The Sounds of Music: Understanding Sound Waves Experiment

1. The sounds of Music Jose Carreras 3 Luciano Pavarotti 1 Yvonne Kenny 4 5 David Hobson Joan Sutherland Madam Melba Placido Domingo Margot Cory 7 2 6 Graeme Wall 9 10 11 Roy Best Maria Prior 8

2. Two students, Pat and Morgan are discussing an experiment • to test the nature of sound waves. • They imagine a loudspeaker with a dust-particle sitting motionless • in front of it, and consider what would happen to the particle • when the speaker is turned on.Dust particle • Pat says that since there is energy transferred by the wave, the particle will gain kinetic energy. The sound wave is like a succession of little pulses that push it away from the loudspeaker. • Morgan says that although energy is carried by the wave, the result of the pressure variations will cause the dust particle to move back and forth about its original position. • Question. • In the box below circle the name of the person whose prediction is most nearly correct. • Pat / Morgan • Question.In the space below explain the reason for your choice. An exam question The speaker cone is vibrating forward and backward. This creates a pressure increase, then a pressure decrease, in sequence. The dust particle is thus moved forward and backward . However there is no net forward motion of the air! Only oscillations.

3. elastic medium Soundwaves need an elastic medium All the parts of the medium are coupled. If I compress one part, this compresses the adjacent part in turn. When I relax, the material returns to original shape Sound does not travel in a vacuum. Medium is usually air (quite elastic) Water is almost inelastic

4. L n = 1 n = 2 n = 3 Standing Waves on a String l = 2L f = f0 Fundamental (1st harmonic) l = L f = 2f0 2nd harmonic 1st overtone 3rd harmonic 2nd overtone l = 2/3L f = 3f0 l = 2/nL f = nf0

5. L n = 1 Antinode Max. motion n = 2 n = 3 Node No motion Node No motion Standing Waves on a String l = 2L f = f0 Fundamental (1st harmonic) l = L f = 2f0 2nd harmonic 1st overtone 3rd harmonic 2nd overtone l = 2/3L f = 3f0 l = 2/nL f = nf0

6. L All harmonics can exist n = 1 l = 2L f = f0 Fundamental (1st harmonic) l = L f = 2f0 2nd harmonic 1st overtone Antinode Max. motion Antinode Max. motion Antinode Max. motion n = 2 3rd harmonic 2nd overtone l = 2/3L f = 3f0 n = 3 Node No motion Node No motion Node No motion Standing Waves in a tube open at both ends Horizontal motion of air l = 2/nL f = nf0

7. L Only odd harmonics exist n = 1 l = 4L f = f0 Fundamental (1st harmonic) l = 4/3L f = 3f0 3rd harmonic 2nd overtone n = 2 5th harmonic 4th overtone l = 4/5L f = 5f0 n = 3 Node No motion Node No motion Node No motion Standing Waves in a tube closed at one end Horizontal motion of air l = 4/(2n-1)L f = (2n-1)f0

8. Effect of harmonics Why does a flute sound different from a clarinet? Flutes are pipes that are open at both ends. Clarinets are pipes that are closed at the reed, and open at the bell. all harmonics can be present only odd harmonics present

9. Flute Waveform Clarinet Waveform Effect of harmonics Why does a flute sound different from a clarinet? all harmonics odd harmonics only

10. Chinook Helicopter Disastrous resonances

11. Tacoma Narrows Bridge Disastrous resonances

12. Standing Waves on a plate 2-D standing waves Any elastic object can produce sound. Standing waves (resonances) will be more complex than a string or pipe. The resonant frequencies will not be simple progressions as for a string of pipe. Example: vibrations of a drum skin

13. Bathroom Divas Why does your voice sound richer in the bathroom? 3-D standing waves

14. The Human voice

15. How do we hear Sound?

16. 5 cm l = 4L Fundamental frequency ~ 3000 Hz

17. 5 cm Threshold of hearing most sensitive frequency l = 4L Fundamental frequency ~ 3000 Hz

18. ear drum (Tympani) oval window into cochlea How do we hear Sound?

19. The cochlear unwound Hairs (or Villi) on surface of basilar membrane

20. the end