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Chapter 15

Chapter 15. Sound. 15.1 Properties of Sound. Sound - longitudinal wave. High pres - compressions Low pres – rarefactions Air molecules collide transmitting changes in air pressure. Frequency - number of oscillations / sec Vel of sound in air depends on temperature.

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Chapter 15

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  1. Chapter 15 Sound

  2. 15.1 Properties of Sound • Sound - longitudinal wave. • High pres - compressions • Low pres – rarefactions • Air molecules collide transmitting changes in air pressure.

  3. Frequency - number of oscillations / sec • Vel of sound in air depends on temperature. • At 20º C, sound travels at 343 m/s. • Sound can be reflected --echoes.

  4. Ex: sonar & echolocation • Can be refracted. • Can be diffracted & interfere to produce dead spots. • Speed of sound is higher in liquids & solids than in gases.

  5. Travels 4x faster in water than in air. • Travels about 11x as fast in steel • Elasticity affects speed; not density. • Ex. Prob 351 Prac Prob 352

  6. Pitch & Loudness • Marin Mersenne & Galileo connected pitch to frequency. • Pitch - frequency of wave. • Loudness depends on amplitude of pressure changes.

  7. Sound level - measured in decibels (dB). • 40 dB is 10 times larger than 20 dB. • Loudness depends on sensitivity of our ears to sounds in different frequency ranges.

  8. The Doppler Shift • Moves toward you, frequency is higher. • Moves away, frequency is lower. • Doppler shift occurs in EM waves & mechanical. • Ex: radar detectors, ultrasound, • Astronomers use it to measure speed of galaxies

  9. 15.2 The Physics of Music • Pythagoras used ratios to define his musical scale. • If ratio is in whole numbers, pleasing sounds result. • Octave - ratio of frequencies is 2:1

  10. Hermann Helmholtz & Lord Rayleigh studied how human voice as well as instruments produce sounds, & how human ear detects sounds. • Sound - produced by vibrating objects. • Human voice - vibrations of vocal cords.

  11. Brass instruments - lip of performer vibrates Fig 15-6 (a) 357 • Reed instruments - reed vibrates Fig 15-6 (b) 357 • Flute, organ, whistle - air is blown across an opening • Stringed instruments - wire or string vibrates • Electric guitars – use electronic devices to amplify vibrations

  12. Resonance in Air Columns • Resonance increases amplitude of vibration by repeatedly applying a small external force at natural frequency. • Closed-pipe resonator - resonating tube with one end closed that resonates when its lengths are at an odd number of quarter wavelengths. • Example: String instruments

  13. Standing wave has pressure nodes & antinodes. Fig 15-9 a 359 • Open-pipe resonator - both ends are open Fig 15-9 b 359 • Examples: saxophone and flute • Standing wave in a pipe can be represented by sine wave. Fig 15-10 359 • Ex Prob 361 Prac Prob 363

  14. Detection of Sound • Sound detectors convert sound energy into other forms of energy. • The ear is sensitive to sounds with frequencies between 1000 & 5000 Hz. Fig 15-14 363 • Three parts: outer - collects sound; penna middle - 3 tiny bones; stirrup, anvil, hammer inner - watery liquid; cochlea hair cells vibrate; sends to brain • Loud sounds can permanently damage ears.

  15. Sound Quality • Timbre - tone color • Beat - oscillation of wave amplitude; frequency of beat is difference in frequencies of two waves. • Dissonance - unpleasant sound • Consonance - pleasant sound • Pythagoras - consonance occurs when wave frequencies have ratios that are small whole numbers.

  16. Fundamental - lowest resonant frequency • Harmonics - wave of frequencies that are whole number multiples of fundamentals • Noise consists of a large # of frequencies with no relationship. • White noise - all frequencies present in equal amplitude; relaxing effect.

  17. Pythagoras used ratios to define his musical scale. • If ratio is in whole numbers (1:2, 2:3 or 3:4), pleasing sounds result. • Octave ratio of frequencies is 2:1 • Ex Prob 366 Prac Prob 367

  18. Sources • http://www.physicsclassroom.com/mmedia/waves/lw.html • http://science.howstuffworks.com/hearing1.htm • http://www.museumca.org/caves/onli_echo_ani.html • http://www.kettering.edu/~drussell/Demos/refract/refract.html • http://www.loa.espci.fr/~michael/en/acel/waveguide/waveguide.html • http://www.walter-fendt.de/ph11e/dopplereff.htm • Encarta • Physics: Principles and Problems, Glencoe. • http://www.walter-fendt.de/ph11e/beats.htm • http://maxwell.ucdavis.edu/~cole/phy9b/movies/fundamental.mov

  19. Sound Waves

  20. Ear

  21. Echo

  22. Echolocation

  23. Refraction

  24. Diffraction

  25. Red & Blue Shift

  26. Pythagoras

  27. Frequency

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