1. Mechanical Waves and Sound
Physical Science

2. Mechanical Waves & Properties of Mechanical Waves
17.1 / 17.2
Physical Science CIA

4. What are mechanical waves? What do you think waves carry?
Can you see waves? Examples?

5. Mechanical Waves
Mechanical waves are disturbances in matter that carry energy from one place to another.
Usually require matter through which to travel
The matter a wave travels through is called a medium.
Medium can be a solid, liquid, or gas
Some waves can travel through space…with no medium!

6. How are mechanical waves created?

7. Creation of mechanical waves:
Need a source of energy!
That energy causes a vibration to travel through the medium

8. Types of Mechanical Waves
Transverse:
A wave that causes the medium to vibrate at right angles to the direction of the wave

9. Parts of a transverse wave:

10. Types of Mechanical Waves, ctd.
Transverse
Longitudinal:
A wave in which the vibration of the medium is parallel to the direction the wave travels

11. Parts of a longitudinal wave:

12. Remember!
A wave doesn’t move the medium…it’s just energy traveling through the medium!

13. Transverse and Longitudinal Wave Which is which?

15. Types of Mechanical Waves, ctd.
Transverse
Longitudinal
Surface:
A wave that
travels along a
surface
separating two
media

17. Period vs Frequency
Period – (T) – The time it takes for one cycle (= how long?)
Measured in seconds
Frequency – (f) – The number of cycles in a given time (= how many?)
Measured in Hertz (Hz)
Frequency is the inverse of the Period
f = 1 / T

18. Speed of a wave We find speed by distance divided by time.
The same holds true for waves!
Speed of a wave = Wavelength/Period
Speed of a wave = Wavelength*Frequency

19. Speed of a Wave

20. Speed of a Wave The speed of a wave is constant within a medium.
The speed can change when a wave enters a new medium
All waves of the same type travel at the same speed
This means wavelength is inversely proportional to frequency!
If the wavelength increases the frequency has to decrease!
If the wavelength decreases the frequency has to increase!

21. Practice Problem #1
A wave in a spring has a wavelength of 0.1 meters and a period of 0.2 seconds. What is the speed of the wave? L G E P S

22. Practice Problem #2
Find the wavelength of a wave in a rope that has a frequency of 2.0 Hz and a speed of 0.4 meters/second. L G E P S

23. Behavior of Waves
17.3 Physical Science CIA

24. What happens when… A wave meets a hard surface like a wall?
A wave enters a new medium?
A wave moves around an obstacle?
A wave meets another wave?

25. A wave meets a hard surface like a wall?
Reflection
A wave bounces off a surface that it can not pass through
Reflection does not change the speed or frequency of the wave, BUT the wave can be flipped upside down!

26. How?

27. A wave enters a new medium?
Refraction
The bending of a wave as it enters a new medium
The wave bends because as it enters a new medium it either speeds up or slows down!

28. Refraction

29. Refraction

30. A wave moves around an obstacle?
Diffraction – the bending of a wave as it moves around an obstacle or passes through a narrow opening

31. Diffraction

32. A wave meets another wave?
Interference – when two or more waves overlap and combine together.
Constructive
Destructive

33. Constructive Interference
Two or more waves combine to produce a wave with a larger displacement

34. Destructive Interference
Two or more waves combine to produce a wave with a smaller displacement

35. Standing Waves Occur because of interference!
A standing wave is a wave that appears to stay in one place.

36. Parts of a Standing wave
Node – Where there is no displacement of the medium in a standing wave
Antinode – Where there is maximum displacement of the medium in a standing wave

37. How do standing waves fit on a string?
Standing waves only form if a half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating string

38. Sound and Hearing
17.4 Physical Science CIA

39. Properties of Sound Waves
What type of waves are sound waves?

40. Properties, ctd.
Sounds behave in certain ways because of the properties of sound waves. Some properties are:
Speed
Intensity & Loudness
Frequency & Pitch

41. Speed Speed of sound = 342 m/s …THAT’S 765 mph!
Sound speed varies, depending on medium
Travel fastest in solids, slowest in gases
Travel fastest in the most dense media
Speed of Sound animation

42. Intensity
Intensity: the rate at which a wave’s energy flows through an area
Sound intensity depends on
Amplitude
Distance from source
Measured in decibels (dB)

44. Loudness
Subjective! (This means it depends on the person who is hearing it.)
Loudness is a personal, physical response to the intensity of sound.
As intensity increases, so does loudness, but loudness also depends on the listener’s ears and brain.

45. Frequency & Pitch
Frequency of a sound wave depends on how fast the source of the sound is vibrating.
Pitch is how we hear frequency of sound waves
Pitch depends on frequency…high frequency sounds are high pitched, and low frequency sounds are low pitched.
Pitch also depends on age and health

46. Frequency and
Pitch animation

47. Ultrasound Most people hear sounds between 20 and 20,000 Hz.
Infrasound – sound at frequencies lower than people usually hear
Ultrasound – sound at frequencies higher than people usually hear
Used in technologies such as sonar and ultrasound imaging

48. Ultrasound, ctd.
Sonar – a technique used to determine the distance to an object under water.
Ultrasound – medical technique used to take pictures of different organs (or a fetus!)

49. The Doppler Effect Where have you heard of the term “Doppler”?
Doppler Effect: a change in sound frequency (pitch) caused by the motion of the sound source, the listener, or both
Why do we observe this?
Doppler Effect animation

50. Hearing and the Ear
Your ear has a membrane that vibrates when sound waves hit it…what is this membrane?
The Ear animation

51. Hearing & the Ear Ear consists of 3 main parts
Outer Ear – gathers and focuses sound
Middle Ear – receives and amplifies vibrations
Inner Ear – uses nerve endings to sense vibrations and send signals to the brain

53. Reproduction of Sound What are some ways that sound is stored?
How do you think sound is stored, and then played back?

54. Reproduction of Sound
To record: sound waves must be converted into electronic signals that can be stored
To reproduce: electronic signals are converted back into sound waves

55. Music
Musical instruments change their pitch by changing the frequency of the waves they produce… how do they do this?
Reflection animation