Vibrations, Waves and Sound
Unit 7: Vibrations, Waves & Sound
Chapter 19: Waves
 19.1
Waves
 19.2
The Motion of Waves
 19.3
Wave Interference and Energy
19.1 Investigation: Waves in Motion
Key Question:
How do waves move?
Objectives:

Explain how waves move.

Compare and contrast transverse and longitudinal waves.

Use knowledge of longitudinal and transverse waves to
describe water waves.
Waves
A
wave is an oscillation that travels from one
place to another.
 If
you poke a floating ball, it oscillates up and
down.
 The
oscillation spreads outward from where it
started.
Why learn about waves?
 Waves
carry useful information
and energy.
 Waves
—
—
—
—
are all around us:
light from the stoplight
ripples in a puddle of
electricity flowing in wires
radio and television and cell
phone transmissions
Recognizing waves around you
 Waves
—
—
—
—
—
are present:
when you see a vibration that moves.
when something makes or responds
to sound.
when something makes or responds
to light.
when technology allows us to “see
through” objects.
when information travels through the
air (or space) without wires.
Waves
 Waves
are a traveling form
of energy because they can
change motion.
 Waves
also carry
information, such as sound,
pictures, or even numbers.
Wave pulses
A
 It
wave pulse is a short ‘burst’ of a traveling wave.
is sometimes easier to see the motion of wave
pulses than it is to see long waves with many
oscillations.
Transverse waves
A
transverse wave has its oscillations
perpendicular to the direction the wave moves. A
A
wave pulse along a rope attached to a wall moves
left to right, while the boys hand moves up and
down.
Longitudinal waves
 The
oscillations of a longitudinal wave are in the
same direction that the wave moves.
Longitudinal waves

A sharp push-pull on the
end of the spring results in a
traveling wave pulse as
portions of the spring
compress, then relax.

Sound waves are
longitudinal waves.

Like a wave pulse on a
spring, air molecules
oscillate back and forth as
sound travels.
Frequency, amplitude, and wavelength
 You
can think of a wave as a moving series of
high points and low points.

A crest is the high point of the wave.

A trough is the low point.
Frequency
 The
frequency of a wave is the rate at which
every point on the wave moves up and down.
 Frequency
means “how often”.
Amplitude
 The
amplitude of a water wave is the maximum
height the wave rises above the level surface.
Wavelength
 Wavelength
is the distance from any point on a
wave to the same point on the next cycle of the
wave.
 The
distance between one crest and the next
crest is a wavelength.
The speed of waves
 The
speed of a water wave is how fast the wave
spreads, NOT how fast the water surface moves
up and down or how fast the dropped ball moves
in the water.
How do we measure the wave speed?
The speed of waves
A
wave moves one
wavelength in each cycle.
 Since
a cycle takes one
period, the speed of the
wave is the wavelength
divided by the period.
The speed of waves
 The
speed is the distance traveled (one
wavelength) divided by the time it takes (one
period).
 We
usually calculate the speed of a wave by
multiplying wavelength by frequency.
Calculating wave speed
A wave has a wavelength of 0.5 meters, and its frequency is
40 hertz. What is the speed of the wave?
1.
Looking for: …speed of the wave.
2.
Given: …wavelength (0.5 m) and frequency (40 Hz).
3.
Relationships: Use formula: speed = ƒ x 
4.
Solution: …speed = 40 Hz × 0.5 m = 40 1/s × 0.5 m
speed = 20 m/s
Cooking with waves

A microwave heats food by
transferring wave energy to the
food.

The magnetron is a device in a
microwave oven that creates a
wave with electricity.

The wave vibrates inside the
cooking space at 2.5
gigahertz- the frequency best
absorbed by water.
Standing waves on a string
A
wave that is confined between boundaries is
called a standing wave.
 With
all waves, resonance and natural frequency
are dependent on reflections from boundaries of
the system containing the wave.
Standing Waves and Harmonics

The standing wave with the
longest wavelength is called the
fundamental.
 The fundamental has the lowest
frequency in a series of standing
waves called harmonics.
 The first five standing wave
patterns of a vibrating string
shows that patterns occur at
multiples of the fundamental
frequency.
Standing waves
 Standing
waves have nodes
and antinodes.
A
node is a point where the
string stays at its equilibrium
position.
 An
antinode is a point where
the wave is as far as it gets
from equilibrium.
Standing waves

It is easy to measure the
wavelength of a standing
wave on a string.
 Two
harmonics equals one
wave!
Unit 7: Vibrations, Waves & Sound
Chapter 19: Waves
 19.1
Waves
 19.2
The Motion of Waves
 19.3
Wave Interference and Energy
19.2 Investigation: Resonance and Standing
Waves
Key Question:
How do we make and control
waves?
Objectives:
Describe how frequency, wavelength, and speed are related.
 Measure the wavelength and frequency of a vibrating string.
 Recognize and apply the concept of harmonics in resonant
systems.
 Define natural frequency and apply methods for changing the
natural frequency of a system.

Waves propagation
 Waves
propagate, which means they spread out
from where they begin.
 When
you drop a ball into water, some of the water
is pushed aside and raised by the ball.
Wave motion
A
wave front is the leading
edge of a moving wave which
is considered to be the crest
for purposes of modeling.
 The
crests of a plane wave
look like parallel lines.
 The
crests of a circular wave
are circles.
Four wave interactions

When a wave encounters a surface, four
interactions can occur:
1.
reflection,
2.
refraction,
3.
diffraction, or
4.
absorption.
Diffraction
 Diffraction
usually
changes the direction
and shape of the wave.
 When
a plane wave
passes through a small
hole diffraction turns it
into a circular wave.
Unit 7: Vibrations, Waves & Sound
Chapter 19: Waves
 19.1
Waves
 19.2
The Motion of Waves
 19.3
Wave Interference and Energy
19.3 Investigation: Exploring Standing Wave
Properties
Key Question:
How does changing the tension
affect a vibrating string?
Objectives:
Apply an understanding of inertia and restoring force to
describe the effects of increasing the tension of a vibrating
string.
 Explain how a string’s tension and mass affects its frequency
and amplitude.
 Describe how wave properties are applicable to musical
instruments.

Superposition principle
 Interference
happens when two or more waves
mix together.
 When
more than one wave is present, the total
oscillation of any point is the sum of the
oscillations from each individual wave.
Noise canceling headphones

Specialized headphones can
create “anti-noise.”

A microphone in the headphone
samples the noise and generates
anti-noise, or sound that is 180
degrees out of phase with the
noise.

The anti-noise uses superposition
to reduce or muffle noise.
Constructive interference
 Constructive
interference happens when waves
add up to make a larger amplitude.
 Suppose
you make two wave pulses on a
stretched string.
 One
comes from the left and the other comes from
the right.
 When
the waves meet, they combine to make a
single large pulse.
Destructive interference
 What
happens when one pulse is on top of the
string and the other is on the bottom?
 When
the pulses meet in the middle, they cancel
each other out.
 During
destructive interference, waves add up
to make a wave with smaller or zero amplitude.
Resonance and light
A
wave has to be caught in a system with boundaries
to show resonance.
 Catch
light between two perfect mirrors and we can
get resonance of light waves.
 This
is exactly how a laser works!
Resonance and elastic string
 Resonance
elastic strings is created by adding new
pulses so that each adds to the reflected pulse in
constructive interference.
Waves and energy
 The
energy of a wave is
proportional to its
frequency.
 Higher
frequency means
higher energy.
Waves and energy
 The
energy of a wave is
also proportional to its
amplitude.
 Given
two standing waves
of the same frequency,
the wave with the larger
amplitude has more
energy.
Waves that Shake the Ground

On January 12, 2010, a 7.0
magnitude earthquake struck the
Caribbean nation of Haiti.

Its capital, Port-au-Prince, was
nearly destroyed.

Many government buildings,
schools, hospitals, and
businesses collapsed.

The powerful earthquake was
caused by waves traveling
through Earth.