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Waves and Sound
WHAT IS A WAVE?
• Waves are everywhere. In fact, you observe many different kinds of waves every day.
• You see waves that move through matter, such as those that cause water to rise and sink in the
ocean.
• You feel waves such as those that produce heat and hear waves such as those that produce
sound.
• A wave is a regular pattern of motion created by a vibratory disturbance.
• A wave can move through matter. Some waves can move through a vacuum (empty space).
• A wave transfers energy as it moves from one place to another.
• You can “see” energy carried by a wave traveling through matter. The movement of a wave is
actually movement of energy through the matter.
• Most waves travel through a medium. A medium is a substance (gas, liquid or solid) through
which a wave travels.
• When a wave travels through a medium, it causes particles in that substance to oscillate.
• An oscillation is a repeating and regular vibration. It is a “back and forth” motion across a point,
called equilibrium. Equilibrium is also known as the rest position.
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A wave does NOT transfer matter.
The wave causes particles to move up and down or left and right. However, they always return
to equilibrium (or rest) once the wave has passed.
Most waves are periodic waves. Periodic waves are waves that move at a constant speed and
oscillate regularly.
TYPES OF WAVES
• There are two main categories of periodic waves:
1. Electromagnetic Waves
2. Mechanical Waves
ELECTROMAGNETIC WAVES
• Electromagnetic (EM) waves are waves created by oscillations of electric and magnetic fields.
• These waves carry electromagnetic energy.
• Electromagnetic waves do not require a medium to travel through. Therefore, they can travel
through a vacuum.
• Light is a type of electromagnetic wave.
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MECHANICAL WAVES
• Mechanical waves are waves produced by a mechanical movement or vibrating object.
• Mechanical waves require a medium (solid, liquid or gas). They cannot travel through a vacuum.
• Mechanical waves transfer energy through a series of collisions. Here how:
1. An initial particle in the medium is disturbed.
2. The particle collides into other particles in the medium.
3. These particles then collide into other particles.
4. The collisions continue so to transfer energy through the medium.
• Sound is a type of mechanical wave.
• There are three types of mechanical waves:
1. Transverse waves
2. Longitudinal waves
3. Surface waves
TRANSVERSE WAVES
• In a transverse wave, the particles of the medium move perpendicular to the direction of the
movement of the wave.
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Example: Rope
When a person disturbs (or shakes) a rope up and down, a transverse wave moves through the
rope. The wave moves left or right while the “particles” of the wave move up and down.
LONGITUDINAL WAVES
• In a longitudinal wave, the particles of the medium move in the same direction as the movement
of the wave.
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Example: Slinky
– When a slinky is disturbed, a longitudinal wave moves through it. The wave moves to the
left or right, causing areas of the slinky to be compressed and spread out.
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SURFACE WAVES
• A surface wave is a combination of a longitudinal and transverse wave.
• Surface waves are called so because they travel along the surface of a medium, such as land and
water. Only particles on the surface of the medium oscillate.
• In a surface wave, the particles oscillate parallel and perpendicular to the motion of the wave.
The combination of these motions causes particles to move in a circular motion.
WAVE PROPERTIES
• Wave properties describe how a wave moves and what a wave looks like. Some of these
properties are:
– Amplitude
– Wavelength
– Frequency
• To best visualize the properties, we draw graphs of waves
• The horizontal axis (x axis) measure length or time
• The vertical axis (y axis) measures height
• The horizontal axis is equilibrium or the rest position. Points along the x axis represent rest or
equilibrium.
• As a wave moves towards an area in the medium, particles in the medium move upward from
their rest position. The maximum upward displacement is called a crest.
• As the disturbance moves away from that area, the particles move downward, past the rest
position. The maximum downward displacement is called a trough.
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Wave Cycle
• A wave cycle is one complete path of a wave. A complete path is from a crest to crest, trough to
trough or a point on one wave cycle to a corresponding point on the next cycle.
• The graph below shows two wave cycles.
Amplitude
• Amplitude is the height of the wave. It is the maximum displacement of a wave from its rest
position or equilibrium.
• Amplitude is usually measured in meters (m).
• For mechanical waves, amplitude is related to the energy of the wave. Waves with a higher
amplitude carry more energy.
• To determine the amplitude of a wave on a graph, measure the distance from equilibrium to the
top of a crest or trough. Do NOT measure the distance from trough to crest.
Wavelength
• Wavelength is the length of one complete wave.
• Wavelength is represented with the Greek letter lambda (λ).
• Wavelength is usually measured in meters (m).
• To determine wavelength of a wave, measure the distance between two corresponding points
on back-to-back wave cycles. Be sure the x axis is measuring distance.
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Frequency
• The frequency of a wave is the number of times a wave cycles in one second.
• Frequency is measured in Hertz (Hz).
• To determine the frequency of a wave, count the number of wave cycles completed in one
second. Be sure the x axis is measuring time.
WAVE PROPERTIES
• There are other important properties of waves. These properties are:
– Period
– Velocity or Speed
PERIOD
• The period of a wave is the time it takes for a wave to complete one wave cycle.
• Period is measured in seconds.
• To determine the period of a wave, measure the time it takes for the wave to complete one
wave cycle. Be sure the x axis is measuring time.
• Period and frequency are related to each other. In fact, you can determine period if you know
the frequency or the frequency if you know the period.
• Period and frequency are reciprocals of each other:
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Period is represented with a capital T.
Frequency is represented with a lowercase f.
VELOCITY
• The velocity (v) or speed of a wave is the rate at which the wave moves through matter or across
space.
• Velocity is measured in meters per second (m/s).
• You can calculate the velocity of a wave if you know the wave’s frequency and wavelength.
• To calculate velocity, use the equation: velocity (v) = wavelength (λ) × frequency (f)
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GRAPHING WAVES (REVIEW)
• Label wave cycle, crest, trough, equilibrium, amplitude, frequency, wavelength and period on the
graph of the wave below.
TRANSVERSE WAVES
• Recall: A transverse wave is a wave that causes the particles of the medium through which the
wave passes to oscillate perpendicular to the motion of the wave.
• As a transverse wave moves across a medium, particles oscillate in a vertical direction. In other
words, as a transverse wave moves to the right or left, the particles of the medium move up and
down.
• When graphed, the x axis represents equilibrium. The x axis can measure time or distance.
• If x axis measures time, you can determine frequency of the wave.
• If x axis measures distance, you can determine the wavelength of the wave.
• The high points of the wave represent crests.
• The low points of the wave represent troughs.
• The distance from equilibrium to a crest or trough is amplitude.
LONGITUDINAL WAVES
• Longitudinal waves are waves that cause particles in a medium to oscillate parallel to the motion
of the wave. In other words, particles in the medium oscillate left and right as the wave moves
left or right.
• As a longitudinal wave moves across a medium, particles oscillate parallel to the direction of the
wave. This produces areas where particles are compressed or spread out.
• As the wave moves towards an area in the medium, particles collide. Particles are pressed
together in this area and thus, they produce an area of compression. We call this area a
compression or condensation.
• As the wave moves away from this area, the particles stop colliding and spread out. The regions
in a longitudinal wave where particles are most separated are called rarefactions.
• A compression is analogous to a crest of a transverse wave.
• A rarefaction is analogous to a trough of a transverse wave.
• Areas of a longitudinal wave where particles are neither compressed nor spread out are
analogous to points of a transverse wave where the wave crosses equilibrium.
• For a longitudinal wave, the wavelength equals the distance between two consecutive
compressions or rarefactions.
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WAVE BEHAVIOR
• When a wave encounters a barrier, it behaves a certain way.
• There are 4 ways to describe how a wave behaves when it encounters a barrier:
1. Reflection
2. Refraction
3. Diffraction
4. Interference
REFLECTION
• When a wave encounters a barrier, it can bounce off the surface of that barrier. This is called
reflection.
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Echo
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Waves reflect best off flat, hard and/or shiny surfaces.
For example, sound waves best reflect off brick walls or tile floors. Light waves best reflect off
calm water or mirrors.
The wave reflects off the surface of the barrier at the same angle it hits the surface.
The angle that the wave hits the surface is called the incident angle. The angle that the wave
reflects off the surface is called the reflective angle.
The reflection of sound produces an echo.
When sound bounces off a flat and hard surface, it can reflect back to the source of the sound.
The reflected noise, called an echo, sounds exactly like the sound produced originally.
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REFRACTION
• When a wave passes across a barrier into a new medium, such as from air into water, the wave
bends. The bending of a wave is called refraction.
• A wave refracts because its speed changes in the new medium. When speed of the wave
changes, the direction of the wave changes and so it bends.
• The picture below shows a light wave refracting as it passes through a piece of glass. The speed
of the light wave is slower in the glass than in air.
DIFFRACTION
• Waves can also bend to get around small barriers or to “squeeze through” small openings within
a barrier. This is called diffraction.
• Waves can diffract if the obstacle is small or if the opening in the barrier is just wide enough to
accommodate the wavelength of the wave.
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Diffraction occurs with water waves.
Breakwaters are structures that protect coasts from erosion and create safe harbors. When
water waves hit breaches in a breakwater, they must diffract to get through.
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INTERFERENCE (AND PHASE)
• When waves come into contact with each other, they interact. This is called interference.
• When waves interfere with each other, they combine to produce one wave called a resultant
wave. The amplitude of the resultant wave depends on the amplitude of the interfering waves
and the phase of those waves.
• The phase of a wave describes the position of a wave. The phase corresponds to the fraction of a
wave cycle a wave has completed.
• The phase of a wave is important to consider when studying how waves interact with each other.
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Waves that have the same position at the same time are said to be in phase.
Waves that have different positions at the same time are said to be out of phase.
Waves that appear to be “opposite” each other are 180 degrees out of phase. These waves are
said to be completely out of phase.
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Interference causes waves to overlay or superimpose each other. Sometimes the waves
superimpose and reinforce each other. Sometimes the waves superimpose and cancel each
other out.
There are two types of interference:
– Constructive interference
– Destructive interference
Phase determines what type of interference occurs.
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Types of Interference
• Constructive interference occurs with waves that are in phase with each other.
• When two in phase waves interfere, the resultant wave has an amplitude higher than the
individual waves.
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Destructive interference occurs with waves that are NOT in phase with each other.
When two out of phase waves interfere, the resultant wave has an amplitude lower than the
individual waves.
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If two waves with the same amplitude are completely out of phase, the waves “cancel each
other out.” In other words, the two waves destroy each other.
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Noise-canceling headphones are headphones that “silence” ambient noise. They operate by
using destructive interference. Here’s how they work:
– The headphones produce sound waves that mimic ambient noise. The sound waves are
completely out of phase with ambient noise. This “cancels out” the ambient noise and
makes it easier to listen to music.
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SOUND
• Sound waves are the most studied mechanical wave. They are produced by mechanical
movement.
• Mechanical movement can cause an object to vibrate. A vibrating object causes particles around
it, including particles in the air, to vibrate. Vibrating air particles create sound waves that can be
heard by humans.
• For example, plucking the strings of a guitar causes the strings to vibrate. This vibration spreads
to particles in air, which can be heard by people.
PITCH
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The frequency of a sound wave determines the pitch of the noise.
Pitch describes the “highness” or “lowness” of sound.
In music, pitch corresponds to the vertical position of a note on a musical scale.
Pitch, like frequency, is measured in Hertz.
When studying sound, the terms frequency and pitch can be used interchangeably.
Sound waves with a low frequency oscillate slowly. These waves produce sound with a low pitch.
Sound waves with a high frequency oscillate fast. These waves produce sound with a high pitch.
A healthy, young person can hear sound with a pitch (or frequency) of 20 to 20,000 Hertz.
People lose the ability to hear sound with higher pitch as they get older. The highest pitch a
middle age adult can hear is 12,000 to 14,000 Hertz.
ULTRASOUND
• Sound with a frequency higher than 20,000 Hz is called ultrasound.
• Although humans cannot hear ultrasound, many animals, including bats, toothed whales and
dolphins, can hear it.
• Bats use ultrasound to hunt and navigate. This is important to bats because they are nocturnal
and cannot rely on vision in the darkness of night.
• Whales and dolphins use ultrasound to hunt and navigate as well. This is important to these
marine animals because the ocean is large and it’s hard to see in the water.
• The use of ultrasound to navigate or capture food is called echolocation. Here’s how
echolocation works with bats:
1. A bat emits ultrasound waves.
2. The ultrasound reflects off prey and “echoes” back to the bat.
3. The time it takes for the waves to get back to the bat and the orientation of the waves
helps the bat locate the prey.
• Humans use ultrasound to produce images of structures inside the body. This is called
sonography. Here’s how it works:
1. A probe is pressed against the skin. This probe emits ultrasound waves into the body.
2. The ultrasound reflects off different types of tissue in the body.
3. The reflected waves are picked up by the probe. The probe relays information on how
the waves reflect to a machine, which produces an image of the structures in the body.
• Sonography is the safest way to produce images of structures inside the body. It does not rely on
x-rays or other forms of high-energy radiation.
• For this reason, sonography is the technique most often used to produce images of a fetus inside
a woman’s uterus during pregnancy.
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VOLUME
• Volume is a measure of loudness.
• Volume is determined by the amplitude of sound waves.
• Volume is measured in decibels.
• A soft sound, like a whisper, produces 15-20 decibels.
• A loud sound, like a jet engine, produces 150 decibels.
• The amplitude of a sound wave is directly related to volume.
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As amplitude increases, volume increases. The higher the volume, the louder the sound.
Loud sound can damage a person’s ability to hear.
Prolonged or repeated exposure to sound with a volume of 85 Db or greater can cause hearing
loss.
Very loud sound can cause pain. The threshold of pain is approximately 130 decibels.
The table below gives examples of sounds and the loudness of those sounds.
Listening to music with or without headphones at a high volume is a major cause of premature
hearing loss in teenagers and young adults.
Listening to an mp3 player at maximum volume has a loudness of 105 decibels. For this reason,
it’s important to listen to music at a low level and not for extended periods of time.
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WAVE VELOCITY
• Different types of waves travel at different speeds.
• For example, the speed of light (an electromagnetic wave) and sound (a mechanical wave) are
significantly different.
• In air, light travels 3.00 x 108 m/s. Sound travels 343 m/s. Light travels nearly 900,000 times
faster than sound!
• You can observe the difference in speed of light and sound during a thunderstorm.
– When lightning is discharged from a cloud, it heats air around it very rapidly. The sudden
increase in temperature produces a rapid expansion of air, which creates the sound of
thunder.
– Although lightning and thunder occur at the same time, thunder seems to come after
lightning. This is because light travels faster than sound. Therefore, you see lightning
before you hear thunder.
• The difference in time between the sight of lightning and the sound of thunder helps you predict
how far away the lightning strike took place.
• Count the number of seconds between the sight of lightning and sound of thunder. Divide by 5.
This calculation will tell you how many miles away the lightning strike took place.
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The velocity of a wave changes as it travels from one medium into another type of medium.
How the velocity of a mechanical wave changes is different compared to how the velocity of an
electromagnetic wave changes.
Velocity of Electromagnetic Waves
• Electromagnetic waves move fastest in a vacuum.
• The particles of matter hinder the propagation of EM waves. EM waves move fastest through
matter that has no or very few particles. In other words, as the density of the medium increases,
the speed of an EM wave decreases.
• On Earth, EM waves travel fastest through gases because particles are most spread out.
Velocity of Mechanical Waves
• Mechanical waves move fastest in solids and slowest in gases.
• Mechanical waves rely on the collision of particles in the medium to propagate. When particles
are packed together closely, the wave can move faster. In other words, as the density of the
medium increases, the speed of the wave increases.
• Mechanical waves cannot move through a vacuum because there are no particles in a vacuum.
• Sound is a mechanical wave. Therefore, its velocity increase as the density of the medium
increases.
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SPEED OF SOUND
• The speed of sound in dry air is 343 meters per second.
• The speed of sound increases as the density of the medium increases:
– Sound travels 4 times faster in water (1,482 m/s).
– Sound travels 13 times faster through steel (4,512 m/s).
SOUND BARRIER
• Some planes (like fighter jets) can travel faster than the speed of sound. Traveling faster than the
speed of sound is called breaking the sound barrier.
• A plane that travels at the speed of sound is said to travel Mach 1.
• A plane that travels twice the speed of sound is said to travel Mach 2 and so on.
• The fastest recorded speed of a plane piloted by a human was 980 meters per second (2,193
mph). That’s traveling almost 3 times the speed of sound!
SHOCK WAVE
• As a plane approaches the speed of sound, sound waves bunch up in front of the plane.
• The sound waves bunch up because the plane “catches up” to the sound waves it produces.
• When a plane travels at the speed of sound (Mach 1), sound waves pile on top of each other in
front of the plane. The sound waves cannot travel ahead of the plane because they move at the
same speed as the plane.
• This piling of waves creates a wave with a large amplitude. This wave is called a shock wave.
• When the plane travels faster than the speed of sound, it “breaks through” the shock wave.
When this happens, you can see and hear the plane break the sound barrier.
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Breaking the sound barrier results in 2 phenomena: sonic boom and vapor cone
SONIC BOOM
• As a plane breaks the sound barrier, the shock wave spreads backward and outward from the
plane. The spread of the shock wave creates a high pressure region followed by a low pressure
region. The change in pressure produces a noise that sounds like an explosion. We call this noise
a sonic boom.
VAPOR CONE
• A vapor cone is a visible cloud of condensed water that forms around the plane as it breaks the
sound barrier. As the plane breaks through the shock wave, it suddenly enters a region of low
pressure and cooler temperature. This causes water on the plane to condense. The plane sheds
the water droplets and forms the vapor cone.
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DOPPLER EFFECT
• The frequency of sound can change for observers if the object producing the sound is moving.
This phenomenon is called the Doppler Effect.
• When an object moves, the sound waves produced by that object are distorted.
– Sound waves bunch up in front of the object. Successive waves are emitted closer to the
previous wave. When the waves bunch up, frequency increases.
– Sound waves “spread out” behind the object. Each successive wave is emitted further
away from the previous wave. When waves spread out, frequency decreases.
• Let’s say you are standing on the side of a road. A car drives past you. The frequency of the
sound you hear changes.
– As the car moves towards you, the sound waves get compressed and the frequency of
the sound increases.
– As the car moves away, the sound waves stretch out and the frequency of the sound
decreases.
– The frequency of the sound produced by the car does NOT change. For the driver and
passengers in the car, the sound of the car has a constant frequency. This is because the
sound produced by the car is NOT moving towards or away from them – they are moving
with the car.
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ACOUSTICS
• Acoustics is the science of sound. Scientists, engineers and musicians study acoustics so to better
understand how sound travels and behaves.
• We want to better understand sound because:
1. Sound and hearing is important to animal survival.
2. Speech, which produces sound, is a vital part of human culture. We rely on our ability to
produce sound to communicate with each other.
3. Applications of acoustics is important to many aspects of modern society, including noise
control and audio industries.
• Acoustics helps engineers design structures such as libraries, lecture halls, auditoriums and
movie theaters.
• Understanding how sound travels helps everyone in an auditorium hear music played by a band
on the stage or helps muffle sound in a library.
• Engineers control how sound travels and behaves in a room by manipulating the materials used
to construct the room.
• Manipulating the materials helps control reverberation and absorption.
Controlling Reverberation
• Reverberation is the persistence of a sound after the sound is produced. It is caused by reflected
sound waves that bounce off surfaces in a room.
• If you construct a room with materials that reverberate sound, you will produce a loud room. For
example, a tile floor reverberates more sound than a carpeted floor.
Controlling Absorption
• Absorption is dampening of a sound. Absorption occurs because sound waves are absorbed
rather than reflected off surfaces in a room.
• If you construct a room with materials that absorb sound, you will produce a quiet room. For
example, carpet and curtains absorb sound and help keep a room quiet.
Acoustics and Animal Behavior
• Studying acoustics helps scientists better understand animal survival and behavior.
– Many animals rely on the ability to hear sound. This is important to survival because
animals that hear are able to detect sound made by predators or other threats.
– Many animals, including humans, rely on the ability to produce sound to communicate
with each other. Birds and frogs use sound in mating rituals and to mark territories.
Humans rely on the ability to speak in order to communicate with each other.
Acoustics and Mechanical Waves
• Acoustics also deals with the science of other mechanical waves, such as ultrasound and seismic
waves.
– Studying ultrasound helps us better understand how some animals navigate and hunt. In
fact, the study of ultrasound has led to the development of sonar. Sonar is a navigational
technique used by ships and submarines to explore and navigate the ocean.
– Seismic waves are waves produces by earthquakes and volcanoes. Studying seismic
waves helps scientists better predict when and where these events occur. Studying
seismic waves also helps locate fossil fuels such as oil and natural gas.
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