Chapter 17
Mechanical Waves & Sound
Waves
A repeating disturbance or
movement that transfers energy
through matter or space.
A wave will travel as long as it
has energy.
Mechanical Waves
 Mechanical Wave: is a disturbance in matter
that carries energy from one place to another.
 Mechanical waves require matter to travel
 Medium: is a material (matter) that
mechanical waves travel through solid liquid
or gas.

Ex. Air, water, aluminum, copper,
 The speed of mechanical waves changes
with different mediums
 Mechanical wave is created when a source of
energy causes a vibration to travel through a
medium
 Vibration: a repeating motion that follows a
pattern
 Sound is created by vibrations

Vocal cords
 There are 3 types of mechanical waves



Transverse waves
Longitudinal waves/ Compressional
Surface waves
Longitudinal
Transverse
 Transverse waves: is a wave that causes the
medium to vibrate at right angles
(perpendicular) to the direction in which the
wave travels
 Up & Down, or Side to Side motion
 Draw a picture of a transverse wave
 Ex.




Water
Rope
Slinky
Electromagnetic (radio waves, infrared, etc)
B. Transverse Waves
 Transverse Waves

medium moves
perpendicular to the
direction of wave
motion
 Crest: is the highest point above the resting
position (top of the wave)
 Trough: is the lowest point below the resting
position (bottom of the wave)
 Resting position: is the flat position of a wave
before it starts moving

Slinky demo
B. Transverse Waves
 Wave Anatomy
corresponds to
the amount of
energy carried
by the wave
crests
wavelength
amplitude
amplitude
nodes
wavelength
troughs
 Longitudinal waves: is a wave in which the
vibration of the medium travels parallel to the
direction of the wave\

Slinky demo
 Compression: a part of a longitudinal wave
where the particles are pushed closely
together
 Rarefaction: a part of a longitudinal wave
where the particles are spaced farther apart
C. Longitudinal Waves
 Longitudinal Waves (a.k.a. compressional)

medium moves in the same direction as wave
motion
C. Longitudinal Waves
 Wave Anatomy
compression
rarefaction
wavelength
wavelength
Amount of compression corresponds to amount of energy 
AMPLITUDE.
 Draw a longitudinal wave
 Ex. of longitudinal waves

Sound
 Surface wave: is a wave that has
characteristics of both transverse and
longitudinal waves
 Up & down movement like a transverse
 Parallel movement of energy like longitudinal
 Ex.


Ocean Waves
Earthquakes (waves through Earth’s surface)
Properties of Waves
 Periodic Motion: is motion that follows a
repeating pattern
 Period: the time period for one interval of
movement
 Frequency: is the number of complete cycles
that pass a point in a given amount of time

Frequency of waves are measured in hertz
(Hz)
 http://www.kettering.edu/physics/drussell/De
mos/waves/wavemotion.html
 Wavelength: is the distance of a complete
cycle (either crest to crest or trough to trough)
 Long wavelength = low frequency
 Short wavelength = high frequency
D. Measuring Waves
 Frequency ( f )

# of waves passing a
point in 1 second

Hertz (Hz)
shorter
wavelength =
higher frequency =
higher energy
1 second
D. Measuring Waves
 Velocity ( v )


speed of a wave as it moves forward
depends on wave type and medium
v=
wave
λ
×f
V:velocity (m/s)
λ:wavelength (m)
ƒ: frequency (Hz)
D. Measuring Waves
 EX: Find the velocity of a wave in a wave pool if its
wavelength is 3.2 m and its frequency is 0.60 Hz.
GIVEN:
v=?
λ= 3.2 m
f = 0.60 Hz v
λ f
WORK:
v=λ×f
v = (3.2 m)(0.60 Hz)
v = 1.92 m/s
D. Measuring Waves
 EX: An earthquake produces a wave that has a
wavelength of 417 m and travels at 5000 m/s. What is
its frequency?
GIVEN:
λ = 417 m
v = 5000 m/s
f=?
v
λ f
WORK:
f=v÷λ
f = (5000 m/s) ÷ (417 m)
f = 12 Hz
 Wave Speed = wavelength X frequency
 Wave speed changes in different mediums
 If waves are traveling the same speed, then
wavelength and frequency are INDIRECTLY
related
 Amplitude: is the distance from the resting
position to either a crest or trough
 Energy and amplitude are DIRECTLY related
 High energy = high amplitude
 Low energy = low amplitude
 Amplitude in sound is called volume
 Light waves travel faster than sound
 Sound waves travel faster in liquids and
solids than gas
 Light waves travel faster in gases and
vacuums than in liquids an solids.
Behavior of Waves
 Reflection: is when a wave bounces off a
surface it can not pass through
 Reflection does not change the speed or
frequency (the wave can be flipped upside
down or side to side) Ex. Mirror
 Law of Reflection: the angle of incidence
(incoming wave) = the angle of reflection
(outgoing wave)
 All waves can be reflected
 The reflection of sound is called an echo
reflection
Reflection terms
normal
 Refraction: is the bending of a
wave as it enters a new medium
 Ex. light waves

Ruler in a beaker of water
 Ex. sound waves

Listening to sound underwater
 Diffraction: is the bending of a wave as is
moves around an obstacle or passes through
a narrow opening
 Page 510
 Eddy: is an area behind a mid-stream boulder
where the water flows in a reverse direction
(provides safety for rafters)
 Chute: is an area of a river where the water is
constricted to a narrow passage
defraction
 Constructive Interference: is when 2 or more
waves combine to form a wave with a larger
displacement (amplitude)
 Destructive interference: is when 2 or more
waves combine to form a wave with a smaller
displacement (amplitude) add together
 Standing wave: is wave or waves that appear
to stay in the same place
 Plucking a guitar string
 Waves in a river
 Node: is the point on a standing wave where
there is no displacement (amplitude)
 Antinodes: are the crests and the troughs on
a standing wave
17.4 Sound Waves
 Sound waves are longitudinal waves
 The speed of sound changes due to different
types of mediums
 Chart 514
 Speed: is the distance traveled in a certain
amount of time
 Meters/second: m/s
 Intensity: depends on the amplitude (volume)
and the distance from the source
 Decibels: (dB) is the unit for sound intensity
 Chart on 515
 Damage to ears around 120 dB
 Frequency: is the number of wave cycles to
pass a given point in one second
 Measured in hertz (Hz)
 Pitch: is the perceived frequency of sound
 Different notes in music
 All the different notes have a unique
frequency
 Ultrasound: use sound to locate objects or
create pictures
 SONAR, fish finders, radar
 Animals use “echo-location”
 Bats, dolphins, whales
 Pregnant ladies get ultra sounds to check the
baby’s health
 Doppler Effect: pitch changes due to the
object creating the sound moving closer or
farther away
 Pic on 516
Human Ear
 Picture on 517
 Outer ear: the collect and funnel the sound
waves into the middle ear
 Middle ear: amplifies the the vibrations
 Inner ear: are where nerve endings receive
and send the signal to the brain
 The brain interprets those signals as sound
 Resonance: waves of the same frequency
combine (constructive interference)
 amplifies the sound
 Resonance can also cause to vibrate
 Every object has a natural frequency, if a
sound wave with the same frequency hits it, it
will cause the object to vibrate
Seismic waves
 Seismic waves are the waves of energy
caused by the sudden breaking of rock
within the earth or an explosion. They are
the energy that travels through the earth
and is recorded on seismographs.
 http://science.howstuffworks.com/nature/natur
al-disasters/earthquake4.htm