1. Waves and Sound

2. What is a Wave?
Wave- a repeating disturbance or movement that transfers energy through matter or space
For example, during earthquakes, energy is transferred by powerful waves that travel through Earth

3. Waves and Energy A pebble falls into a pool of water and ripples form
Because it is moving, the falling pebble has energy
As it splashes into the pool, the pebble transfers some of its energy to nearby water molecules, causing them to move
What you see is energy traveling in the form of a wave on the surface of the water

4. Waves and Matter
Imagine you’re in a boat on a lake, and approaching waves bump against your boat
The waves don’t carry the boat along with them as they pass, or even the water.
Only the energy carried by the waves is transferred
All waves have this propertythey transfer energy without transferring matter from place to place.

5. Making Waves
Suppose you are holding a rope at one end, and you give it a shake
You would create a pulse that travels along the rope to the other end, and then the rope would be still again

6. Making Waves
It is the up-and-down motion of your hand that creates the wave
Anything that moves up and down or back and forth in a rhythmic way is vibrating
The vibrating movement of your hand at the end of the rope created the wave (in fact, all waves are produced by something that vibrates)

7. Mechanical Waves
The matter the waves travel through is called a medium
The medium can be a solid, a liquid, a gas, or a combination of these
Not all waves need a medium
Some waves, such as light and radio waves, can travel through space

8. Mechanical Waves
Mechanical waves- waves that are caused by a disturbance or vibration in matter that can travel only through matter
The two types of mechanical waves are transverse waves (i.e. water) and compressional waves (i.e. sound) NOTE: THESE ARE ALSO CALLED LONGITUDINAL

9. Transverse Waves
Transverse wave- matter in the medium moves back and forth at right angles to the direction that the wave travels
For example, a wave on a rope travels horizontally as the rope moves vertically up and down
Animation

10. Compressional Waves
Compressional wave- matter in the medium moves back and forth along the same direction that the wave travels
You can model these waves with a coiled spring toy
Squeeze several coils together at one end of the spring and then let go of the coils
Animation

11. Compressional Waves
As the wave moves, it looks as if the whole spring is moving toward one end
The wave carries energy, but not matter, forward along the spring

12. Check for Understanding
Which type of mechanical wave is represented here?
A. compressional
B. electromagnetic
C. seismic
D. transverse

13. Check for Understanding
The answer is A. The wave travels horizontally along the spring, in the same direction that the coils move

14. Sound Waves Sound waves are compressional waves
When a noise is made, such as when a locker door slams shut and vibrates, nearby air particles are pushed together by the vibrations
The air particles are squeezed together like the coils in a coiled spring
The compressions travel through the air to make a wave

15. Sound in Other Materials
Sound waves also can travel through other mediums, such as water and wood
When a sound wave reaches your ear, it causes your eardrum to vibrate
Your inner ear then sends signals to your brain, and your brain interprets the signals as sound
Sound
and
Nascar
Guitar strings
Eardrum

16. Water Waves
A water wave causes water to move back and forth, as well as up and down
Water is pushed back and forth to form the crests and troughs
The low point of a water wave is formed when water is pushed aside and up to the high point of the wave

17. Water Waves
Ocean waves are formed most often by wind blowing across the ocean surface
The size of the waves that are formed depend on the wind speed, the distance over which the wind blows, and how long the wind blows

18. Seismic Waves
Forces in Earth’s crust can cause tectonic plates to suddenly slide past each other
This sudden sliding between tectonic plates produces seismic waves that carry energy outward
Tsunami

19. Seismic Waves
Seismic waves are a combination of compressional and transverse waves that can travel through Earth and along Earth’s surface
The more the crust moves during an earthquake, the more energy is released
Click image to view movie

20. The Parts of a Wave
Waves can differ in how much energy they carry and in how fast they travel
Waves also have other characteristics that make them different from each other
A transverse wave has alternating high points, called crests, and low points, called troughs

21. The Parts of a Wave
On the other hand, a compressional wave has no crests and troughs
When you make compressional waves with a coiled spring, a compression is a region where the coils are close together
The coils in the region next to a compression are spread apart, or less dense which is called a rarefaction

22. Check for Understanding
Which is represented by area A?
A. compression
B. crest
C. rarefaction
D. trough

23. Check for Understanding
The answer is C. A rarefaction is the less-dense region of a compressional wave

24. Wavelength
A wavelength (λ) is the distance between one point on a wave and the nearest point just like it
For transverse waves the wavelength is the distance from crest to crest or trough to trough

25. Wavelength
A wavelength in a compressional wave is the distance between two neighboring compressions or two neighboring rarefactions
The wavelengths of sound waves that you can hear range from a few centimeters for the highest-pitched sounds to about 15 m for the deepest sounds

26. Frequency and Period
The frequency (f) of a wave is the number of wavelengths that pass a fixed point each second
You can find the frequency of a transverse wave by counting the number of crests or troughs that pass by a point each second
For a compressional wave, such as sound, the frequency is the number of compressions or the number of rarefactions that pass by each second
Frequency is expressed in hertz (Hz)=1/sec

27. Frequency and Period
The period of a wave is the amount of time it takes one wavelength to pass a point
As the frequency of a wave increases, the period decreases
Period has units of seconds

28. Wavelength is Related to Frequency
As frequency increases, wavelength decreases
The frequency of a wave is always equal to the rate of vibration of the source that creates it
If you move the rope down, up, and back down in 1 s, the frequency of the wave you generate is 1 Hz (1 wave/second)

29. Wavelength is Related to Frequency
The speed of a wave depends on the medium it is traveling through
Sound waves usually travel faster through liquids and solids than they do through gases
However, light waves travel more slowly through liquids and solids than they do through gases or through a vacuum
Sound waves usually travel faster through warmer materials than through cooler materials

30. Calculating Wave Speed
You can calculate the speed of a wave represented by v by multiplying its frequency times its wavelength

31. Amplitude and Energy
Amplitude is related to the amount of disturbance from a wave
The greater the wave’s amplitude is, the greater the disturbance that the wave produces
Amplitude is measured differently for compressional and transverse waves

32. Amplitude of Compressional Waves
The amplitude of a compressional wave is related to how tightly the medium is pushed together at the compressions
The denser the medium is at the compressions, the larger the wave’s amplitude is and the more disturbance the wave creates

33. Amplitude of Compressional Waves
The closer the coils are in a compression, the farther apart they are in a rarefaction
So the less dense the medium is at the rarefactions, the more energy the wave carries

34. Amplitude of Transverse Waves
The amplitude of any transverse wave is the distance from the crest or trough of the wave to the rest position of the medium

35. Check for Understanding
What is the distance from the rest position to the crest in the diagram called?
A. compression
B. wavelength
C. amplitude
D. frequency

36. Check for Understanding
The answer is C. The distance from the rest position to the crest or from the trough to the rest position is the amplitude

37. Check for Understanding
What is the speed of a wave that has a wavelength of 5.0 m and a frequency of 2 Hz?
A m/s
B m/s
C. 10 m/s
D m/s

38. Check for Understanding
The answer is C. Wave speed is equal to the frequency multiplied by wavelength
= 5 m X 2 Hz

39. Reflection
Reflection occurs when a wave strikes an object and bounces off of it
All types of wavesincluding sound, water and light wavescan be reflected
How does the reflection of light allow you to see yourself in the mirror?
First, light strikes your face and bounces off
Then, the light reflected off your face strikes the mirror and is reflected into your eyes

40. Echoes A similar thing happens to sound waves when your footsteps echo
Sound waves form when your foot hits the floor and the waves travel through the air to both your ears and other objects
Sometimes when the sound waves hit another object, they reflect off that object and come back to you
Your ears hear the sound again, a few seconds after you first heard your footstep

41. The Law of Reflection
The line perpendicular to the surface of the mirror is called the normal
The angle formed by the incoming beam and the normal is the angle of incidence (i)
The angle formed by the reflected beam and the normal is the angle of reflection (r)
According to the law of reflection, the angle of incidence is equal to the angle of reflection
Ken Karp for McGraw-Hill Companies
All reflected waves obey this law

42. Refraction
When a wave passes from one medium to another (such as when a light wave passes from air to water) it changes speed
Refraction the bending of a wave caused by a change in its speed as it moves from one medium to another

43. Refraction of Light in Water
Light waves travel slower in water than in air. This causes light waves to change direction when they move from water to air or air to water
When light waves travel from air to water, they slow down and bend toward the normal

44. Check for Understanding
Which would you predict will happen if the flashlight is submerged and the beam directed upward?

45. Check for Understanding
A. The light waves would slow down and bend away from the normal
The light waves would speed up and bend away from the normal
C. The light waves would slow down and bend toward the normal
D. The light waves would speed up and bend toward the normal

46. Refraction of Light in Water
When light waves travel from water to air, they speed up and bend away from the normal

47. Check for Understanding
The answer is B. Light waves travel slower in water than in air, so if the flashlight were submerged and directed upward, the light waves speed up and bend away from the normal

48. Refraction of Light in Water
You may have noticed that objects underwater seem closer to the surface than they really are
In the figure, the light waves reflected from the swimmer’s foot are refracted away from the normal and enter your eyes
The light waves that enter your eyes seem to have come from a foot that was higher in the water

49. Diffraction When waves strike an object, several things can happen:
The waves can bounce off, or be reflected
If the object is transparent, light waves can be refracted as they pass through it
Waves also can behave another way when they strike an object…they can bend around the object

50. Diffraction
Diffraction- when an object causes a wave to change direction and bend around it
Diffraction and refraction both cause waves to bend
The difference is that refraction occurs when waves pass through an object, while diffraction occurs when waves pass around an object

51. Diffraction
Waves also can be diffracted when they pass through a narrow opening
After they pass through the opening, the waves spread out

52. Diffraction and Wavelength
The amount of diffraction that occurs depends on how big the obstacle or opening is compared to the wavelength
When an obstacle is smaller than the wavelength, the waves bend around it
If the obstacle is larger than the wavelength, the waves do not diffract as much. In fact, if the obstacle is much larger than the wavelength, almost no diffraction occurs

53. Hearing Around Corners
You’re walking down the hallway and you can hear sounds coming from the lunchroom before you reach the open lunchroom door
The wavelengths of sound waves are similar in size to a door opening. Sound waves diffract around the door and spread out down the hallway

54. Interference
Interference- when two or more waves overlap and combine to form a new wave
Interference can either be constructive or destructive

55. Constructive Interference
With constructive interference, the waves add together
This happens when the crests of two or more transverse waves arrive at the same place at the same time and overlap
The amplitude of the new wave that forms is equal to the sum of the amplitudes of the original waves

56. Destructive Interference
With destructive interference, the waves subtract from each other as they overlap
This happens when the crests of one transverse wave meet the troughs of another transverse wave
The amplitude of the new wave is the difference between the amplitudes of the waves that overlapped (out of phase)

57. What produces sound?
Every sound is produced by an object that vibrates and causes compressional waves
Your friends’ voices are produced by the vibrations of their vocal cords, and music from a speaker are produced by vibrating speakers

58. Sound Traveling as a Wave
Compressions and rarefactions move away from the speaker as particles of air collide with their neighbors
A series of compressions and rarefactions forms that travels from the speaker to your ear and transfers energy to your eardrum
This sound wave is what you hear

59. Sound Traveling Through Materials
Most sounds you hear travel through air to reach your ears
If you’ve ever been swimming underwater and heard garbled voices, you know that sound also travels through water
Sound waves can travel through any type of matter, or mediumsolid, liquid, or gas
Sound waves cannot travel through a vacuum

60. Check for Understanding
In which environment would sound waves not travel?
A. at altitudes of 10,000 – 15,000 m
B. in solid aluminum
C. on the Moon
D. under water

61. Check for Understanding
The answer is C. Sound waves require a medium through which to travel. Sound waves cannot travel through a vacuum.

62. The Speed of Sound through Different Materials
The speed of a sound depends on the substance the medium is made of, its temperature and whether the medium is solid, liquid, or gas
In general, sound travels the slowest through gases, faster through liquids, and even faster through solids

63. The Speed of Sound through Different Materials
Sound travels faster in liquids and solids than in gases because the molecules in a liquid or solid are closer together than the molecules in a gas
However, the speed of sound doesn’t depend on the loudness of the sound
Loud sounds travel through a medium at the same speed as soft sounds

64. A Model for Transmitting Sound
A line of people passing a bucket is a model for molecules transferring the energy of a sound wave
When the people are far away from each other, like the molecules in gas, it takes longer to transfer the bucket of water from person to person
The closer the particles, the faster they can transfer energy from particle to particle

65. Temperature and the Speed of Sound
As the temperature of a substance increases, its molecules move faster
This makes them more likely to collide with each other and transfer energy

66. Check for Understanding
Use the table to estimate how far a sound will travel through a steel rail in 5s
A. 5 km
B. 10 km
C. 20 km
D. 30 km

67. Check for Understanding
The answer is D. Remember that distance equals speed multiplied by time

68. Intensity and Loudness
What happens to the sound waves from your radio when you adjust the volume? The notes sound the same as when the volume was higher, but something about the sound changes
The difference is that quieter sound waves do not carry as much energy as louder sound waves do
The amount of energy a wave carries corresponds to its amplitude
For a compressional wave, amplitude is related to how close together the particles are that make up the compressions and rarefactions

69. Intensity and Loudness
When an object vibrates strongly with a lot of energy, it makes sound waves with tight, dense compressions
When an object vibrates with low energy, it makes sound waves with loose, less dense compressions

70. Intensity and Loudness
The density of particles that make up the rarefactions behaves in the opposite way
It is important to remember that matter is not transported during the compression and rarefaction of a compressional waveonly energy is transported
The matter compresses and expands as the wave passes through that medium

71. Intensity
Intensity- is the amount of energy that is transferred across a certain area in a specific amount of time
When you turn down the volume of your radio, you reduce the energy carried by the sound waves, so you also reduce their intensity
Intensity influences how far away a sound can be heard

72. Intensity Decreases with Distance
Sound intensity decreases with distance for two reasons
First, the energy that a sound wave carries spreads out as the sound wave spreads out
Second, some of a sound wave’s energy converts to other forms of energy, usually thermal energy, as the sound travels through matter

73. Loudness
Loudness is the human perception of sound volume and depends primarily on sound intensity
When sound waves of high intensity reach your ear, they cause your eardrum to move back and forth a greater distance than sound waves of low intensity do

74. A Scale for Loudness
The intensity of sound can be described using a measurement scale
Each unit on the scale for sound intensity is called a decibel, abbreviated dB
On this scale, the faintest sound that most people can hear is 0 dB

75. Pitch
If you were to sing a scale, your voice would start low and become higher with each note
Pitch is how high or low a sound seems to be
The pitch of a sound is related to the frequency of the sound waves—high pitch = high frequency
Mosquitos

76. The Doppler Effect
The change in pitch or wave frequency due to a moving wave source is called the Doppler effect
The Doppler effect occurs when the source of a sound wave is moving relative to a listener OR if a listener is moving relative to a sound
Doppler effect sounds

77. Moving Sound
As a race car moves, it sends out sound waves in the form of compressions and rarefactions
The race car creates a compression, labeled A that moves through the air toward the flagger
By the time compression B leaves the race car, the car has moved

78. Moving Sound
Because the car have moved since compression A, compressions A and B are closer together than if the car had stood still
As a result, the flagger hears a higher pitch because of the increased frequency in the beginning and a lower one in the end

79. Using the Doppler Effect
The Doppler effect also occurs for other waves besides sound waves
The frequency of electromagnetic waves, such as waves from a radar gun, change when they reflect off a moving car depending on how fast the car is going.
Weather radar also uses the Doppler shift to show the movement of winds in storms, such as a tornado

80. What is music?
Music and noise are caused by vibrationswith some important differences
Noise has random patterns and pitches
Music is made of sounds that are deliberately used in a regular pattern

81. Echolocation
At night, bats swoop around in darkness without bumping into anything
Their senses of sight and smell help them navigate
Many species of bats also depend on echolocation-the process of locating objects by emitting sounds and interpreting the sound waves that are reflected back

83. Sonar
Sonar is a system that uses the reflection of underwater sound waves to detect objects

84. Check for Understanding
Sonar is used to locate an underwater object. At approximately what depth would you expect to find the object if the speed of sound in water is 1,439 m/s and the total time taken for the sonar pulse was 1.6 s? Tricky!!!!
A m
1,200 m
C. 1,800 m
D. 2,400 m

85. Check for Understanding
The answer is B. Remember that the time it takes for the sonar pulse to reach the bottom of the ocean is half the total time

86. Ultrasound in Medicine
One of the important uses of ultrasonic waves is in medicine
Using special instruments, medical professionals can send ultrasonic waves into a specific part of a patient’s body
Reflected ultrasonic waves are used to detect and monitor conditions such as pregnancy, certain types of heart disease, and cancer