1. UNIT 5: VIBRATIONS, WAVES & SOUND

2. Waves and Oscillations
Pendulum swinging back and forth shows how oscillations can create waves.

3. WAVES Energy transfer: by doing work, by heat, or by waves!
Wave: a disturbance (vibration) that travels
mechanical waves require a material medium (solid, liquid, or gas) – particles vibrate in simple harmonic motion (water, sound, earthquake waves)
2 types – transverse and longitudinal
electromagnetic waves travel through a material or a vacuum – vibrating electric and magnetic fields (radio, microwave, infrared, visible light, ultraviolet, x-ray, gamma rays)

4. WAVES
Transverse waves: vibrations are perpendicular to wave direction

5. WAVES Longitudinal waves: vibrations parallel to wave direction
rarefaction

6. WAVES
Frequency, f: number of waves each second, unit: Hertz (Hz) Hz = 1 wave/sec
Period, T: time for one wave to pass, unit: s f=1/T
Wavelength, l: distance between identical points on two waves, unit: m

7. WAVES Amplitude, A: maximum displacement from equilibrium, unit: m
Wave speed, v: speed of the wave, not the particles, unit: m/s v=fl
use difference in wave speeds to find distance
ex: lightning & thunder

8. WAVE INTERACTIONS
Reflection: waves "bounce back" at boundary

9. WAVE INTERACTIONS
Law of Reflection: qi = qr i: incidence, r: reflection

10. WAVE INTERACTIONS
Refraction: wave path bends as wave crosses boundary. Note that speed & wavelength change as wave moves into new medium, but frequency remains constant.

11. Refraction Examples
1. Dish filled with water Light through glass

12. Refraction Rules
When a wave goes from fast to slow mediums the wave will bend toward the normal.
When a wave goes from slow to fast mediums the wave will bend away from the normal.
Vi Sin qr = Vr Sin qi

13. WAVE INTERACTIONS
Diffraction wave spreads out or “bends” beyond edge of barrier

14. WAVE INTERACTIONS
Diffraction greatest when l is greater than or equal to the size of opening or object

15. SOUND INTERACTIONS Resonance (sympathetic vibration)
objects have natural vibrating frequency
sending waves to an object at at its natural frequency will make it vibrate
pushing a child on a swing
using microwaves to heat up water

16. SOUND WAVES
Source: a vibrating object (vocal cord, string, reed, etc.)

17. SOUND INTERACTIONS

18. WAVE INTERACTIONS
Interference: waves pass through each other without changing each other, but their displacements add together

19. WAVE INTERACTIONS
constructive interference: combined wave displacement is greater than individual waves

20. WAVE INTERACTIONS
destructive interference: combined wave displacement is less than individual waves

21. WAVE INTERACTIONS
Standing Waves: interference of two identical waves going opposite directions makes waves appear to vibrate in place

22. WAVE INTERACTIONS Standing Waves: nodes: no displacement
antinodes: maximum displacement
Harmonic number is how many crests are trapped

23. SOUND WAVES Pitch: musical tone or note – frequency of a wave
sonic spectrum:
musical scale: specific proportional frequencies
C major scale C D E F G A B
frequency (Hz)
264
297
330
352
396
440
495
528

24. MUSICAL INSTRUMENTS l=2L/n L: length of string, and n is 1,2,3…
f=v/l v: wave speed in string
v=√TL/m T: tension, m: mass of string

25. MUSICAL INSTRUMENTS Stringed Instruments
quality: mixture of fundamental and harmonics (makes different instruments sound different)
sound boards & boxes: more air surface contact - amplifiers

26. MUSICAL INSTRUMENTS

27. MUSICAL INSTRUMENTS Wind Instruments
pitch = frequency of vibration of column of air
f = v/l v: sound speed in air = 340 m/s l: wavelength, depends on length of air column

28. MUSICAL INSTRUMENTS open-end tube: each end of tube is antinode
l = 2L/n L: length of tube and n is 1,2,3…
Examples: flutes, saxophones, some organ pipes

29. MUSICAL INSTRUMENTS closed-end tube: closed end of tube is node
l=4L/n L: length of tube and n is 1,3,5
Examples: clarinets, some pipe organs

30. UNIT 5: VIBRATIONS, WAVES & SOUND
PHYSICS
UNIT 5: VIBRATIONS, WAVES & SOUND

31. SOUND INTERACTIONS
The Doppler Effect: apparent change in frequency due to motion of source or listener

32. SOUND INTERACTIONS
Wave speed stays constant, remember wave speed depends on medium.
Frequency changes and wavelength changes
When source or observer moves toward each other wavelength decreases and frequency increases.
When source or observer moves away from each other wavelength increases and frequency decreases

33. SOUND INTERACTIONS
Radar: uses Doppler Effect in radio waves reflected off an object to determine its speed (speed traps, locating enemy aircraft)
Red shift (decreased frequency) and Blue shift (increased frequency) of light tells astronomers whether a star or galaxy is moving toward or away from Earth.

34. SOUND INTERACTIONS

35. SOUND INTERACTIONS The Doppler Effect
sound barrier: “pile-up” of sound waves (pressure) in front of object traveling Mach 1
sonic boom: cone-shaped pressure pulse following an object traveling at supersonic speeds (bow wave, also called water wake, following a speedboat)

36. SOUND INTERACTIONS

37. UNIT 5: VIBRATIONS, WAVES & SOUND
PHYSICS
UNIT 5: VIBRATIONS, WAVES & SOUND

38. QUIZ 5.4
The speed of sound in earth is 3500 m/s. An earthquake wave, frequency 5 Hz, travels from its source to a distant mountain range and returns in 3.4 minutes.
(a) How far away is the mountain range?
(b) What is the wavelength of the earthquake wave?
(c) If the mountain range was moving away at 0.50 m/s. what would be the frequency of the reflected wave?
357,000 m
700 m
5.00 Hz

39. UNIT 5 REVIEW f = 1/T v = fl qi = qr visinqr = vrsinqi node dist = l/2
loop height = 4A
v = T
I = P/4pr2
b = 10log(I/I0)
I0 = 1×
10-12 W/m2
open pipe l = 2L
closed pipe l = 4L
x = vt