Chapter 17
Mechanical Waves & Sound

How does a disturbance produce
waves?
 Procedure
 Fill a clear plastic container with water.
 Observe the surface of the water by looking down at
an angle to the container. Use the pipet to release a
drop of water from a height of 3 cm above the
surface of the water.
 Repeat Step 2 with a drop released from each of
these heights: 10, 20, 50, 60, 70, 90 cm. Create a
table to record your observations after each drop.
 These observations will be QUALITATIVE (or
descriptive, and not mumeric)
Analysis Questions
 Which drop produced the highest wave?
 Write a general statement (or conclusion) about
how the distance a drop falls affects the wave
produced in the container.
 Using your knowledge of energy, conservation of
energy, and energy transfer, explain why the
distance a drop falls affects the height of the
wave produced. Thoroughly explain your answer.
Mechanical Waves
17.1 Notes

Inquiry Activity
 How does a disturbance produce waves?
 P 499
 Complete activity, answer questions as a group
 Submit one paper per group with all
observations recorded and questions answered!
What are mechanical waves?
 Mechanical Wave
 A disturbance in matter that carries energy
from one place to another
 Require a medium (or matter) in order to
carry energy
 All waves carry energy!
What is a Medium?
 The material through which a wave travels
 Can be a solid, a liquid or a gas
 Space is NOT a medium
 Why?
How are mechanical waves
created?
 A source of energy causes a vibration to
travel through a medium
Types of Mechanical Waves
Transverse
 Medium moves
perpendicularly (or at
right angles to the
direction the wave
travels)
Longitudinal
(Compressional)
 Medium moves
parallel to the
direction the wave
travels
Transverse Waves Demos
 Rope (with ribbon attached)
 Student line (arms over shoulders)
Parts of a Transverse Wave
Crest
 Highest point of the
wave
Trough
 Lowest point of the
wave
Compressional Wave Demos
 Slinkys!
 Hip Bump
Parts of a Compressional Wave
Compression
 Area where the
particles in a medium
are spaced close
together
Rarefaction
 An area where the
particles in a medium
are spread out
Waves transfer ENERGY!
 Waves DO NOT transfer MATTER
 Waves ONLY transfer ENERGY
 Example: THE HUMAN WAVE
Surface Waves
 A wave that travels along a surface that
separates two media (or two types of
matter)
 An object resting on a surface wave will
move up and down, and back and forth
 These two motions result in a circular motion
for the object
Wave Animation
 http://njscuba.net/biology/misc_waves_w
eather.html
Breaking Waves
Exit Exercise
 With your group, make a Venn Diagram
that compares and contrast Transverse
and Compressional Waves.
Properties of Mechanical Waves
17.2

Periodic Motion
 Any motion that repeats at regular time
intervals
 Period
 The time required for one cycle, a complete
motion that returns to its starting point
Wavelength
 Distance between a point on one wave
and the same point on the next cycle of
the wave
 Between adjacent Crests (or troughs), or
compressions (or rarefactions)
One complete wave cycle
One Wavelength

Frequency
 A periodic motion has a frequency
 Frequency
 The number of complete cycles in a given
time
 For waves, this is the number of wave cycles
that pass a point in a given time
 Measured in cycles per second, or Hertz (Hz)
Frequency & Wavelength
 As frequency increases, what happens to
wavelength?
 Use the slinky at your table to determine the
answer to this question, then respond using
Socrative
 HINT: You can make either TRANSVERSE,
or COMPRESSIONAL waves with your slinky
Frequency Formula
Frequency =
1
period
*Remember, period is the amount of time it
takes for a wave to complete one full cycle
Socrative Graph #1
Socrative Graph #2
t in
seconds
Socrative Questions…
 Two calculating frequency/ period
questions
Surfing
 Science of Surf - Episode 1
Wave Speed
 REMEMBER
 v = d/t
 Think of one wavelength as DISTANCE
 Think of period as TIME
 Wave Speed = wavelength / period
 OR Wave Speed = wavelength x frequency
Wave Speed Example
 One end of a rope is vibrated to produce a
wave with a wavelength of 0.25 meters.
The frequency of the wave is 3.0 Hertz.
What is the speed of the wave?
 FORMULA
 Speed = Wavelength x Frequency
Socrative Practice
 Wave Speed Questions
Amplitude
 The maximum displacement of the
medium from its rest position
 The more energy a wave has, the greater
its amplitude
Behavior of Waves
17.3

Reflection
 Occurs when a wave bounces off a surface
that it cannot pass through
 Does not change wave speed or frequency,
but does change wave direction
Refraction
 Bending of a wave as it enters a new
medium
 Occurs because one side of the wave
moves more slowly than the other side
Refraction Example
 Pencil in water demonstration
Diffraction
 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
Diffraction Animation
 Human moving with arms out
 http://www.acoustics.salford.ac.uk/fescho
ols/waves/diffract3.htm
Interference
 Occurs when two or more waves overlap
and combine together
 In interference, waves DO NOT bounce off
one another, but rather move PAST each
other
Types of Interference
Constructive



Occurs when two or more
waves combine to produce a
wave with a larger
displacement
This occurs when two crests
meet, or when two troughs
meet
Wave amplitudes are added
together, producing a larger
wave during the time the waves
overlap
Destructive
 Occurs when 2 or more
waves combine to
produce a wave with a
smaller displacement
 This occurs when a crest
meets a trough
 Produces a waves with
reduced amplitude
Wave Superposition
 When two waves interfere, the resulting
displacement of the medium at any
location is the sum of the displacements
of the individual waves at that same
location.
Animations
 Constructive Interference
 Destructive Interference
Standing Waves
 A wave that appears to stay in one place
and does not appear to move through the
medium
 Only certain points on the wave are
stationary, not the entire wave
 Happens only at certain frequencies
Standing Waves
Nodes
 A point on a standing
wave that has no
displacement from the
rest position
 Complete destructive
interference between
incoming and
reflected waves
Antinodes
 A point where a crest
or a trough appears
midway between 2
nodes
Animation
 Standing Waves
 Rope/Slinky Example
Chapter 1 Midterm Review
 You water three sunflower plants with salt
water. Each plant receives a different
concentration of salt solutions. A fourth plant
receives pure water. After a two week period,
the height is measured.
 Identify the independent variable and the
dependent variable in the experiment above.
Chapter 1 Review #2
 One tank of gold fish is fed the normal amount
of food once a day, a second tank is fed twice
a day, and a third tank four times a day during
a six week study. The fish’s weight is recorded
daily
 Identify the independent variable and the
dependent variable in the experiment above.
17.4 Sound & Hearing

Sound Waves
 Longitudinal (or Compressional) Waves
 Have compressions & rarefactions
 Cause matter to vibrate in a direction that is
parallel to the direction the wave is moving
Sound Speed
 In dry air at 20 degrees Celsius, the speed of sound is
342 m/s
 Can you think of an example when you’ve experienced
a sound delay?
 In general, sound travels fastest in solids and slowest
in gases
 Why?
Sound Intensity
 The amount of energy that is transported past
a given area of the medium per unit of time
 Sound intensity refers to how much energy
the sound waves is transporting
 Can be measured in decibels (dB)
Sound Loudness
 Subjective human response to sound
 Depends on sound intensity
Sound Frequency
 Depends on how fast a sound is vibrating
 Most people hear sounds between 2o Hz
and 20,000 Hz
Infrasound
Ultrasound
 Infra means “below”
 Ultra means “above”
 Sound frequencies
 Sound frequencies
that are below what
humans can hear
that are above what
humans can hear
 Used in sonar and
ultrasound
technologies
Upper-range Frequencies
 http://www.audiocheck.net/audiotests_fr
equencychecklow.php
 http://www.audiocheck.net/audiotests_fr
equencycheckhigh.php
Ultrasound
 Sounds are bounced off parts of the body
and then the reflections are used to create
an image of the body part
Sonar
 Stands for sound navigation and ranging
 Sounds are bounced off an object and
then the time that the sound waves takes
to return to the object is measured
 Uses echoes
Echo
 Echoes occur when sound waves reflect
off of objects they cannot pass through
 Echolocation
Doppler Effect
 Occurs when a source that is producing waves
(like an ambulance’s siren produces sound waves)
is moving with respect to any observers (like a
person on the side walk watching an ambulance
drive by)
 There is an apparent upward shift in wave
frequency for observers when the sound source
is moving towards them, and an apparent
downward shift in frequency for observers when
the sound source is moving away from them
Doppler Effect (Continued)
 The Doppler effect can be observed for
any type of wave - water wave, sound
wave, light wave, etc.
Doppler Effect
 Big Band Theory Clip
How We Hear

Step 1: The outer ear collects sound (acoustic) energy and directs it through
the ear canal to the eardrum

Step 2: The incoming waves of sound energy cause the eardrum to vibrate,

Step 3: The vibration of the eardrum causes three smaller bones (known as
the hammer, anvil and stirrup) to vibrate as well

Step 4: Sound energy is transferred to the middle ear, which amplifies the
sound

Step 5: Sound travels through the inner ear, eventually causing thousands of
tiny sensory hair cells to vibrate

Step 6: The motion of the cells triggers chemical-electrical signals that are
transmitted through to the brain along the auditory nerve pathway. The
brain can then translate the impulses of energy into recognizable sound
patterns.