PHYS16 – Lecture 36 & 37

1. PHYS16 – Lecture 36 & 37 Sound December 5 & 7, 2010

2. Outline for Sound • Sound • Longitudinal Pressure Waves • Sound Velocity and Reflection • Resonance and Standing waves • Intensity and Sound Level • Interference and Beats • Doppler

3. Resonance and Standing Waves

4. Guitar Demonstration

5. Example Question • A guitar string with a greater mass density has a/an: • Greater velocity • Higher frequency • Smaller wavelength • Unchanged wavelength

6. Example Question • So what does the hole do?

7. Standing Waves with Sound • Need two “ends” where sound reflects • Reflections constructively and destructively interfere to setup up standing waves

8. Ruben’s Tube Demo http://en.wikipedia.org/wiki/Rubens'_tube

9. Standing Waves with Sound • Helmholtz Resonator A As A increases, Wavelength decreases and Frequency increases! http://www.diracdelta.co.uk/science/source/h/e/helmholtz%20resonator/image001.gif

10. Bottle Demo http://www.physicsclassroom.com/class/sound/u11l4a3.gif

11. Intensity and Sound Level

12. Wave Intensity • Intensity is the power per unit area in a wave http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/imgaco/isc2.gif

13. Sound Level • Sound Level (β) - a relative intensity in decibels (dB) • 0 dB = smallest sound that can be heard • 50 dB = background noise • 70 dB = traffic • 110 dB = rock concert

14. Example Question • If a Rock Concert is 110 dB, what is the intensity level in W/m2? A) 0.1 W/m2 B) 10 W/m2 C) 1011 W /m2 D) 1098 W/m2

15. Example Question • A bell is rung in a room and can be assumed to be a point source. If you sit three times the distance your friend is sitting from the bell, what is the decibel drop? A) 0.1 dB B) 1 dB C) 10 dB D) 100 dB

16. Interference and Beats

17. Sound Interference • Sound waves can interfere destructively or constructively

18. Dead Zones • Places of destructive interference are called Dead Zones • For Sound we usually don’t get dead zones because sounds are composed of many different frequencies • How does having many frequencies in a sound eliminate dead zones?

19. t t t Beats • Beats occur when two sine waves (two notes) with very similar frequency interfere Y Y I

20. Example Question • What is the beat frequency between 440 Hz and 442 Hz? A) 441 Hz B) 2 Hz C) -2Hz D) 1 Hz

21. Doppler Effect

22. Doppler Effect • Frequency of sound changes when relative velocity changes • Frequencies on approach sound higher • Frequencies that recede sound lower

23. Example Question • The frequency of an ambulance siren is 1200 Hz. The ambulance is going 60 mph (26.8 m/s). As the ambulance approaches your stopped car, you hear what frequency? fapproach = 1300 Hz

24. Sound + Light = Optical Communication http://www.scientificamerican.com/media/inline/F8CDD0E6-DFB3-6D63-DEE9270DAAFA852D_1.jpg

25. Main Points - Sound • Sound– a longitudinal pressure wave that moves through a medium • Velocity – 343 m/s in air • Reflection – occurs when velocity changes • Resonance and Standing Waves – depends on open/closed ends

26. Main Points - Sound • Intensity and Sound Level • Interference and Beats • Doppler Effect