound waves • λ = 1.14 m • v = 343 m/s; λ = 0.57 m

  11. Sound waves • Sound waves transmit energy. • The rate at which they transmit energy per unit area is called intensity. • Intensity is responsible for a sound’s loudness level.

  12. Beats • When two sound waves that are similar in frequency interfere, a beat frequency is produced. • A beat is perceived as a modulation in amplitude (loud then soft then loud then soft). • The beat has its own frequency, measured by: fbeat = | f1 – f2 |

  13. Beats A piano tuner uses a tuning fork to adjust the key that plays the A note above middle C ( which is 440 Hz). The tuning fork emits a perfect 440 Hz tone. When the tuning fork and the piano key are struck, beats of a frequency of 3 Hz are heard. • What is the frequency of the piano key? • If the piano’s key frequency is too high, should the piano tuner tighten or loosen the string?

  14. Beats • either 437 Hz or 443 Hz • loosen

  15. Resonance • Standing waves can also occur for sound waves. • There are two typical situations: • Closed tube (closed at one end) • Open tube (open at both ends) http://www.physics.smu.edu/~olness/www/03fall1320/applet/pipe-waves.html

  16. Resonance

  17. Resonance A closed-end tube resonates at a fundamental frequency of 440.0 Hz. The air in the tube is at a temperature of 20C, and it conducts sound at a speed of 343 m/s. • What is the length of the tube? • What is the next higher harmonic frequency? • Answer the questions assuming that the tube were open at both ends.

  18. Doppler effect • When a source of sound and the observer are stationary, does the frequency of the sound change? • What will happen if the source or the observer moves? Ripple tank simulation

  19. Doppler effect • The shift in frequency and wavelength that occurs when the source and detector are in relative motion is called the Doppler effect.

  20. Doppler effect • The frequency perceived by the observer is expressed as follows: where f ‘ is the frequency perceived f is the frequency at which sound is emitted v is the speed of sound vOis the speed of the observer vS is the speed of the source

  21. Doppler effect • If the detector is moving towards the source, you expect an upward shift in frequency therefore you use addition. • If the source is moving towards the detector you expect an upward shift therefore you use subtraction (smaller denominator).

  22. Doppler effect A source of 4 kHz sound waves travels at 1/9 the speed of sound towards a detector that’s moving at 1/9 the speed of sound toward the source. • What is the frequency of the waves as they’re received by the detector? • How does the wavelength of the detected waves compare to the wavelength of the emitted waves?

  23. Doppler effect • 5 kHz • shorter

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