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Chapter 15

Chapter 15. The Nature of Sound. What is Sound???. Sound is a Longitudinal Wave traveling through matter. Longitudinal Waves. Longitudinal Waves Matter vibrates in the same direction as the wave travels. Compression. Longitudinal Waves. Rarefaction. λ. Sound from a Tuning Fork.

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Chapter 15

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  1. Chapter 15

  2. The Nature of Sound What is Sound??? Sound is a Longitudinal Wave traveling through matter.

  3. Longitudinal Waves Longitudinal Waves Matter vibrates in the same direction as the wave travels.

  4. Compression Longitudinal Waves Rarefaction

  5. λ

  6. Sound from a Tuning Fork

  7. Speed of Sound Sound is transmitted through matter. The Velocity of Sound depends on the matter that carries it.

  8. Sound travels at a velocity of 332m/s in air at 0C. • Sound travels faster through warm air than through cold air. • The velocity of sound increases about 0.6m/s for each degree in temperature. • At 20C sound travels at 344m/s. • Sound travels much faster through liquids and solids than through gases.

  9. Comparing Media

  10. Human Hearing Frequency of Sound 20 Hz to 20,000 Hz. Sound above 20,000 Hz - Ultrasonic Sound less than 20 Hz – Subsonic (Infrasonic)

  11. FrequencyisPitch

  12. Detection of Pressure Waves

  13. Ear Drum Detection of Pressure Waves

  14. Intensity and Loudness Intensity of Sound Depends on the amplitude of the wave. Loudness Describes a person’s response to sound intensity.

  15. Loudness is measured in Decibels(dB) For every 10dB change the sound doubles!! 70dB is twice 60dB 80dB is four times 60dB

  16. Faintest Sound Heard 0dB Whisper 15dB Rustling Leaves 20dB Purring Cat 25dB Average Home 50dB Vacuum Cleaner 75dB Noisy Restaurant 80dB Power Mower 100dB Chain Saw 115dB ------Painful ------- 120dB Jet Plane Taking Off 150dB

  17. Louder Sound Interference Constructive Interference Occurs when the compressions and rarefactions of two or more waves come together.

  18. Quieter Sound Interference Destructive Interference Occurs when a compression of one wave arrives at the same time as a rarefaction of another wave.

  19. Interference Beats The result of compressions and rarefactions of two slightly different frequencies reaching your ears together. Beats

  20. Beats f1 = 512Hz f2 = 514Hz Beats = f1 - f2 = 514Hz - 512Hz Beats = 2Hz (beats/s)

  21. The Doppler Effect The change in wave frequency caused by the motion of the sound source or the motion of the observer.

  22. The Doppler Effect Shorter WavelengthHigher Frequency

  23. The Doppler Effect Longer WavelengthLower Frequency

  24. Speed of Sound

  25. Greater than the Speed of Sound

  26. Resonance A resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object.

  27. Harmonics Vibrations which occur at a particular frequency is known as a harmonic.

  28. First Harmonic The lowest possible frequency at which a string could vibrate to form a standing wave pattern is known as the fundamental frequency or the first harmonic.

  29. First Harmonic

  30. Second Harmonic

  31. Third Harmonic

  32. L Resonance in Air Columns Closed Air Column λ = 4L λ = 4/3L λ = 4/5L

  33. L Resonance in Air Columns Open Air Column λ = 2L λ = L λ = 2/3L

  34. Example A tuning fork is placed above an open-pipe resonator in which the length can be changed. The loudest sound is heard at a length of 67cm and the next loudest was heard at 100.5cm. If the temperature of the air is 20°C what is the frequency of the tuning fork?

  35. 67cm = λ Example 33.5cm = ½λ 67cm 2•33.5cm = λ 100.5cm (100.5 - 67)= 33.5cm

  36. Example f = 343m/s 0.67m λ = 67cm = 0.67m v@20°C = 343m/s v = λf f = v/λ f = 512Hz

  37. Music to Your Ears A back and forth motion is set up in a string, resulting in a regular vibration. The vibration is called a standing wave the location of the crests and troughs are always in the same place.

  38. In a wind instrument, holes are opened and closed, changing the length of the vibrating column of air. This changes the size of the standing wave.

  39. Noise Sound with no regular pattern or definite pitch.

  40. Tone Quality The differences among sounds of the same pitch and loudness.

  41. Music Musical Sounds Based on a series of notes called a musical scale.

  42. The Sound Spectrum:Fundamental and Harmonics

  43. Open Air Column L λ = 2L λ = L λ = 2/3L f1 = v/λ f2 = v/L f3 = v/2/3L f3 = 3f1 f1 = v/2L f2 = 2f1

  44. Fundamental Frequency 262Hz First Overtone 524Hz Second Overtone 786Hz Third Overtone 1048Hz

  45. L Closed Air Column λ = 4L λ = 4/3L λ = 4/5L f3 = v/4/5L f1 = v/4L f2 = v/4/3L f2 = 3f1 f3 = 5f1

  46. Fundamental Frequency 256Hz First Overtone 768Hz Second Overtone 1280Hz Third Overtone 1792Hz

  47. Harmony Notes that sound pleasing together.The ratio of the frequencies of tones that are in harmony are small whole numbers. ·Notes that are one octave apart. Middle C and C 524/262 = 2/1 ·Notes E and C 330/262 = 5/4

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