1. Chapter 19 Vibrations & Waves
Part IV: Sound
Chapter 19 Vibrations & Waves
24-Apr-17
Physics 1 (Garcia) SJSU

2. Vibrations Vibrations are a common phenomenon.
Oscillating motion of a mass on a spring
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3. Period
Time required for a full oscillation (one round trip) is called the period of oscillation.
Pendulum that is about one meter long has a period of two seconds per oscillation.
Note: Measure the period of a pendulum in lab.
24-Apr-17
Physics 1 (Garcia) SJSU

4. Frequency Frequency is the inverse of the period, (Frequency) = 1
For example, for a period of 2 seconds per oscillation, the frequency is ½ oscillation per second or ½ Hertz.
1 Hertz = 1 oscillation per second 1
(Period)
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Physics 1 (Garcia) SJSU

5. Amplitude
The distance from the rest position is the amplitude of oscillation.
Amplitude

6. Waves
Concept of vibrations extends into the phenomenon of wave motion.
String
Sound
Water waves
Radio
Light waves
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Physics 1 (Garcia) SJSU

7. Transverse Waves
For transverse waves the wave’s amplitude is perpendicular to the wave’s motion.
Amplitude
Wave Motion
Amplitude
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8. Doing “The Wave”
Synchronized standing and sitting by a stadium’s crowd is an example of a transverse wave.
Video analysis indicates that it takes only a few dozen fans leaping to their feet with their arms up to trigger a wave. Once started, it usually rolls in a clockwise direction at a rate of about 40 feet per second, or about 20 seats per second. At any given time, the wave pulse is about 15 seats wide.
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Physics 1 (Garcia) SJSU

9. Longitudinal Waves
For longitudinal waves, amplitude and wave motion are parallel.
Wave Motion
Amplitude
Amplitude
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A crowd can do a longitudinal wave by moving side-to-side instead of up-and-down

10. Wavelength
Wavelength is distance between crests or between troughs of waves.
Longitudinal
Transverse
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Physics 1 (Garcia) SJSU

11. Water waves at the beach move a few miles per hour
Wave Speed
The speed at which waves travel is called the wave speed.
Speed of sound = 330 m/s = 725 mi/hr
Speed of light = 300,000,000 m/s
Water waves at the beach move a few miles per hour
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Physics 1 (Garcia) SJSU

12. Tsunami Waves
Tsunamis are ordinary water waves, just like waves in your bathtub, but because they are typically generated by deep sea earthquakes they carry huge amounts of energy and momentum, traveling at almost 500 mph while in the deep ocean.
The tsunami of 26 December 26th 2004 was produced by an earthquake whose epicenter was located off the coast of Indonesia in the Indian Ocean. The death toll is estimated at over a quarter of a million persons.
Animation by Vasily V. Titov
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Physics 1 (Garcia) SJSU

13. Wave Relations Wave speed, wavelength & frequency related.
(Wave speed) = (Wavelength) x (Frequency)
(Wave length) =
(Frequency) =
(Wave speed)
(Frequency)
(Wave speed)
(Wavelength)
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Physics 1 (Garcia) SJSU

14. Check Yourself What is the wave length? What is the wave speed?
What is the wave’s frequency?
What is the wave’s period?
6 m/s
2 m
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Physics 1 (Garcia) SJSU

15. Demo: Hearing Sound
Range of human hearing is roughly 20 Hertz to 20,000 Hertz.
Wave speed for sound is 330 m/s
Wavelength of 20 Hertz is 16 m (about 50 ft)
Wavelength of 20,000 Hz is 1.6 cm (½ inch)
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Physics 1 (Garcia) SJSU

16. Hearing in Animals
Frequency range varies widely, depending on natural adaptation using sound to communicate, locate food, avoid predators, etc.

17. Constructive Interference
Two waves in phase add together, which is called constructive interference.
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Physics 1 (Garcia) SJSU

18. Destructive Interference
Two waves out of phase cancel each other out, which is destructive interference.
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19. In & Out of Phase

20. Demo: In & Out of Phase
Pair of speakers constructively interfere when they are in phase (oscillating together).
When out of phase (reverse wires on one of the speakers) then they destructively interfere.
Out of
Phase
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Physics 1 (Garcia) SJSU

21. Noise-Canceling Headphones
Noise-canceling headphones use a microphone that listens for noise and a speaker that produces the same noise but out of phase (cancellation by destructive interference)
External Noise
Canceling Sound

22. Demo: Speaker Baffle
Why are speakers mounted behind a baffle and inside an enclosure?
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23. Standing Waves
When a wave interferes with its reflection, this may create a standing wave.

24. Standing Waves and Phase
Standing waves are another example of constructive and destructive interference.
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25. Doppler Effect
Sound coming from a moving object has a different wavelength and frequency than if it were stationary.
If moving towards you, wavelength shorter and frequency higher.
If moving away, wavelength longer and frequency lower.
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Physics 1 (Garcia) SJSU

26. Demo: Doppler Shift
Hear frequency as higher when buzzer is moving towards you and hear it as lower when moving away from you.
Shorter Wavelength
Higher
Frequency
Lower
Frequency
Longer Wavelength
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Physics 1 (Garcia) SJSU