Mechanical Waves and Sound Waves

Contributed by:
Jonathan James
Mechanical waves, Properties of mechanical waves, Behaviour of waves, Standing waves, Sound and hearing, Doppler's effect
1. Chapter 17:
Mechanical
Waves and Sound
Jennie L. Borders
2. Warm-Up Nov. 4
1. What is a wave?
2. What is the highest
point on a wave called?
3. What is reflection?
3. Section 17.1 –
Mechanical Waves
 A mechanical wave is a disturbance in
matter that carries energy from one
place to another.
 Mechanical waves require matter to
travel through.
 The material through which a wave
travels is called a medium.
 A mechanical wave is created when a
source of energy causes a vibration to
travel through a medium.
4. Types of Mechanical
Waves
 The three main types of mechanical
waves are transverse waves,
longitudinal waves, and surface
waves.
 A point on the wave vibrates up and
down between a maximum and
minimum height.
5. Transverse Waves
 A transverse wave is a wave that
causes the medium to vibrate at
right angles to the direction in which
the wave travels.
6. Transverse Waves
 The highest point of the wave above
the rest position is the crest.
 The lowest point below the rest
position is the trough.
7. Longitudinal Waves
 A longitudinal wave is a wave in which
the vibration of the medium is parallel
to the direction the wave travels.
 An area where the particles in a
medium are spaced close together is
called a compression.
 An area where the particles in a
medium are spread out is called a
rarefaction.
8. Surface Waves
 A surface wave is a wave that travels
along a surface separating two
media.
 The motion of the particles is in a
circle.
9. Section 17.1 Assessment
 List the three main types of
mechanical waves.
 For each type of wave, compare the
vibration of the medium to the
direction of the wave.
10. Warm-Up Nov. 6
1. What are the 3 types of waves?
2. How is a wave made?
3. What is the lowest point of the
wave called?
11. Section 17.2 – Properties of
Mechanical Waves
 Any motion that repeats at regular
time intervals is called periodic
motion.
 The time required for one cycle, a
complete motion that returns to its
starting point, is called the period.
12. Frequency
 Any periodic motion has a
frequency, which is the number of
complete cycles in a given time.
 Frequency is measured in cycles per
second, or hertz (Hz).
 A wave’s frequency equals the
frequency of the vibrating source
producing the waves.
13. Wavelength
 Wavelength is the distance between
a point on one wave and the same
point on the next cycle of the wave.
 Increasing the frequency of a wave
decreases the wavelength.
14. Wave Speed
 Formula for the speed of waves
Speed = Wavelength x Frequency
v=x
Speed = m/s
Wavelength = m
Frequency = 1/s or Hz
15. Sample Problem
 One end of a rope is vibrated to
produce a wave with a wavelength of
0.25m. The frequency of the wave is
3.0Hz. What is the speed of the wave?
v=x
v=? v=x
 = 0.25m v = 0.25m x 3.0(1/s)
 = 3.0Hz v = 0.75m/s
16. Practice Problems
A wave on a rope has a wavelength

of 2.0m and a frequency of 2.0Hz.
What is the speed of the wave?
v=x v = 2.0m x 2.0(1/s) = 4.0m/s
A wave in the water has a frequency

of 4Hz and a wavelength of 0.1m.
What is the speed of the wave?
v=x v = 0.1m x 4(1/s) = 0.4m/s
17. Practice Problems
What is the wavelength of an

earthquake wave if it has a speed of
5km/s and a frequency of 10Hz?
=x  = v/  = 5km/s / 10(1/s) = 0.5k
 A wave travels at 1.8m/s and have a
wavelength of 1.2m. What is the
frequency of this wave?
=  x   = v/  = 1.8m/s / 1.2m = 1.5H
18. Speed
 If you assume that waves are
traveling at a constant speed, then
wavelength is inversely proportional
to frequency.
 If wavelength increases, then
frequency decreases.
 If wavelength decreases, then
frequency increases.
19. Amplitude
 The amplitude of a wave is the
maximum displacement of the
medium from its rest position.
 The more energy a wave has, the
greater is its amplitude.
20. Section 17.2 Assessment
 How is wavelength related to
frequency for waves moving at a
constant speed?
 How is the energy of a wave related
to its amplitude?
 If you double the frequency of a
wave, what is the effect on its
wavelength?
21. Section 17.2 Assessment
 A wave on a rope has a frequency of
3.3Hz and a wavelength of 1.2m.
What is the speed of the wave?
v = x  v = 1.2m x 3.3(1/s) = 4.0m/s
22. Warm-Up Nov.7
1. What is the area of a
longitudinal wave where the
particles are close together?
2. How do the particles of a
surface wave move?
3. What is the lowest point on a
transverse wave called?
23. Section 17.3 – Behavior
of Waves
 Reflection occurs when a wave
bounces off a surface that is cannot
pass through.
 Reflection does not change the
speed or frequency of a wave, but
the wave can be flipped upside
down.
24. Refraction
 Refraction is the bending of a wave
as it enters a new medium at an
angle.
 When a wave enters a medium at an
angle, refraction occurs because one
side of the wave moves more slowly
than the other side.
25. Diffraction
 Diffraction is the bending of a wave
as it moves around an obstacle or
passes through a narrow opening.
 A wave diffracts more if its
wavelength is large compared to the
size of an opening or obstacle.
26. Interference
 Interference occurs when two or
more waves overlap and combine
together.
 Two types of interference are
constructive interference and
destructive interference.
27. Constructive
Interference
 Constructive Interference occurs
when two or more waves combine to
produce a wave with a larger
displacement.
28. Destructive Interference
 Destructive interference occurs
when two or more waves combine to
produce a wave with a smaller
displacement.
29. Standing Waves
 A standing wave is a wave that
appears to stay in one place.
 A node is a point on a standing wave
that has no displacement from the
rest position.
 An antinode is a point where a crest
or trough occurs midway between
two nodes.
30. Section 17.3 Assessment
 How is a wave changed by reflection?
 What causes refraction when a wave
enters a medium at an angle?
 What determines how much a wave
diffracts when it encounters an opening
or an obstacle?
 How does the frequency of a reflected
wave compare with the frequency of
the incoming wave?
 What is the amplitude of a wave that
results when two identical waves
interfere constructively?
31. Warm-Up Nov. 11
1. What is reflection?
2. What is refraction?
3. What is diffraction?
32. Section 17.4 – Sound and
Hearing
 Sound waves are longitudinal waves
that travel through a medium.
 Many behaviors of sound can be
explained using a few properties –
speed, intensity and loudness, and
frequency and pitch.
33. Speed
 In dry air at 20oC, the speed of sound
is 342 m/s.
 In general, sound waves travel
fastest in solids, slower in liquids,
and slowest in gases.
 This is due to the fact that particles
in a solid tend to be closer together
than particles in a liquid or a gas.
34. Intensity and Loudness
 Intensity is the rate at which a wave’s
energy flows through a given area.
 Sound intensity depends on both the
wave’s amplitude and the distance from
the sound source.
 The decibel (dB) is a unit that compares
the intensity of different sounds.
 Loudness is a physical
response to the intensity
of sound.
35. Frequency and Pitch
 Pitch is the frequency of a sound as
you perceive it.
 High-frequency sounds have a high
pitch, and low-frequency sounds
have a low pitch.
36. Ultrasound
 Ultrasound is sound at frequencies
higher than most people hear.
 Ultrasound is used in a variety of
applications, including sonar and
ultrasound imaging.
37. Sonar
 Sonar is a technique for determining
the distance to an object
underwater.
 Sonar stands for sound navigation
and ranging.
38. Doppler Effect
 The Doppler effect is a change in
sound frequency caused by motion of
the sound source, motion of the
listener, or both.
 As a source of sound approaches, an
observer hears a higher frequency.
When the sound source moves away,
the observer hears a lower frequency.
39. Section 17.4 Assessment
 List five properties used to explain
the behavior of sound waves.
 Names two uses for ultrasound.
 What is the Doppler effect?
 If workers in a distant stone quarry
are blasting, why can you feel the
explosion in your feet before you
hear it?