Vibrations & Waves & Sound

1. Vibrations & Waves & Sound ‘”a periodic motion of the particles of an elastic body or medium in alternately opposite directions from the position of equilibrium when that equilibrium has been disturbed” When things vibrate, they move the air and create sound waves Music, talking, sirens, etc.

2. Vibrations & Waves

3. wavelength amplitude amplitude nodes wavelength Transverse Waves • Wave Anatomy crest trough

4. Vibrations & Waves Wavelength - Distance between peaks Amplitude - Max Height above resting spot Frequency - Number of Nodes-Crests-Troughs/second Period - Time of a complete vibration Period = 1 / Frequency Frequency = 1 / Period

5. Vibrations & Waves Water Waves – bounce swimmers up and down Sound Waves - move air back and forth

6. Wave Motion Consider a pebble thrown in the water Creates circular waves that move out from the disturbance Water moves up and down, the wave moves outward away from the disturbance Waves carry energy from the impact A bug in the water will move and and down

7. Wave Speed Wavelength is distance between peaks Frequency is number of peaks per second going past a point in the water Frequency depends on wavelength and how fast the wave moves!!! Speed = Wavelength x Frequency

8. Longitudinal or Transverse?

9. compression wavelength rarefaction wavelength Longitudinal Waves • Wave Anatomy

10. Sound Waves

11. Human Hearing sound wave vibrates ear drum amplified by bones converted to nerve impulses in cochlea

12. Sound Waves Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.

13. Human Hearing • Pitch • highness or lowness of a sound • depends on frequency of sound wave • human range: 20 - 20,000 Hz ultrasonic waves subsonic waves

14. B. Human Hearing • Intensity • volume of sound • depends on energy (amplitude) of sound wave • measured in decibels (dB)

15. Human Hearing DECIBEL SCALE 120 110 100 80 70 40 18 10 0

16. Sound Waves

17. Depends on the material of the vibrating medium Sound can vibrate water, wood (speaker enclosures, pianos), metal, plastic, etc. Sound speed in dry air is 330 meters/second at 0o C Faster in warm air, slower in cold Water 4 times faster, steel 15 times faster Speed of Sound

18. When a sound wave reflects from a surface we generate an echo Wave reflection from surfaces depends on the characteristics of the surface Smooth hard surfaces reflect best Rough soft surfaces reflect poorly Energy not reflected is absorbed or transmitted through the material Wave Reflection

19. Think of arrows pointing in the direction of the wave motion We can trace the path of these arrows Wave Reflection Angles Equal

20. Wave Reflection Acoustics of room design is very interesting. Need some reflections to “liven” the room. Too many reflections and the sound gets mushy. Look in a concert hall or auditorium to see the different sound treatments

21. If there is a change in the characteristics of a medium, waves are bent This occurs because different parts of the wave front travel at different speeds Think of a marching around a curved track The inside people have to move more slowly than the outside people to keep the lines straight Wave Refraction

22. Wave Refraction

23. The combination of reflection and refraction enables imaging Ultrasonic medical imaging Naval SONAR for detecting submarines Bats catch mosquitoes Catch discussion of dolphins on p. 261 Wave Reflection & Refraction

24. Objects have “natural” frequencies based on their size and structure Guitar strings are an example Timpani heads Air columns Natural Frequencies

25. Can externally impose a vibration on an object Guitars and violins and pianos Set the wood into motion at the frequency of the string This provides a larger surface to interact with the air Harp vs. Piano Forced Vibrations

26. When the forced vibration matches a natural frequency we get a “resonance” condition Think about a swing on a playground You go high when you pump the swing at its natural vibration frequency Sympathetic vibrations in tuning forks Famous Tacoma Narrows bridge collapse Resonance

27. Next Time • We continue the chapter on waves and sound.

28. Principle of Superposition Works for both longitudinal waves and for transverse waves Wave Interference

29. Simply align the waves in time and add the amplitudes Amplitudes can be either positive or negative If the amplitudes are of the same sign, the wave is reinforced and grows bigger If the amplitudes are of opposite sign, the wave is diminished and grows smaller Wave Interference

30. Wave Interference

31. Wave Interference

32. So far, we have only looked at waves that have the same wavelength or frequency What happens if the frequencies are different? According to the Superposition Principle, we simply add the two waves together to see what results Wave Interference

33. Produces “Beats” This is how musicians “tune” their instruments to match in the orchestra Wave Interference

34. Wave Interference 10% Frequency Difference 20% Frequency Difference

35. Standing Waves

36. Standing Waves

37. C. Doppler Effect • Doppler Effect • change in wave frequency caused by a moving wave source • moving toward you - pitch sounds higher • moving away from you - pitch sounds lower

38. Doppler Effect

39. Wave Barriers The object is moving at the speed of waves in the medium. See how the waves pile up at the source.

40. Shock Waves The source is moving faster than the wave speed in the medium. A shock wave is formed and it is very difficult to break through the previous wave barrier. These waves produce sonic booms.

41. Music vs. Noise • Music • specific pitches and sound quality • regular pattern • Noise • no definite pitch • no set pattern

42. Guitar Strings

43. Guitar Strings A combination wave composed of the 1st harmonic and the third harmonic.

44. What makes instruments unique is the combination of harmonics produced by the different instruments. Flutes produce primarily the 1st harmonic They have a very pure tone Oboes produce a broad range of harmonics and sound very different Music

45. Combining Waves Revisited

46. Combining Waves

47. Three ways to make sound Vibrate a string Vibrate an air column Vibrate a membrane Musical Instruments

48. Violin, viola, cello, string bass Guitars Ukuleles Mandolins Banjos All vibrate a structure to amplify the sound Vibrating Strings

49. Pipe Organs Brass Instruments Woodwinds Whistles Vibrating Air Columns

50. Percussion Instruments Snare Drum Bass Drum Bongos Timpani are unique in that they are tuned to produce particular 1st harmonics You see the timpanist tunes each drum during a performance Vibrating Membranes