1. Mechanical Waves and Sound
Chapter 17
Physical Science

2. Mechanical Waves
17-1

3. 17-1 Learning Targets
Compare and contrast the three types of mechanical waves

4. Mechanical Waves
A disturbance in matter that carries energy from one place to another
Require a medium
Medium- material through which a wave travels
Solid, liquid, or gas
All waves except electromagnetic waves

5. Mechanical Waves
Created when a source of energy causes a vibration to travel though a medium
3 types: Longitudinal, transvers, and surface

6. Wave moves horizontally from left to right
Transverse Waves
Wave moves horizontally from left to right
Medium moves at right angles compared to the direction of the wave

7. Crest- highest point of transverse wave
Trough-lowest point of transverse wave

8. Transverse wave

9. Longitudinal Wave
Vibration of the medium is parallel to the direction the wave travels
Like a spring
Made of alternating compressions and rarefactions
Sound Waves
P waves produced by earthquakes

10. Longitudinal Wave

11. Compression-particles in medium spaced closed together
Rarefaction- particles in medium are spread out

12. Surface Waves A wave that travels along a surface separating two media
Objects move in circular pattern due to combination of movement ( from transverse and longitudinal like wave)

15. Properties of Waves
17-2

16. 17-2 Learning Targets
Determine how frequency, wavelength, and speed are related.
Determine how amplitude and energy are related.

17. Periodic motion - motion that repeat at regular time intervals
Pendulums
Period- time required for one cycle
complete motion that returns to its starting point

18. Frequency
Number of full wavelengths that pass a point in a given amount of time
Measures how rapidly vibrations occur in the medium or source of wave
Symbol is f
SI unit for frequency is Hertz (Hz) – vibrations per second

20. Wavelength
Distance between two neighboring crests or troughs in transverse wave
Distance between two neighboring compressions or rarefactions in a longitudinal wave
λ (lambda)

21. Wavelength is indirectly related to frequency
As Frequency increases, wavelength decreases

22. v= fλ Period T= 1/f Wave Speed (m/s) (waves/ s) (m/wave)
Wave velocity (v)= frequency (f) x wavelength(λ)
(m/s) (waves/ s) (m/wave)
v= fλ
Period
T= 1/f

23. v= fλ Wave Speed Problem V= 0.1 Hz x 15.0 m V= 1.50 m/s
The average wavelength in a series of ocean waves is 15.0 m. A wave crest arrives at the shore on average every 10s, so the frequency is Hz. What is the average speed of the waves?
v= fλ
V= 0.1 Hz x 15.0 m
V= 1.50 m/s

24. rearrange to find λ = v/ f λ= 340 m/s 220 Hz λ= 1.7 m
The speed of sound in air is about 340 m/s. What is the wavelength of a sound wave with a frequency of 220 Hz?
v= fλ
rearrange to find λ = v/ f
λ= 340 m/s
220 Hz
λ= 1.7 m

25. What is the period of a 5.2 Hz wave?
T= 1/f
T= 1/ 5.2 Hz
T= 0.19 s

26. Amplitude
The maximum displacement of the medium from its rest position
Transverse wave = vertical distance between the line of origin and crest or trough
Longitudinal wave= difference in pressure between maximum compression and resting state

28. Amplitude and energy is direct relationship
The more energy a wave has, the greater the amplitude

30. Behavior of Waves
17-3

31. 17-3 Learning Targets Describe reflection Describe refraction
Describe factors that affect diffraction
Describe two types of interference
Describe what will make standing waves

32. Reflection
Occurs when a wave bounces off a surface that is cannot pass through
Does not change speed or frequency of wave
Wave may be flipped upside down
Transverse wave – if hits fixed boundary the reflected wave is upside down compared to original wave

35. Refraction
Bending of light waves as they pass from one medium to another
Due to changing speed of wave (one side of wave moves more slowly than other side)
Objects may appear to be bent if light waves

36. Diffraction
Bending of a wave as it moves around an obstacle or through a narrow opening
Longer wavelengths are better at bending
Think of radio waves

37. Diffraction

38. Wave Interference
Mechanical waves are not matter, but displacement of matter.
Two wave can occupy the same space at the same time
Known as principle of superposition
Superposition principle is the method of summing the displacement of waves
When two or more waves travel through a medium, the resultant wave is the sum of the displacements of the individual waves at each point

39. Constructive interference
2 or more waves combine to make a resulting wave bigger than the original waves
New wave has greater amplitude
Crests align with crests

40. Destructive interference
2 or more waves combine so the resulting wave is smaller than the largest original wave
Complete destructive interference is when two pulses completely cancel each other
Crests align with troughs

42. Standing wave A wave that appears to stay in one place
Combination of two waves moving in opposite directions
Each having the same amplitude and frequency
Interference of reflected wave on self
Plucked strings

43. Standing wave forms only if half wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord
Or if frequency is doubled or triples

44. Standing Waves Node- point on standing wave that has no displacement
Antinode- point where crest or trough occurs midway between two nodes

47. Sound and Hearing
17-4

48. What affects the speed of sound?
1- Medium
Density
sounds moves best through more dense materials
Material
sounds travel fastest through solids and slowest through gases
2-Temperature
increase in temperature= increase in sound speed

50. Intensity
Rate at which a wave’s energy fowls through a given area

51. Decibels (dB) Unit used to measure the sound intensity or loudness
Sound greater than 120dB can cause pain

52. Loudness Loudness is just human perception
Determined by intensity –amplitude

53. Hair dryer, vacuum cleanerdB
Loudness of Sound
Faintest sound heard 15
Whisper 50
Normal conversation 75
Hair dryer, vacuum cleaner 80
Noisy restaurant
100
Lawn mower
120
Threshold of pain
150
Jett taking off

54. Pitch Highness of lowness of sound Determined by frequency
The greater the vibrations the higher the pitch
Human hearing range is ,000 Hz

55. Infrasound- slow vibrations, frequency lower than 20 Hz
Below human hearing range
Ultrasound- sound wave with frequency above 20,000 Hz
Above human hearing range
Used for Sonar

57. Doppler Effect
Change in pitch and frequency as the source of sound moves
Frequency and pitch increase as source of sound approaches and decrease as source moves away
Occurs for light, sound and other types of waves