Characteristics of Sound Waves

Intro and Characeristics of Sound
Presented by:Clark L. Felisilda, BTVTED-ELX II

Subtopic
• Pressure and Intensity of sound waves
• Pitch, Resonance effect in sound systems, and Helmholtz
resonator
• Reflection and diffraction of sound waves

• 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.

• What Is Sound?

• 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!

• 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.

• 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.

Slinky

• 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 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.

• 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.

Visual comparison of longitudinal and transverse waves.

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.

Characteristics of Sound Waves
There are five main characteristics of sound waves: wavelength, amplitude,
frequency, time period, and velocity.
• The time period of a sound wave is the amount of time required to create
a complete wave cycle. 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.

Pressure and Intensity of sound waves
By the end of this section, you will be able to:
• Define intensity, sound intensity, and sound pressure
level.
• Calculate sound intensity levels in decibels (dB).

• Intensity is defined to be the power per unit area carried by a
wave. Power is the rate at which energy is transferred by the
wave. In equation form, intensity I is
where P is the power through an area A. The SI unit for I is W/m2.
The intensity of a sound wave is related to its amplitude squared
by the following relationship:

• Here Δp is the pressure variation or pressure amplitude (half the
difference between the maximum and minimum pressure in the
sound wave) in units of pascals (Pa) or N/m2. The energy (as kinetic
energy mv2/2) of an oscillating element of air due to a traveling
sound wave is proportional to its amplitude squared. In this equation,
ρ is the density of the material in which the sound wave travels, in
units of kg/m3, and Vw is the speed of sound in the medium, in units
of m/s. The pressure variation is proportional to the amplitude of the
oscillation, and so I varies as (Δp)2 (Figure 2). This relationship is
consistent with the fact that the sound wave is produced by some
vibration; the greater its pressure amplitude, the more the air is
compressed in the sound it creates.

Figure 2. Graphs of the gauge pressures in
two sound waves of different intensities. The
more intense sound is produced by a source
that has larger-amplitude oscillations and has
greater pressure maxima and minima.
Because pressures are higher in the greater-
intensity sound, it can exert larger forces on
the objects it encounters.
Amplitude

• Sound intensity levels are quoted in decibels (dB) much
more often than sound intensities in watts per meter
squared (W/m2). How our ears perceive sound can be
more accurately described by the logarithm of the
intensity rather than directly to the intensity. The sound
intensity level β in decibels of a sound having an
intensity I in watts per meter squared is defined to be

• where I0 = 10−12 W/m2 is a reference intensity. In
particular, I0 is the lowest or threshold intensity of sound a
person with normal hearing can perceive at a frequency of
1000 Hz. Sound intensity level is not the same as
intensity. Because β is defined in terms of a ratio, it is a
unitless quantity telling you the level of the sound relative
to a fixed standard (10−12 W/m2, in this case). The units of
decibels (dB) are used to indicate this ratio is multiplied by
10 in its definition. The bel, upon which the decibel is
based, is named for Alexander Graham Bell, the inventor
of the telephone.

Table 1 gives levels
in decibels and
intensities in watts
per meter squared
for some familiar
sounds.
The decibel level of
a sound having the
threshold intensity
of 10−12 W/m2 is
β = 0 dB, because
log101 = 0. That is,
the threshold of
hearing is 0 db.

• It should be noted at this point that there is another decibel scale in
use, called the sound pressure level, based on the ratio of the
pressure amplitude to a reference pressure. This scale is used
particularly in applications where sound travels in water. It is beyond
the scope of most introductory texts to treat this scale because it is not
commonly used for sounds in air, but it is important to note that very
different decibel levels may be encountered when sound pressure
levels are quoted. For example, ocean noise pollution produced by
ships may be as great as 200 dB expressed in the sound pressure
level, where the more familiar sound intensity level we use here would
be something under 140 dB for the same sound.

Pitch, Resonance effect in sound systems, and Helmholtz resonator
• Frequency And Pitch Of Sound
—A frequency's sensation is generally referred to as a
sound's pitch. A high-frequency sound wave corresponds
to a high pitch sound, whereas a low-frequency sound
wave corresponds to a low pitch sound.
—The pitch of sound is determined by the frequency of
vibration of the sound waves that produce them. A high
frequency (e.g., 880 Hz) is seen as a high pitch, while a
low frequency (e.g., 55 Hz) is regarded as a low pitch.

• Frequency And Pitch Of Sound
—Low-frequency sounds include a bass drum, thunder, and
a man's deep voice. High-frequency sounds include a
high-pitched whistle, squeak, and a child's voice. The
loudness or softness of a sound is measured in intensity.

FREQUENCY OF SOUND
• Sound waves are measured in hertz (Hz), which is the number of waves that pass
through a particular place in a second.
• Sounds with a frequency of 20 Hz to 20,000 Hz are generally audible to humans.
• Infrasound refers to sounds having frequencies less than 20 hertz.
• Humans cannot hear infrasound because it is too low-pitched.
• Ultrasound refers to sounds with frequencies greater than 20,000 hertz. Humans
cannot hear ultrasound because it is too high-pitched.
• Other species are capable of hearing noises in the ultrasonic spectrum.
• Dogs, for example, can detect sounds with frequencies of up to 50,000 Hz.
• You may have seen special whistles that dogs can hear but not humans.
• Whistles make noises that are too high in frequency for the human ear to detect.
• Even higher-frequency sounds are audible to other animals. Bats can detect
sounds with frequencies exceeding 100,000 Hz.

PITCH OF SOUND
• Pitch is the quality of sound that distinguishes an acute (or strident) note
from a grave or flat note.
• In music, the phrase 'pitch' is the frequently used.
• It is determined by the sound wave's frequency.
• A higher frequency note has a higher pitch than a lower frequency note.
• This is determined by the frequency of the waves' vibrations.
• The sound is harsh and has a high pitch if the frequency of vibration is
higher.
• On the other hand, if a sound is described to have a lower pitch, it means
that it vibrates at a lower frequency.
• Pitch is influenced by changes in frequency. The shrillness of the sound
increases as the frequency rises.

Resonance
• An object free to vibrate tends to do so at a specific rate
called the object's natural, or resonant frequency. (This
frequency depends on the size, shape, and composition
of the object.) Such an object will vibrate strongly when it
is subjected to vibrations or regular impulses at a
frequency equal to or very close to its natural frequency.
This phenomenon is called resonance. Through
resonance, a comparatively weak vibration in one object
can cause a strong vibration in another.

Resonance has earned a reputation as a destructive force, capable of
shattering nearly anything.

• In music, resonance is used to increase the intensity
(loudness) of a sound. The comparatively weak vibrations
produced at the end of an organ pipe, for example, cause
a column of air in the pipe to vibrate in resonance, thus
greatly increasing the loud-ness of the sound. This
principle also applies to the human voice, in which the
vibrations of the vocal cords are reinforced by resonant
vibrations in the oral and nasal passages.

• Every material (such as glass, steel, concrete) has a
natural frequency at which it vibrates, called a resonant
frequency. If you put energy into the substance at its
resonant frequency, you will force it to vibrate or resonate
(resonance is a forced vibration).
• If you impart enough energy to the glass at its resonant
frequency, you can cause the glass to shatter.
• Some singers can sing a note equal to the resonant
frequency of a wine glass and cause it to shatter

• Resonance has also been shown to cause bridges to
collapse. Marching troops of soldiers will often break
cadence when crossing a bridge to prevent a resonance
collapse. The most famous example of resonance was
the Tacoma Narrows Bridge in Washington State (also
called Galloping Gertie). In 1940, just months after its
completion, winds in the Tacoma Narrows matched the
bridge's resonant frequency and caused the suspension
bridge to sway uncontrollably. Within hours, the bridge
collapsed.

Helmholtz Resonators: Tools for the Analysis of Sound
• Invented by Herman von Helmholtz, these instruments were used to
analyze the composition of musical and speech sounds by means of
'resonances'. Later improved by Rudolph Koenig (1832-1901), they
were an essential tool for the 19th century acoustician.
Set of four Helmholtz resonators by Max Kohl,
c. 1900. (Wh.HC15).

Original use of Helmholtz resonators
• Helmholtz designed and used these resonators
to identify and estimate the relative strengths
of the partials present in sounds. The
resonators were designed to have a very
precise natural frequency and in general, the
larger the resonator the lower the frequency.
Each resonator, like a wine bottle, has a
mouth, a neck and a main cavity, but they also
have a thin 'nipple' opening at the back. By
inserting the resonator's nipple into his ear,
Helmholtz was able to detect when sounds of
the specific frequency of his resonator were
present.
Glass Helmholtz resonator,
mid to late 19th-century.

Other uses
• Helmholtz resonators were used in a multitude
of different apparatus, including Helmholtz's
apparatus for the synthesis of sound and
Koenig's apparatus for the analysis of sound.
In modern times Helmholtz resonances are
used to amplify bass frequencies in hi-fi
speakers and headphones.
Glass Helmholtz resonator,
mid to late 19th-century.

Reflection and diffraction of sound waves
What is reflection and diffraction of sound waves?
• Reflection of sound waves occurs when it strikes the
surface of another medium and bounces back in some
other direction, causing echoes more than 0.1 seconds
after the original sound wave was heard.
• Diffraction is the bending of sounds waves around
obstacles and openings.

Reflection of sound
• Just like the reflection of light, the reflection of sound is similar as it follows
the laws of reflections, where the angle of reflection is equal to the angle of
incidence and the reflected sound, the incident sound, and the normal sound
belong in the same plane. Sound bounces off the surface of the medium
which can be a solid or a liquid. In order to make the reflection of sound to
occur, the surface can be of large size and can be either rough or polished.
Laws of Reflection of Sound
• The angle of reflection is always equal to the angle of incidence .
• The reflected sound, the incident sound, and the normal sound belong in the
same plane.

Applications of reflection of sound
• Echo:
The sound heard after reflections from a rigid surface such as a cliff or a wall is called an echo
creating a persistence of sound even after the source of sound has stopped vibrating. The echo is
used by bats (echolocation) and dolphins to detect obstacles or to navigate. The same principle is
used in SONAR (Sound Navigation And Ranging technique), used in oceanographic studies. SONAR
is used for the detection and location of unseen underwater objects, such as submerged submarines,
sunken ships, and icebergs. In SONAR, ultrasonic waves are sent in all directions from the ship and
the received signal is analysed.

• Hearing Aid:
A hearing aid is a device used by people with difficulty in hearing. Here, the
sound waves are received by the hearing aid and are reflected in a
narrower area leading to the ear.

• Megaphone:
Megaphones are horn-shaped tubes that prevent the spreading out of
sound waves by successive reflections, thus confining them to the air in
the tube.

• Sound Board
Sound boards are curved surfaces that are placed in such a way that the
sound source is at the focus. The sound waves are made to reflect equally
throughout the hall or an auditorium thus enhancing their quality.

Diffraction Of Sound Waves
• Diffraction of sound waves is the phenomenon of bending of this wave
around obstacles. These obstacles can be in the form of walls of a
room, a table, or any other object we see around us. It is due to this
phenomenon that we can hear the sound of a boy standing on the
other side of a wall but we cannot see him.
Obstacle
Diffracted sound waves

In the figure, we see a receiver
standing on the other side of a wall/
screen. Sound from a source which is
placed behind the screen / wall is
creating sound waves. After diffraction
through a small hole, the sound is
reaching the receiver. Thus, he can
hear the sound. Same thing happens
in case of light also but the level of
diffraction in light is very small. Thus,
diffraction is the cause of violation of
law of linear propagation of light or
sound.

Factors affecting the diffraction of sound waves:
• Wavelength:
Wavelength of the incident wave plays an important role in determining the
magnitude of diffraction in sound wave. This can be explained by the
following illustration. For the same aperture (hole) size, the sound wave
having a longer wavelength will be diffracted more (large angle of
diffraction) and the sound wave having a shorter wavelength will have a
less diffraction capacity (less angle of diffraction).

Factors affecting the diffraction of
sound waves:
• Size of aperture:
Now, keeping the wavelength of the
incident sound wave constant and
changing the size of the aperture (hole), it
is observed that the sound wave
diffracting off the aperture of smaller size
diffracts to a much larger extent as
compared to the aperture with a larger
size. It can be concluded that, the smaller
the size of the opening, the greater will be
the effect of diffraction.

Online Reference
• https://www.pasco.com/products/guides/sound-
waves#:~:text=Sound%20is%20produced%20when%20an%20object%20vibrates%2C%20creating%20a
%20pressure,sound%20further%20through%20the%20medium.
• https://courses.lumenlearning.com/atd-austincc-physics1/chapter/17-3-sound-intensity-and-sound-
level/#:~:text=The%20intensity%20of%20a%20sound,)%20or%20N%2Fm2.
• https://www.inspiritvr.com/general-physics/sound/frequency-and-pitch-of-sound-study-guide
• https://acousticstoday.org/8-the-world-through-sound-
resonance/#:~:text=Resonance%20occurs%20when%20a%20system,periodic%20force%20at%20that%2
0frequency.
• https://science.howstuffworks.com/resonance-
info.htm#:~:text=In%20music%2C%20resonance%20is%20used,loud%2Dness%20of%20the%20sound.
• https://www.whipplemuseum.cam.ac.uk/explore-whipple-collections/acoustics/herman-von-
helmholtz/helmholtz-resonators-tools-analysis-sound
• https://artofsmart.com.au/hsctogether/reflection-diffraction-resonance-and-superposition-of-sound-
waves/#:~:text=Reflection%20of%20sound%20waves%20occurs,waves%20around%20obstacles%20and
%20openings.
• https://byjus.com/physics/sound-reflection/
• https://www.chegg.com/learn/physics/introduction-to-physics/diffraction-of-sound-waves

Characteristics of Sound Waves

Recommended

Recommended

More Related Content

Similar to Characteristics of Sound Waves

Similar to Characteristics of Sound Waves (20)

Recently uploaded

Recently uploaded (20)

Characteristics of Sound Waves

Editor's Notes