1. © 2010 Pearson Education, Inc. Conceptual Physics 11 th Edition Chapter 20: SOUND

2. © 2010 Pearson Education, Inc. Ch 20 Objective: This lecture will help you understand: Nature and Origin of Sound Media That Transmit Sound Reflection of Sound Refraction of Sound Energy in Sound Waves Forced Vibrations Natural Frequency Resonance Interference Beats Warm-Up Consider the situation on the next slide

3. © 2010 Pearson Education, Inc. Consider a person attending a concert that is being broadcast over the radio. The person sits about 45 m from the stage and listens to the radio broadcast with a transistor radio over one ear and a nonbroadcast sound signal with the other ear. Further suppose that the radio signal must travel all the way around the world before reaching the ear. Reflection of Sound A situation to ponder…

4. © 2010 Pearson Education, Inc. Which signal will be heard first? A.Radio signal B.Nonbroadcast sound signal C.Both at the same time. D.None of the above. A situation to ponder… CHECK YOUR NEIGHBOR

5. © 2010 Pearson Education, Inc. Which signal will be heard first? A.Radio signal B.Nonbroadcast sound signal C.Both at the same time. D.None of the above. Explanation: A radio signal travels at the speed of light, 3  10 8 m/s. Time to travel 45 m at 340 m/s ≈ 0.13 s. Time to travel 4  10 7 m (Earth’s circumference) at 3  10 8 m/s ≈ 0.13 s. Therefore, if you sit farther back at the concert, the radio signal would reach you first! A situation to ponder… CHECK YOUR ANSWER

6. © 2010 Pearson Education, Inc. Soundwaves are longitudinal (parallel to direction of movement)

7. © 2010 Pearson Education, Inc.

8. Sound in Air Wavelength of sound Distance between successive compressions or rarefactions

9. © 2010 Pearson Education, Inc. Speed of sound The speed of sound in dry air at 0 o C is about 330 m/s Each degree above 0 o C the speed increases by 0.6 m/s

10. © 2010 Pearson Education, Inc. Reflection of Sound Reflection Process in which sound encountering a surface is returned….. Often called an echo Multiple reflections—called reverberations Too many reflections can be ear piercing Too little can cause the sound to be muffled

11. © 2010 Pearson Education, Inc. Reflection of Sound Acoustics Study of sound Example: A concert hall aims for a balance between reverberation and absorption. Some have reflectors to direct sound (which also reflect light— so what you see is what you hear).

12. © 2010 Pearson Education, Inc. Refraction of Sound- reflection (bounce) vs refraction (bend) Refraction Bending of waves—caused by changes in speed affected by –wind variations. –temperature variations.

13. © 2010 Pearson Education, Inc. Reflection and Refraction of Sound Multiple reflection and refractions of ultrasonic waves Device sends high-frequency sounds into the body and reflects the waves more strongly from the exterior of the organs, producing an image of the organs. Used instead of X-rays by physicians to see the interior of the body.

14. © 2010 Pearson Education, Inc. Reflection and Refraction of Sound Dolphins emit ultrasonic waves to enable them to locate objects in their environment.

15. © 2010 Pearson Education, Inc. 1. You watch a person chopping wood and note that after the last chop you hear it 1 second later. How far away is the chopper? A. 330 m B. More than 330 m C. Less than 330 m D. There’s no way to tell. 2. You hear thunder 2 seconds after you see a lightning flash. How far away is the lightning? A.340 m/s B. 660 m/s C. More than 660 m/s 3. Reverberations are best heard when you sing in a room with A.carpeted walls. B. hard-surfaced walls. C. open windows. 4. When air near the ground on a warm day is warmed more than the air above, sound tends to bend…. 5. In the evening, when air directly above a pond is cooler than air above, sound across a pond tends to bend…. Exit Ticket CHECK YOUR NEIGHBOR HW: Study Quiz tomorrow

16. © 2010 Pearson Education, Inc. This lecture will help you understand: Objective: This lecture will help you understand: Nature and Origin of Sound Media That Transmit Sound Reflection of Sound Refraction of Sound Energy in Sound Waves Forced Vibrations Natural Frequency Resonance Interference Beats Warm-Up Quiz

17. © 2010 Pearson Education, Inc. Forced vibrations and resonance Pluck a guitar string and the vibration is forced to the bridge (forced vibrations) which is forced to the body (sympathetic) which allows energy exchange between string and air which increases intensity Pendulum’s natural frequency depends on length. If each pendulum has same natural frequency then the frequency transfers= resonance –A bridge can do this, wind causes the bridge to vibrate and high winds cause bridge to oscillate at it’s natural frequency which increases until it breaks. http://www.bing.com/vide os/search?q=tuning+fork +resonance&view=detail &mid=008444A92CB7D6 752AB1008444A92CB7 D6752AB1&FORM=VIR E3 http://www.bing.com/vide os/search?q=tuning+fork +resonance&view=detail &mid=008444A92CB7D6 752AB1008444A92CB7 D6752AB1&FORM=VIR E3

18. © 2010 Pearson Education, Inc. Forced Vibrations Forced vibration Setting up of vibrations in an object by a vibrating force Example: factory floor vibration caused by running of heavy machinery

19. © 2010 Pearson Education, Inc. Natural Frequency Natural frequency Own unique frequency (or set of frequencies) Dependent on –elasticity –shape of object ** When you tune a radio you are actually adjusting the natural frequency of the electronic to the surrounding signals.

20. © 2010 Pearson Education, Inc. Resonance- sounding again the frequency of forced vibrations on an object matches the object’s natural frequency Examples: Swinging in rhythm with the natural frequency of a swing Troops marching in rhythm with the natural frequency of a bridge (a no-no!)

21. © 2010 Pearson Education, Inc. Resonance Dramatic example of wind-generated resonance http://www.bing.com/videos/search?q=troops+marching+in+natural+frequency+of+bridge&view =detail&&&mid=D8B80A1C110379BC596AD8B80A1C110379BC596A&rvsmid=E89802D6 82BC6C89F3E6E89802D682BC6C89F3E6&fsscr=0&ajf=70

22. © 2010 Pearson Education, Inc. Exit ticket HW: Pg 364 5, 7, 8, 10, 11 Pg 365 1 (ranking) Pg 365 10, 15, 21, 22, 33 Pg 367 1, 2, 8

23. © 2010 Pearson Education, Inc. This lecture will help you understand: Objective: This lecture will help you understand: Nature and Origin of Sound Media That Transmit Sound Reflection of Sound Refraction of Sound Energy in Sound Waves Forced Vibrations Natural Frequency Resonance Interference Beats Warm-Up Check HW

24. © 2010 Pearson Education, Inc. Interference Property of all waves and wave motion Superposition of waves that may either reinforce or cancel each other

25. © 2010 Pearson Education, Inc. Interference Two patterns of interference Constructive interference –increased amplitude when the crest of one wave overlaps the crest of another wave Destructive interference –reduced amplitude when the crest of one wave overlaps the trough of another wave

26. © 2010 Pearson Education, Inc. Interference

27. © 2010 Pearson Education, Inc. Interference Application of sound interference Destructive sound interference in noisy devices such as jackhammers that are equipped with microphones to produce mirror-image wave patterns fed to operator’s earphone, canceling the jackhammer’s sound

28. © 2010 Pearson Education, Inc. Application of sound interference (continued) Sound interference in stereo speakers out of phase sending a monoaural signal (one speaker sending compressions of sound and other sending rarefactions) As speakers are brought closer to each other, sound is diminished. Interference

29. © 2010 Pearson Education, Inc. Beats Periodic variations in the loudness of sound due to interference Occur with any kind of wave Provide a comparison of frequencies

30. © 2010 Pearson Education, Inc. Beats Applications –Piano tuning by listening to the disappearance of beats from a tuning fork and a piano key –Tuning instruments in an orchestra by listening for beats between instruments and piano tone

31. © 2010 Pearson Education, Inc. Interference is a property of A.sound. B.light. C.Both A and B. D.Neither A nor B. Interference CHECK YOUR NEIGHBOR

32. © 2010 Pearson Education, Inc. Interference is a property of A.sound. B.light. C.Both A and B. D.Neither A nor B. Explanation: See Figure 12.21 for illustrations of both light and sound interference. Interestingly, the presence of interference tells a physicist whether something is wavelike. All types of waves can interfere. Interference CHECK YOUR ANSWER HW: Study Quiz Monday