Class 9 Science Notes on Chapter 10 Sound Waves: Characteristics and Applications

Sound is a form of energy produced by vibrating objects and plays an important role in communication and interaction with the environment. Class 9 Science Notes on Chapter 10 Sound Waves: Characteristics and Applications cover the concepts related to the production, propagation, and behaviour of sound. Also, students will learn about sound waves, compressions and rarefactions, wavelength, frequency, amplitude, intensity, speed of sound, and human perception of sound. 

The chapter also explains reflection of sound, echo, reverberation, ultrasonic and infrasonic waves, echolocation, and SONAR, along with their practical and reali-life applications. These notes provide a clear and concise explanation of all key concepts for effective learning and revision.

Important Topics Covered in Class 9 Science Notes on Chapter 10 Sound Waves: Characteristics and Applications

Production of Sound	Human Perception of Sound
Sound Production in Humans and Animals	Pitch
Propagation of Sound	Loudness
Sound Waves	Noise and Noise Pollution
Longitudinal Waves	Speed of Sound
Mechanical Waves	Intensity of Sound
Energy of Sound Waves	Reflection of Sound
Graphical Representation of Sound Waves	Echo
Crest and Trough	Reverberation
Characteristics of Sound Waves	Ultrasonic Waves
Wavelength	Infrasonic Waves
Frequency	Applications of Ultrasonic Waves
Time Period	Echolocation
Amplitude	SONAR (Sound Navigation and Ranging)

Complete Class 9 Science Notes on Chapter 10 Sound Waves: Characteristics and Applications

Production of Sound

Sound is one form of energy that we experience every day. We hear different sounds which are produced by people talking, birds chirping, vehicles moving, mobile phones ringing, and many other sources around us. 

What Produces Sound

Sound is produced due to the vibration of an object. A vibrating object moves repeatedly to and fro about its mean position. This repeated motion is called vibration or oscillation.

When a stretched rubber band is plucked, it starts moving rapidly back and forth. During this vibration, sound is produced. As soon as the vibration stops, the sound also disappears. This shows that vibration is necessary for the production of sound.

How Do Humans Produce Sound

Humans produce sound using vocal cords, which are located inside the larynx (voice box) in the throat.

When air from the lungs passes through the vocal cords, they vibrate and produce sound. The sound is then modified by the tongue, lips, mouth cavity, and nasal cavity to form speech and music.

Important parts involved in speech production

Part	Function
Vocal Cords	Produce sound through vibrations
Tongue	Helps form different words
Lips	Shape speech sounds
Mouth Cavity	Modifies sound
Nasal Cavity	Contributes to voice quality

If you gently place your fingers on your throat while speaking, you can feel vibrations. These vibrations are produced by the vocal cords.

How Do Animals Produce Sound

Many animals produce sound using organs similar to vocal cords. Birds, dogs, cats, and cows are common examples.

Examples of Sound Production in Animals

Animal	Method of Sound Production
Human	Vibrating vocal cords
Bird	Vibrating vocal organs
Cricket	Rubbing wings together
Grasshopper	Rubbing legs and wings

Tuning Fork

A tuning fork is an instrument commonly used in sound experiments. It consists of a U-shaped metal bar attached to a stem and is usually made of steel or aluminium.

The two arms of the U-shaped part are called prongs or tines. When a prong is struck against a soft surface, it begins to vibrate. These vibrations produce sound.

In Kongthong, a village in Meghalaya, every person has a unique musical tune instead of a spoken name. This tradition is known as Jingrwai Iawbei.

Propagation of Sound

The movement of sound from its source to a listener is called propagation of sound. Sound reaches our ears after travelling through a material substance.

Sound can travel through different media such as solids, liquids and gases.

1. Sound Travels Through Solids: When a person places an ear against a desk while someone taps it from the other side, the sound is heard clearly.

Sound can travel through solid materials. Solids allow vibrations to pass from one particle to another.

2. Sound Travels Through Liquids: When two metal spoons are struck together under water, the sound can still be heard.

Sound can travel through water and other liquids. Liquids act as a medium for sound propagation.

3. Sound Travels Through Gases: Air is the most common medium through which sound travels.

Examples include human speech, musical instruments, vehicle horns and thunder. In all these cases, sound reaches us through air.

Vacuum

A vacuum is a space where there is no matter. Since there are no particles in a vacuum, sound cannot travel through it.

Sound in Outer Space: Outer space is nearly a vacuum because sound cannot travel without a medium. 

Astronauts cannot hear sounds directly in space. They use special communication devices in their spacesuits to talk to each other.

Sound Waves

Sound is produced by vibrating objects, but it can only reach our ears if it travels through a medium. Air is the most common medium for sound, although sound can also travel through liquids and solids.

A sound wave carries energy from one place to another without transporting the particles of the medium along with it.

Propagation of Sound

When a source produces sound, the disturbance spreads through the surrounding medium. Sound does not travel as individual particles moving from the source to the listener. Instead, the particles of the medium vibrate about their mean positions and pass the disturbance to neighbouring particles.

Sound generally spreads in many directions from its source. The direction and pattern of propagation may depend on the shape of the source.

Sound Waves in Air

To understand sound propagation in air, imagine a long tube filled with air and a piston placed at one end.

Formation of Compression

When the piston moves forward, it pushes nearby air particles; air particles come closer together, and the density and pressure of the air increase in that region.

This high-density region is called a compression.

Compression: Compression is a region of a sound wave where the particles of the medium are closer together than normal.

Characteristics of Compression

• High density
• High pressure
• Particles are crowded together

Formation of Rarefaction

When the piston moves backward, air particles move away from one another, and the density and pressure decrease in that region. This low-density region is called a rarefaction.

Rarefaction: Rarefaction is a region of a sound wave where the particles of the medium are farther apart than normal.

Characteristics of Rarefaction

• Low density
• Low pressure
• Particles are spread apart

Formation of a Sound Wave

As the piston continues to move forward and backward repeatedly:

• Compressions and rarefactions are produced alternately.
• These regions move through the medium.
• The air particles only oscillate about their mean positions.
• Energy is transferred from one particle to another.

A series of alternating compressions and rarefactions forms a sound wave.

A sound wave is a disturbance consisting of alternating compressions and rarefactions that travels through a medium without causing the actual flow of the medium's particles.

Direction of Propagation

The direction in which a wave travels through a medium is called the direction of propagation.

In a sound wave:

• The disturbance moves forward through the medium.
• The particles of the medium vibrate around their mean positions.

Longitudinal Waves

A longitudinal wave is a wave in which the particles of the medium oscillate parallel to the direction of wave propagation.

Characteristics of Longitudinal Waves

• Consists of compressions and rarefactions.
• Particle vibration is parallel to wave propagation.
• Require a material medium.
• Sound waves are longitudinal waves.

Sound as a Mechanical Wave

Sound cannot travel through empty space because there are no particles to transfer the disturbance. A material medium is necessary for the propagation of sound.

Mechanical Waves: Waves that require a material medium for propagation are called mechanical waves.

Since sound requires a medium such as air, water, or solids to travel, it is classified as a mechanical wave.

Energy of Sound Waves

Sound is not just something we hear; it is also a form of energy. Like other forms of energy, sound can cause changes in objects and transfer energy from one place to another.

When a sound-producing object vibrates, it transfers energy to the particles of the surrounding medium. This energy travels through the medium in the form of sound waves.

How Sound Transfers Energy

When a sound-producing object vibrates:

1. It transfers energy to nearby particles of the medium.
2. These particles begin to vibrate.
3. Through collisions, they transfer energy to neighbouring particles.
4. The process continues as the sound wave moves forward.

The particles themselves do not travel from the source to the listener. Only the disturbance and the energy associated with it move through the medium.

During the propagation of sound, energy is transferred, not the particles of the medium.

Sound Energy and Particle Motion

The particles of a medium are never completely at rest. They are always in random motion due to the thermal energy present in the medium.

When a sound wave passes through the medium:

• The particles vibrate more than their usual random motion.
• This extra vibration is caused by the energy carried by the sound wave.
• After the sound wave passes, the particles return to their normal random motion.

This shows that sound temporarily transfers energy to the particles of the medium.

Conversion of Sound Energy

Sound energy can be converted into other forms of energy and vice versa.

Microphone: Sound Energy to Electrical Energy

A microphone is a device that converts sound energy into electrical energy.

When sound waves enter the microphone, they make a thin membrane called a diaphragm vibrate. These vibrations are converted into electrical signals, which can then be amplified, recorded, or transmitted. 

Speaker: Electrical Energy to Sound Energy

A speaker performs the opposite function of a microphone.

When electrical signals are supplied to a speaker, they cause its diaphragm or cone to vibrate. These vibrations create sound waves in the surrounding air, reproducing the original sound. 

Graphical Representation of a Sound Wave

As a sound wave propagates through a medium, the density of the medium changes periodically. At any instant, some regions have a higher density than the average density, while others have a lower density. These regions correspond to compressions and rarefactions.

A sound wave can be represented graphically by plotting the density of the medium against distance. In such a graph, the average density is represented by a horizontal line. The density rises above the average value in the region of compression and falls below the average value in the region of rarefaction.

The point corresponding to the maximum density is called the crest, while the point corresponding to the minimum density is called the trough.

Crest

A crest is the highest point on the wave graph and represents the region of maximum density or maximum compression.

Trough

A trough is the lowest point on the wave graph and represents the region of minimum density or maximum rarefaction.

Another way to represent a sound wave graphically is by plotting the density of the medium against time at a fixed location. This graph shows how the density at a particular point changes as successive compressions and rarefactions pass through that point.

Characteristics of a Sound Wave

Sound waves are described using certain physical quantities that help us understand their behaviour. The most important characteristics of a sound wave are wavelength, frequency, and time period.

Wavelength

The wavelength of a sound wave is the distance between two consecutive crests or two consecutive troughs. It represents the length of one complete wave.

The wavelength is denoted by the Greek letter λ (lambda).

SI Unit: metre (m)

A sound wave with a larger wavelength has its compressions and rarefactions farther apart, while a sound wave with a smaller wavelength has them closer together.

Frequency

As a sound wave passes through a medium, the density at a fixed point changes repeatedly between maximum and minimum values. One complete change from maximum density to minimum density and back to maximum density is called one oscillation.

The frequency of a sound wave is the number of complete density oscillations occurring at a fixed point in one second.

Frequency is represented by the Greek letter ν (nu).

SI Unit: hertz (Hz)

1 Hz = 1 oscillation per second

A higher frequency means more oscillations occur every second, while a lower frequency means fewer oscillations occur every second.

Time Period

The time period of a sound wave is the time taken for one complete density oscillation at a fixed point.

It is represented by T.

SI Unit: second (s)

The time period indicates how long one oscillation takes to complete.

Relationship Between Frequency and Time Period

Frequency and time period are inversely related.

\nu = \frac{1}{T}

Where ν is frequency (Hz), and T is time period (s).

A shorter time period corresponds to a higher frequency, while a longer time period corresponds to a lower frequency.

Sound and Musical Notes

Most sounds heard in daily life consist of a mixture of many frequencies. However, nearly single-frequency sounds can be produced by sources such as a tuning fork or by whistling.

Each musical note has a characteristic frequency. As we move from Sa to Re, Ga, Ma, Pa, Dha, Ni, and the higher Sa, the frequency gradually increases. This difference in frequency gives each musical note its unique sound.

Amplitude and Intensity of Sound Waves

Sound travels through a medium as a series of compressions and rarefactions. The amplitude of a sound wave is the maximum change in the density of the medium from its average density. A greater change in density indicates a larger amplitude.

The intensity of sound is the amount of sound energy passing through a unit area perpendicular to the direction of propagation in one second. Intensity gives a measure of how much sound energy reaches a particular area.

Speed of Sound

The speed of sound is the rate at which sound waves travel through a medium. It represents how fast compressions and rarefactions move from one place to another.

The speed of sound can be defined as the distance travelled by a point on the wave, such as a crest or a trough, in unit time.

For a sound wave, one wavelength (λ) is travelled in one time period (T). 

\nu = \frac{1}{T}ν=T1

Relation Between Speed, Wavelength and Frequency

v = \lambda \nu

This relation shows that the speed of a sound wave is equal to the product of its wavelength and frequency.

Factors Affecting the Speed of Sound

The speed of sound depends on the nature of the medium through which it travels.

Medium	Speed of Sound
Solids	Fastest
Liquids	Faster than gases
Gases	Slowest

Sound travels approximately 4-5 times faster in water and about 15-20 times faster in solids than in air.

The speed of sound in air is also affected by temperature and humidity. As temperature increases, the particles of air move more rapidly, allowing sound to travel faster. Similarly, an increase in humidity also increases the speed of sound.

Human Perception of Sound

The physical properties of sound, such as wavelength, frequency, amplitude, time period, and speed, can be measured accurately. However, the way humans experience sound is subjective and depends on how our ears and brain interpret these properties.

Pitch

Pitch is the sensation by which we distinguish between high and low sounds. It is related to the frequency of a sound wave.

Sounds that are sharp or shrill, such as a whistle, a siren, or a bird's chirp, are perceived as having a high pitch. Sounds that are deep, such as thunder or the rumbling of an aircraft, are perceived as having a low pitch.

Why Do Different People Have Different Voices?

Every person's voice is unique. Although frequency plays an important role, the quality of a voice also depends on how sound is modified by the throat, mouth, tongue, and nasal cavities.

During adolescence, the vocal cords of boys become longer and thicker. As a result, they vibrate less frequently, causing the voice to become deeper. 

Loudness

Loudness is the human perception of the amplitude of a sound wave. Sounds with larger amplitudes are heard as louder, while those with smaller amplitudes are heard as softer. Loudness decreases as the distance from the source increases.

Loudness is commonly measured in decibels (dB). Very loud sounds can damage hearing if a person is exposed to them for long periods.

Noise and Noise Pollution

Noise is any unwanted or unpleasant sound. Excessive noise in the environment causes noise pollution, which can affect health, sleep, concentration, and hearing. People with hearing loss may use hearing aids that contain a microphone, amplifier, and speaker.

Hearing Mechanism

When sound enters the ear, it makes the eardrum vibrate. These vibrations are amplified by tiny bones in the ear and converted into electrical signals by the cochlea. The brain interprets these signals as sound.

Tone, Musical Note and Timbre

A tone is a sound with a single frequency, such as the sound produced by a tuning fork.

A musical note consists of a fundamental frequency and higher frequencies called overtones. Together, they produce a pleasant and rich sound.

Timbre is the quality that helps us distinguish between sounds produced by different instruments, even when they have the same pitch and loudness.

Octave

An octave is the interval between two notes whose fundamental frequencies differ by a factor of two. For example, 200 Hz and 400 Hz are one octave apart.

Reflection of Sound

When sound waves strike a surface and bounce back into the same medium, the phenomenon is called reflection of sound. Sound follows the same laws of reflection as light.

 Echo

An echo is the repetition of a sound caused by its reflection from a distant surface.

For an echo to be heard separately, the reflected sound must reach the listener at least 0.1 s after the original sound.

Hard and smooth surfaces produce clear echoes, while soft surfaces absorb sound 

Reverberation

Reverberation is the persistence of sound due to multiple reflections from walls, ceilings, and other surfaces after the source has stopped producing sound.

A controlled amount of reverberation improves sound quality in auditoriums and concert halls. Excessive reverberation can make speech and music unclear. 

Ultrasonic and Infrasonic Waves and Their Applications

Humans can hear sounds with frequencies between 20 Hz and 20 kHz. Sound waves with frequencies outside this range are called infrasonic and ultrasonic waves.

Infrasonic Waves

Infrasonic waves have frequencies less than 20 Hz. These waves can travel long distances and are used to detect natural events such as earthquakes, volcanic eruptions, and severe storms.

Ultrasonic Waves

Ultrasonic waves have frequencies greater than 20 kHz. They have many applications in medicine, industry, and technology.

Some important uses of ultrasonic waves include:

Application	Use
Ultrasonography	Imaging internal organs without surgery
Medical Treatment	Breaking kidney stones into smaller pieces
Industrial Cleaning	Cleaning delicate machine parts
Ultrasonic Welding	Joining materials using high-frequency vibrations
Non-Destructive Testing	Detecting defects inside metal blocks
Object Detection	Locating objects using reflected sound waves

Echolocation

Echolocation is the process of locating objects by using reflected sound waves.

Bats emit short bursts of ultrasonic waves that reflect from nearby objects. By analysing the returning echoes, they can determine the position of obstacles and prey, allowing them to fly safely even in complete darkness.

SONAR (Sound Navigation and Ranging)

Humans use the same principle of echolocation in SONAR. In this technique, ultrasonic waves are sent into water, and the reflected waves are analysed to determine the distance, direction, and speed of underwater objects.