Waves are repeating and periodic disturbance that travels through a medium (e.g.

water) from
one location to another location.
Wave: The repeating and periodic disturbance that travels through a medium (e.g. water) from
one location to another location.

Wave Crest: The highest part of a wave.

Wave Trough: The lowest part of a wave.

Wave Height: The vertical distance between the wave trough and the wave crest.

Wave Length: The distance between two consecutive wave crests or between two consecutive
wave troughs.

Wave Frequency: The number of waves passing a fixed point in a specified period of time.

Wave Period: The time it takes for two successive crests (one wavelength) to pass a specified
point. The wave period is often referenced in seconds, e.g. one wave every 6 seconds.

Fetch: The uninterrupted area or distance over which the wind blows (in the same direction).
The greater the fetch, the greater the wave height.

What are Waves?

A wave transmits information or energy from one point to another in the form of signals, but no
material object makes this journey. The frequency of a wave is obtained by including a factor of
time in the mix. We are completely dependent on waves for all of our wireless communications.
For example, you make a call to your friend in another city with your mobile phone, the entire
communication is happening via audio but the entire process of transmission of a signal from the
talker to the receiver occurs as a waveform. The phone converts your voice into an electrical

1
signal which then propagates either through copper wires or through antennae in wireless
communication.
Wave is a flow or transfer of energy in the form of oscillation through a medium – space or
mass. Sea waves or tides, a sound which we hear, a photon of light travelling and even the
movement of small plants blown by the wind are all examples of different types of waves. A
simple wave illustration is as follows.

Types of Waves in Physics

Different types of waves have a different set of characteristics. Based on the orientation of
particle motion and direction of energy, there are three categories:

 Mechanical waves
 Electromagnetic waves
 Matter waves

Mechanical Wave

 A mechanical wave is a wave that is an oscillation of matter and is responsible for the
transfer of energy through a medium.
 The distance of the wave’s propagation is limited by the medium of transmission. In this
case, the oscillating material moves about a fixed point, and there is very little
translational motion. One intriguing property of mechanical wave is the way they are
measured, which is given by displacement divided by wavelength. When this
dimensionless factor is 1, it results in the generation of harmonic effects; for example,
waves break on the beach when this factor exceeds 1, resulting in turbulence.

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There are two types of mechanical waves:

 Longitudinal waves – In this type of wave, the movement of the particle are parallel to
the motion of the energy i.e. the displacement of the medium is in the same direction to
which the wave is moving. Example – Sound Waves, Pressure Waves.
 Transverse waves – When the movement of the particles is at right angles or
perpendicular to the motion of the energy, then this type of wave is known as Transverse
wave. Light is an example of a transverse wave. Some of the other examples are –
‘Polarized’ waves & Electromagnetic waves.

Water waves are an example of a combination of both longitudinal and transverse motions.

 Surface waves – In this type, the particles travel in a circular motion. These waves
usually occur at interfaces. Waves in the ocean and ripples in a cup of water are examples
of such waves.

Electromagnetic Wave

 Electromagnetic waves are created by a fusion of electric and magnetic fields. The light
you see, the colours around you are visible because of electromagnetic waves.
 One interesting property here is that unlike mechanical waves, electromagnetic waves do
not need a medium to travel. All electromagnetic waves travel through a vacuum at the
same speed, 299,792,458 ms-1.

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Following are the different types of electromagnetic waves:

 Microwaves
 X-ray
 Radio waves
 Ultraviolet waves

Difference Between Mechanical Wave and Non-Mechanical Wave

Mechanical Waves vs Non-Mechanical Waves

Mechanical Wave Non-Mechanical Wave

Mechanical waves are waves that need a Non-mechanical waves are waves that
medium for propagation. do not need a medium for propagation.

Sound waves, water waves and seismic The electromagnetic wave is the only
waves are some examples of mechanical non-mechanical wave.
waves.

Mechanical waves cannot travel through Non-mechanical waves can travel

vacuum through vacuum

Properties of Waves
The prime properties of waves are as follows:
Amplitude – Wave is an energy transport phenomenon. Amplitude is the height of the wave,
usually measured in meters. It is directly related to the amount of energy carried by a wave.
Wavelength – The distance between identical points in the adjacent cycles of crests of a wave is
called a wavelength. It is also measured in meters.
Period – The period of a wave is the time for a particle on a medium to make one complete
vibrational cycle. As the period is time, hence is measured in units of time such as seconds or
minutes.
Frequency – Frequency of a wave is the number of waves passing a point in a certain time. The
unit of frequency is hertz (Hz) which is equal to one wave per second.
The period is the reciprocal of the frequency and vice versa.
Period=1FrequencyPeriod=1Frequency
OR
Frequency=1PeriodFrequency=1Period
Speed – The speed of an object means how fast an object moves and is usually expressed as the
distance travelled per time of travel. The speed of a wave refers to the distance travelled by a
given point on the wave (crest) in a given interval of time. That is –
Speed=DistanceTimeSpeed=DistanceTime
Speed of a wave is thus measured in meter/second i.e. m/s.

4
What Is Sound
In physiology, sound is produced when an object’s vibrations move through a medium until they
enter the human eardrum. In physics, sound is produced in the form of a pressure wave. When an
object vibrates, it causes the surrounding air molecules to vibrate, initiating a chain reaction of
sound wave vibrations throughout the medium. While the physiological definition includes a
subject’s reception of sound, the physics definition recognizes that sound exists independently of
an individual’s reception. You may recognize this section from our blog post, “What is a Sound
Wave in Physics?” Keep reading for a more in-depth look at sound waves.

Types of Sound
There are many different types of sound including, audible, inaudible, unpleasant, pleasant, soft,
loud, noise and music. You’re likely to find the sounds produced by a piano player soft, audible,
and musical. And while the sound of road construction early on Saturday morning is also
audible, it certainly isn’t pleasant or soft. Other sounds, such as a dog whistle, are inaudible to
the human ear. This is because dog whistles produce sound waves that are below the human
hearing range of 20 Hz to 20,000 Hz. Waves below 20 Hz are called infrasonic waves
(infrasound), while higher frequencies above 20,000 Hz are known as ultrasonic waves
(ultrasound).
Infrasonic Waves (Infrasound)
Infrasonic waves have frequencies below 20 Hz, which makes them inaudible to the human ear.
Scientists use infrasound to detect earthquakes and volcanic eruptions, to map rock and
petroleum formations underground, and to study activity in the human heart. Despite our
inability to hear infrasound, many animals use infrasonic waves to communicate in nature.
Whales, hippos, rhinos, giraffes, elephants, and alligators all use infrasound to communicate
across impressive distances – sometimes hundreds of miles!

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Ultrasonic Waves (Ultrasound)
Sound waves that have frequencies higher than 20,000 Hz produce ultrasound. Because
ultrasound occurs at frequencies outside the human hearing range, it is inaudible to the human
ear. Ultrasound is most often used by medical specialists who use sonograms to examine their
patients’ internal organs. Some lesser-known applications of ultrasound include navigation,
imaging, sample mixing, communication, and testing. In nature, bats emit ultrasonic waves to
locate prey and avoid obstacles.

How is Sound Produced?

Sound is produced when an object vibrates, creating a pressure wave. This pressure wave causes
particles in the surrounding medium (air, water, or solid) to have vibrational motion. As the
particles vibrate, they move nearby particles, transmitting the sound further through the medium.
The human ear detects sound waves when vibrating air particles vibrate small parts within the
ear.
In many ways, sound waves are similar to light waves. They both originate from a definite
source and can be distributed or scattered using various means. Unlike light, sound waves can
only travel through a medium, such as air, glass, or metal. This means there’s no sound in space!

How Does Sound Travel?

Mediums
Before we discuss how sound travels, it’s important to understand what a medium is and how it
affects sound. We know that sound can travel through gases, liquids, and solids. But how do
these affect its movement? Sound moves most quickly through solids, because its molecules are
densely packed together. This enables sound waves to rapidly transfer vibrations from one
molecule to another. Sound moves similarly through water, but its velocity is over four times
faster than it is in air. The velocity of sound waves moving through air can be further reduced by
high wind speeds that dissipate the sound wave’s energy.

6
Mediums and the Speed of Sound
The speed of sound is dependent on the type of medium the sound waves travel through. In dry
air at 20°C, the speed of sound is 343 m/s! In room temperature seawater, sound waves travel at
about 1531 m/s! When physicists observe a disturbance that expands faster than the local speed
of sound, it’s called a shockwave. When supersonic aircraft fly overhead, a local shockwave can
be observed! Generally, sound waves travel faster in warmer conditions. As the ocean warms
from global climate, how do you think this will affect the speed of sound waves in the ocean?
Propagation of Sound Waves
When an object vibrates, it creates kinetic energy that is transmitted by molecules in the medium.
As the vibrating sound wave comes in contact with air particles passes its kinetic energy to
nearby molecules. As these energized molecules begin to move, they energize other molecules
that repeat the process. Imagine a slinky moving down a staircase. When falling down a stair, the
slinky’s motion begins by expanding. As the first ring expands forward, it pulls the rings behind
it forward, causing a compression wave. This push and pull chain reaction causes each ring of
the slinky’s coil to be displaced from its original position, gradually transporting the original
energy from the first coil to the last. The compressions and rarefactions of sound waves are
similar to the slinky’s pushing and pulling of its coils.
Compression & Rarefaction
Sound waves are composed of compression and rarefaction patterns. Compression happens when
molecules are densely packed together. Alternatively, rarefaction happens when molecules are
distanced from one another. As sound travels through a medium, its energy causes the molecules
to move, creating an alternating compression and rarefaction pattern. It is important to realize
that molecules do not move with the sound wave. As the wave passes, the molecules become
energized and move from their original positions. After a molecule passes its energy to nearby
molecules, the molecule’s motion diminishes until it is affected by another passing wave. The
wave’s energy transfer is what causes compression and rarefaction. During compression there is
high pressure, and during rarefaction there is low pressure. Different sounds produce different
patterns of high- and low-pressure changes, which allows them to be identified. The wavelength
of a sound wave is made up of one compression and one rarefaction.

Sound waves lose energy as they travel through a medium, which explains why you cannot hear
people talking far away, but you can hear them whispering nearby. As sound waves move
through space, they are reflected by mediums, such as walls, pillars, and rocks. This sound
reflection is better known as an echo. If you’ve ever been inside a cave or canyon, you’ve

7
probably heard your echo carry much farther than usual. This is due to the large rock walls
reflecting your sound off one another.
Types of Waves
So what type of wave is sound? Sound waves fall into three categories: longitudinal waves,
mechanical waves, and pressure waves. Keep reading to find out what qualifies them as such.
Longitudinal Sound Waves
A longitudinal wave is a wave in which the motion of the medium’s particles is parallel to the
direction of the energy transport. Sound waves in air and fluids are longitudinal waves, because
the particles that transport the sound vibrate parallel to the direction of the sound wave’s travel.
If you push a slinky back and forth, the coils move in a parallel fashion (back and forth).
Similarly, when a tuning fork is struck, the direction of the sound wave is parallel to the motion
of the air particles.
Mechanical Sound Waves
A mechanical wave is a wave that depends on the oscillation of matter, meaning that it transfers
energy through a medium to propagate. These waves require an initial energy input that then
travels through the medium until the initial energy is effectively transferred. Examples of
mechanical waves in nature include water waves, sound waves, seismic waves and internal water
waves, which occur due to density differences in a body of water. There are three types of
mechanical waves: transverse waves, longitudinal waves, and surface waves.
Why is sound a mechanical wave? Sound waves move through air by displacing air particles in a
chain reaction. As one particle is displaced from its equilibrium position, it pushes or pulls on
neighboring molecules, causing them to be displaced from their equilibrium. As particles
continue to displace one another with mechanical vibrations, the disturbance is transported
throughout the medium. These particle-to-particle, mechanical vibrations of sound conductance
qualify sound waves as mechanical waves. Sound energy, or energy associated with the
vibrations created by a vibrating source, requires a medium to travel, which makes sound energy
a mechanical wave.

Teaching Tools
Wireless Sound Sensor
The Wireless Sound Sensor features two key sensors in one portable package: a sound wave
sensor for measuring relative changes in sound pressure and a sound level sensor with both dBA-
and dBC-weighted scales. With live data reporting and a wide range of displays (FFT, scope,
digits), the Wireless Sound Sensor’s simple design makes it easy to use for introductory sound
explorations, while its onboard memory and robust software features support higher-level
investigations into the science of sound.

8
Pressure Sound Waves
A pressure wave, or compression wave, has a regular pattern of high- and low-pressure regions.
Because sound waves consist of compressions and rarefactions, their regions fluctuate between
low and high-pressure patterns. For this reason, sound waves are considered to be pressure
waves. For example, as the human ear receives sound waves from the surrounding environment,
it detects rarefactions as low-pressure periods and compressions as high-pressure periods.
Transverse Waves
Transverse waves move with oscillations that are perpendicular to the direction of the wave.
Sound waves are not transverse waves because their oscillations are parallel to the direction of
the energy transport; however sound waves can become transverse waves under very specific
circumstances. Transverse waves, or shear waves, travel at slower speeds than longitudinal
waves, and transverse sound waves can only be created in solids. Ocean waves are the most
common example of transverse waves in nature. A more tangible example can be demonstrated
by wiggling one side of a string up and down, while the other end is anchored (see standing
waves video below). Still a little confused? Check out the visual comparison of transverse and
longitudinal waves below.

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 Visual
comparison of longitudinal and transverse waves.
How to Create Standing Waves
With PASCO’s String Vibrator, Sine Wave Generator, and Strobe System, students can create,
manipulate and measure standing waves in real time. The Sine Wave Generator and String
Vibrator work together to propagate a sine wave through the rope, while the Strobe System can
be used to “freeze” waves in time. Create clearly defined nodes, illuminate standing waves, and
investigate the quantum nature of waves in real-time with this modern investigative approach.
You can check out some of our favorite wave applications in the video below.
4 Properties of Sound
What makes music different from noise? A bird’s call is more melodic than a car alarm. And, we
can usually tell the difference between ambulance and police sirens - but how do we do this? We
use the four properties of sound: pitch, dynamics (loudness or softness), timbre (tone color), and
duration.
Frequency (Pitch)
Pitch is the quality that enables us to judge sounds as being “higher” and “lower. It provides a
method for organizing sounds based on a frequency-based scale. Pitch can be interpreted as the
musical term for frequency, though they are not exactly the same. A high-pitched sound causes
molecules to rapidly oscillate, while a low-pitched sound causes slower oscillation. Pitch can
only be determined when a sound has a frequency that is clear and consistent enough to
differentiate it from noise. Because pitch is primarily based on a listener’s perception, it is not an
objective physical property of sound.
Amplitude (Dynamics)
The amplitude of a sound wave determines it relative loudness. In music, the loudness of a note
is called its dynamic level. In physics, we measure the amplitude of sound waves in decibels
(dB), which do not correspond with dynamic levels. Higher amplitudes correspond with louder

10
sounds, while shorter amplitudes correspond with quieter sounds. Despite this, studies have
shown that humans perceive sounds at very low and very high frequencies to be softer than
sounds in the middle frequencies, even when they have the same amplitude.
Timbre (Tone Color)
Timbre refers to the tone color, or “feel” of the sound. Sounds with various timbres produce
different wave shapes, which affect our interpretation of the sound. The sound produced by a
piano has a different tone color than the sound from a guitar. In physics, we refer to this as the
timbre of a sound. It’s what allows humans to quickly identify sounds (e.g. a cat’s meow,
running water, the sound of a friend’s voice).
Duration (Tempo/Rhythm)
In music, duration is the amount of time that a pitch, or tone, lasts. They can be described as
long, short, or as taking some amount of time. The duration of a note or tone influences the
timbre and rhythm of a sound. A classical piano piece will tend to have notes with a longer
duration than the notes played by a keyboardist at a pop concert. In physics, the duration of a
sound or tone begins once the sound registers and ends after it cannot be detected.
Creating Music with the 4 Properties of Sound
Musicians manipulate the four properties of sound to make repeating patterns that form a song.
Duration is the length of time a musical sound lasts. When you strum a guitar, the duration of the
sound is stopped when you quiet the strings. Pitch is the relative highness or lowness that is
heard in a sound and is determined by the frequency of sound vibrations. Faster vibrations
produce a higher pitch than slower vibrations. The thicker strings of the guitar produce slower
vibrations, creating a deeper pitch, while the thinner strings produce faster vibrations and a
higher pitch. A sound with a definite pitch, or specific frequency, is called a tone. Tones have
specific frequencies that reach the ear at equal time intervals, such as 320 cycles per second.
When two tones have different pitches, they sound dissimilar, and the difference between their
pitches is called an interval. Musicians frequently use an interval called an octave, which allows
two tones of varying pitches to share a similar sound. Dynamics refers to a sound’s degree of
loudness or softness and is related to the amplitude of the vibration that produces the sound. The
harder a guitar string is plucked, the louder the sound will be. Tone color, or timbre, describes
the overall feel of an instrument’s produced sound. If we were to describe a trumpet’s tone color,
we may refer to it as bright or brilliant. When we consider a cello, we may say it has a rich tone
color. Each instrument offers its own tone color, and new tone colors can be created by layering
instruments together. Furthermore, modern music styles like EDM have introduced new tone
styles, which were unavailable prior to digital music creation.
What Makes Sound Music or Noise?
Acousticians, or scientists who study sound acoustics, have studied how different sound types,
primarily noise and music, affect humans. Randomized, unpleasant sound waves are often
referred to as noise. Alternatively, constructed patterns of sound waves are known as music.
Studies have shown that the human body responds differently to noise and music, which may
explain why road construction on a Saturday morning makes us more tense than a pianist’s song.
Acoustics
Acoustics is an interdisciplinary science that studies mechanical waves, including vibration,
sound, infrasound and ultrasound in various environments, such as solids, liquids and gases.
Professionals in acoustics can range from acoustical engineers, who investigate new applications
for sound in technology, to audio engineers, who focus on recording and manipulating sound, to
acousticians, who are scientists concerned with the science of sound.

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Teaching Tools

Resonance Air Column

Whether you’re in need of an all-in-one wave demonstrator or an affordable apparatus that lets
students experiment hands-on with resonance and harmonics, the Resonance Air Column is your
go-to tool. The Resonance Air Column consists of a hollow tube with a piston inside. As the
piston is moved through the Resonance Air Column, a loud tone is emitted each time it
encounters a node. Using meter sticks and the strap-on rings, students can identify, measure and
mark the location of nodes and antinodes throughout the Resonance Air Column – all while
viewing real-time data using Capstone’s FFT display. After exploring the resonant frequency,
nodes and antinodes, students can compare their experimental measurements with the expected
measurements using their own graphs and calculations.
Characteristics of Sound Waves
There are five main characteristics of sound waves: wavelength, amplitude, frequency, time
period, and velocity. The wavelength of a sound wave indicates the distance that wave travels
before it repeats itself. The wavelength itself is a longitudinal wave that shows the compressions
and rarefactions of the sound wave. The amplitude of a wave defines the maximum displacement
of the particles disturbed by the sound wave as it passes through a medium. A large amplitude
indicates a large sound wave. The frequency of a sound wave indicates the number of sound
waves produced each second. Low-frequency sounds produce sound waves less often than high-
frequency sounds. The time period of a sound wave is the amount of time required to create a
complete wave cycle. Each vibration from the sound source produces a wave’s worth of sound.
Each complete wave cycle begins with a trough and ends at the start of the next trough. Lastly,
the velocity of a sound wave tells us how fast the wave is moving and is expressed as meters per
second.

12
Sound wave diagram. A wave cycle occurs between two troughs.
Units of Sound
When we measure sound, there are four different measurement units available to us. The first
unit is called the decibel (dB). The decibel is a logarithmic ratio of the sound pressure compared
to a reference pressure. The next most frequently used unit is the hertz (Hz). The hertz is a
measure of sound frequency. Hertz and decibels are widely used to describe and measure sounds,
but phon and sone are also used. A sone is the perceived loudness of a sound and a phon is the
unit of loudness for pure tones. Additionally, the phon refers to subjective loudness, while the
sone is the perceived loudness.
Sound Wave Graphs Explained
Sound waves can be described by graphing either displacement or density. Displacement-time
graphs represent how far the particles are from their original places and indicates which direction
they’ve moved. Particles that show up on the zero line in a particle displacement graph didn’t
move at all from their normal position. These seemingly motionless particles experience more
compressions and rarefactions than other particles. Since pressure and density are related, a
pressure versus time graph will display the same information as a density versus time graph.
These graphs indicate where the particles are compressed and where they are very expanded.
Unlike displacement graphs, particles along the zero line in a density graph are never squished or
pulled apart. Instead, they are the particles that move back and forth the most.

13
Sound Pressure
Sound pressure describes the local pressure deviation from the ambient atmospheric pressure as a
sound wave travels. It’s important to recognize that sound pressure and air pressure are not the
same concept. Overall, the speed of sound is not influenced by air pressure. As sound waves pass
from the sound source through the air, they alter the pressure experienced by air nearby particles.
Sound Level
Sound level is a comparison of the sound wave’s pressure relative to the reference point. Sound
level is measured in decibels, with higher decibels correlating to higher sound levels. Some
sound instruments measure sound level in dBc, which is the power ratio (decibels) of a signal to
its carrier signal. Other sound instruments measure the relative loudness of sounds as perceived
by the human ear using a-weighted decibels, known as dBa. When dBa is used, sounds at low
frequencies have their decibel values reduced and compared to unweighted decibels.

Sound Level is a comparison of the sound wave’s pressure relative to the reference point. A dBc
meter measures high and low frequencies, while a dBa meter measures mid-level frequencies.

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Sound Intensity
Sound intensity is the power per unit area carried by a sound wave. The more intense the sound
is, the larger the amplitude oscillations will be. As sound intensity increases, the pressure exerted
by the sound waves on nearby objects also increases. Decibels are used to measure the ratio of a
given intensity (I) to the threshold of hearing intensity, which typically has a value of 1000 Hz
for the human ear.

Sound Intensity is the power per unit area carried by a sound wave. The more intense the sound
is, the larger the amplitude oscillations will be. As sound intensity increases, the pressure
exerted by the sound waves on nearby objects also increases.
Sound Intensity in an Air Column
An air column is a large, hollow tube that is open on one side and closed on the other. The
conditions created by an air column are especially useful for investigating sound characteristics
such as intensity and resonance. Check out the video below to see how air columns can be used
to investigate nodes, antinodes and resonance.

Sound waves are longitudinal or compression waves that transmit sound energy from the source
of the sound to an observer. Sound waves are typically drawn as transverse waves, with the
peaks and troughs representing the areas of compression and decompression of the air. Sound
waves can also move through liquids and solids, but this article focuses on sound waves in air.

15
Rights: The University of Waikato Te Whare Wānanga o Waikato
Longitudinal and transverse waves
Diagram illustrating longitudinal and transverse waves. The high points of the transverse waves
(peaks) represent more-dense areas of the longitudinal waves, and the low points (troughs)
represent less-dense areas. The arrows show the directions of wave material movement.
When a sound wave travels out from a source, it travels outwards like a wave produced when a
stone is dropped into water. The sound wave from a single clap is similar to a stone dropped in
water – the wave spreads out over time. The wave pattern formed by a series of steady vibrations
would look like a series of concentric circles centred on the source of the vibration.

Rights: The University of Waikato Te Whare Wānanga o Waikato

Sound waves from single clap and a constant sound source

16
The three diagrams on the left show the result of a single sound, like a clap, spreading out over
time – similar to a stone dropped into water. The two diagrams on the right show wave patterns
from a continuous sound source.

Detecting sound waves

Sound waves are not visible. To detect them, we can use our ears or we can position a
microphone (probe) and observe the sound using an oscilloscope, computer or smartphone app.
In the image below, the microphone is detecting the sound of the note A at two different octaves
– one vibrating at 440 vibrations per second and the other at 220. The number of vibrations per
second is also known as frequency or pitch and is measured in hertz, which has the symbol Hz
(named after Heinrich Hertz).

440 Hz and 220 Hz waves with microphone probes

The note A at two different octaves – one vibrating at 440 vibrations per second and the other at
220.
Notice that the wavelength for the 220 Hz wave is longer than the wavelength for the 440 Hz
wave. As the wavelength increases, the frequency decreases according to the formula:

17
Wave interference
When two or more sound waves occupy the same space, they affect one another. The waves do
not bounce off of each, but they move through each other. The resulting wave depends on how
the waves line up.

Constructive and destructive interference

Two identical sound waves can add constructively or destructively to give different results
(diagrams A and B). Diagram C shows addition of waves with different frequencies. Diagram D
shows addition of waves with nearly the same frequency, which forms beats.
With constructive interference, two waves with the same frequency and amplitude line up – the
peaks line up with peaks and troughs with troughs as in diagram A above. The result is a wave
that has twice the amplitude of the original waves so the sound wave will be twice as loud.
Destructive interference is when similar waves line up peak to trough as in diagram B. The result
is a cancellation of the waves. Noise-cancelling headphones work on this principle. They detect
the sounds coming into the ear and produce sounds with equal volume but with the peaks and
troughs reversed, resulting in near silence.
The result of any combination of sound waves is simply the addition of the various waves. When
we hear the sound of two different musical notes, as shown in diagram C, we hear a complex
waveform we think of as harmony.
Diagram D shows beats – when two sound waves are nearly the same frequency but slightly
different. The resulting wave has points of constructive interference and destructive interference.
A sound wave with the beat pattern in diagram D will have a volume that varies at a regular rate
– you can hear a pulse or flutter in the sound.

18
Sound waves and pitch
Because sound travels outwards from a central source, waves interact in interesting patterns.
When the same pitch or frequency sound wave is produced from two sources, a pattern of
interference is produced.
In the image below, two sources – labelled Sound 1 and 2 – are aligned one above the other. The
waves interfere with each other so that there is constructive interference in some areas (left-hand
picture) and destructive interference in other areas (right-hand picture).

19
Two-source interference in sound waves

Sound wave interference showing zones of constructive interference (left) and destructive
interference (right) with a microphone probe.

As the spacing between the sources is increased, the interference pattern changes and more zones
of destructive interference are created.

standing wave, also called stationary wave, combination of two waves moving in opposite
directions, each having the same amplitude and frequency. The phenomenon is the result of
interference; that is, when waves are superimposed, their energies are either added together or
canceled out.
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change
in frequency of a wave in relation to an observer who is moving relative to the wave source.[3] It
is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.
A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn
approaches and recedes from an observer. Compared to the emitted frequency, the received
frequency is higher during the approach, identical at the instant of passing by, and lower during
the recession.[4]
The reason for the Doppler effect is that when the source of the waves is moving towards the
observer, each successive wave crest is emitted from a position closer to the observer than the

20
crest of the previous wave. Therefore, each wave takes slightly less time to reach the observer
than the previous wave. Hence, the time between the arrivals of successive wave crests at the
observer is reduced, causing an increase in the frequency. While they are traveling, the distance
between successive wave fronts is reduced, so the waves "bunch together". Conversely, if the
source of waves is moving away from the observer, each wave is emitted from a position farther
from the observer than the previous wave, so the arrival time between successive waves is
increased, reducing the frequency. The distance between successive wave fronts is then
increased, so the waves "spread out".
For waves that propagate in a medium, such as sound waves, the velocity of the observer and of
the source are relative to the medium in which the waves are transmitted. The total Doppler
effect may therefore result from motion of the source, motion of the observer, or motion of the
medium. Each of these effects is analyzed separately. For waves which do not require a medium,
such as electromagnetic waves or gravitational waves, only the relative difference in velocity
between the observer and the source needs to be considered. When this relative velocity is not
negligible compared to the speed of light, a more complicated relativistic Doppler effect arises.

Bow wave
Progressive disturbance propagated through a fluid such as water or air as the result of
displacement by the foremost point of an object moving through it at a speed greater than the
speed of a wave moving across the water.
In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating
disturbance that moves faster than the local speed of sound in the medium. ... The sonic
boom associated with the passage of a supersonic aircraft is a type of sound wave produced by
constructive interference.

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