UNIT 7 Waves, Vibrations, and Sound

1. UNIT 7Waves, Vibrations,and Sound

2. Thursday February 2nd WAVES, VIBRATIONS, AND SOUND

3. TODAY’S AGENDA Thursday, February 2 • More on Harmonics • Problem Quiz #2 UPCOMING… • Fri: Inertial Balance Lab • Mon: Problem Quiz #3 • Tue: Speed of Sound Lab

4. ConcepTest 14.12aPied Piper I (1) the long pipe (2) the short pipe (3) both have the same frequency (4) depends on the speed of sound in the pipe You have a long pipe and a short pipe. Which one has the higher frequency?

5. ConcepTest 14.12aPied Piper I (1) the long pipe (2) the short pipe (3) both have the same frequency (4) depends on the speed of sound in the pipe You have a long pipe and a short pipe. Which one has the higher frequency? A shorter pipe means that the standing wave in the pipe would have a shorter wavelength. Since the wave speed remains the same, the frequency has to be higher in the short pipe.

6. ConcepTest 14.12bPied Piper II A wood whistle has a variable length. You just heard the tone from the whistle at maximum length. If the air column is made shorter by moving the end stop, what happens to the frequency? 1) frequency will increase 2) frequency will not change 3) frequency will decrease

7. ConcepTest 14.12bPied Piper II A wood whistle has a variable length. You just heard the tone from the whistle at maximum length. If the air column is made shorter by moving the end stop, what happens to the frequency? 1) frequency will increase 2) frequency will not change 3) frequency will decrease Ashorter pipemeans that the standing wave in the pipe would have ashorter wavelength. Since the wave speed remains the same, and since we know thatv = fl, then we see that thefrequency has to increase when the pipe is made shorter.

8. ConcepTest 14.12cPied Piper III If you blow across the opening of a partially filled soda bottle, you hear a tone. If you take a big sip of soda and then blow across the opening again, how will the frequency of the tone change? 1) frequency will increase 2) frequency will not change 3) frequency will decrease

9. ConcepTest 14.12cPied Piper III If you blow across the opening of a partially filled soda bottle, you hear a tone. If you take a big sip of soda and then blow across the opening again, how will the frequency of the tone change? 1) frequency will increase 2) frequency will not change 3) frequency will decrease By drinking some of the soda, you have effectively increased the length of the air column in the bottle. Alonger pipemeans that the standing wave in the bottle would have alonger wavelength. Since the wave speed remains the same, and since we know thatv = fl,then we see that thefrequency has to be lower. Follow-up: Why doesn’t the wave speed change?

10. Chapter 12 Sound

11. Units of Chapter 12 • Characteristics of Sound • Intensity of Sound: Decibels • The Ear and Its Response; Loudness • Sources of Sound: Vibrating Strings and Air Columns • Quality of Sound, and Noise; Superposition • Interference of Sound Waves; Beats • Doppler Effect

12. Units of Chapter 12 • Shock Waves and the Sonic Boom • Applications: Sonar, Ultrasound, and Medical Imaging

13. Standing Waves; String The frequencies of the standing waves on a particular string are called resonant frequencies. They are also referred to as the fundamental and harmonics.

14. Standing Waves; Strings The wavelengths and frequencies of standing waves are:

15. Standing Waves in Air Columns • If one end of the air column is closed, a node must exist at this end since the movement of the air is restricted • If the end is open, the elements of the air have complete freedom of movement and an antinode exists

16. Tube Open at Both Ends

17. Resonance in Air Column Open at Both Ends • In a pipe open at both ends, the natural frequency of vibration forms a series whose harmonics are equal to integral multiples of the fundamental frequency

18. Tube Closed at One End

19. Resonance in an Air Column Closed at One End • The closed end must be a node • The open end is an antinode • There are no even multiples of the fundamental harmonic

20. Beats • Beats are alternations in loudness, due to interference • Waves have slightly different frequencies and the time between constructive and destructive interference alternates • The beat frequency equals the difference in frequency between the two sources:

21. Quality of Sound – Tuning Fork • Tuning fork produces only the fundamental frequency

22. Quality of Sound – Flute • The same note played on a flute sounds differently • The second harmonic is very strong • The fourth harmonic is close in strength to the first

23. Quality of Sound – Clarinet • The fifth harmonic is very strong • The first and fourth harmonics are very similar, with the third being close to them

24. Timbre • In music, the characteristic sound of any instrument is referred to as the quality of sound, or the timbre, of the sound • The quality depends on the mixture of harmonics in the sound

25. Pitch • Pitch is related mainly, although not completely, to the frequency of the sound • Pitch is not a physical property of the sound • Frequency is the stimulus and pitch is the response • It is a psychological reaction that allows humans to place the sound on a scale

26. Doppler Effect As can be seen in the previous image, a source moving toward an observer has a higher frequency and shorter wavelength; the opposite is true when a source is moving away from an observer.

27. Shock Waves and the Sonic Boom If a source is moving faster than the wavespeed in a medium, waves cannot keep up and a shock wave is formed. The angle of the cone is: (12-5)

28. Shock Waves and the Sonic Boom Shock waves are analogous to the bow waves produced by a boat going faster than the wave speed in water.

29. Shock Waves and the Sonic Boom Aircraft exceeding the speed of sound in air will produce two sonic booms, one from the front and one from the tail.

30. Applications: Sonar, Ultrasound, and Medical Imaging Sonar is used to locate objects underwater by measuring the time it takes a sound pulse to reflect back to the receiver. Similar techniques can be used to learn about the internal structure of the Earth. Sonar usually uses ultrasound waves, as the shorter wavelengths are less likely to be diffracted by obstacles.

31. Applications: Sonar, Ultrasound, and Medical Imaging Ultrasound is also used for medical imaging. Repeated traces are made as the transducer is moved, and a complete picture is built.

32. Applications: Sonar, Ultrasound, and Medical Imaging Ordinary ultrasound gives a good picture; high-resolution ultrasound is excellent.

33. Summary of Chapter 12 • Sound is a longitudinal wave in a medium. • The pitch of the sound depends on the frequency. • The loudness of the sound depends on the intensity and also on the sensitivity of the ear. • The strings on stringed instruments produce a fundamental tone whose wavelength is twice the length of the string; there are also various harmonics present.

34. Summary of Chapter 12 • Wind instruments have a vibrating column of air when played. If the tube is open, the fundamental is twice its length; if it is closed the fundamental is four times the tube length. • Sound waves exhibit interference; if two sounds are at slightly different frequencies they produce beats. • The Doppler effect is the shift in frequency of a sound due to motion of the source or the observer.

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