CBSE Class 9 Science Revision Notes Chapter 12

CBSE Class 9 Science Revision Notes Chapter 12 – Sound

In these Class 9 Science Chapter 12 notes, students will learn about the concept of sound. In addition, in the Class 9 Chapter 12 Science notes, students will get to know the significant details of the Chapter that are important for their final examination. Along with Chapter 12 Science Class 9 notes, Extramarks will provide students with essential questions that can be asked to prepare them quickly. Moreover, Class 9 Science notes Chapter 12 will be a student’s last-minute revision guide, providing all the necessary information. These notes are based on the syllabus as decided by CBSE


You’ve learned that one kind of energy is sound. Sound occurs due to vibrations. Sound waves are longitudinal waves. They have to be transmitted through a material medium because they are elastic waves. They cannot be communicated in a vacuum. They are capable of navigating through solids, liquids, and gases. Their velocity is highest in solids while it is lowest in gases.

We are exposed to a wide range of sounds in our everyday lives, including musical, loud, high-pitched, and others. In this Chapter, we’ll examine the distinctions between pleasant and unpleasant sounds and the factors that affect pitch, loudness, and other aspects of Sound.

Sound as a wave

Sound wave is the particle’s motion in a medium. Let’s examine how wave properties can be applied to sound. Our ears pick up sound as a type of energy that is transmitted as waves. As we speak, our vocal cords vibrate. For example, the spring in a guitar moves back and forth while we play, creating sound. A tuning fork’s vibrations also produce sound. When a body vibrates, its vibrations produce sound. Since sound waves can’t move through a vacuum, they must move through a material medium. Because sound waves result in the vibration of eardrums when they enter your ear, you can hear. Then, the vibrations reach your brain by nerves. Finally, brian translates messages into sound.

Propagation of Sound

Sound waves caused by tuning fork

Propagation of Sound waves in the Air from a Tuning Fork

In a longitudinal wave, the particles of the medium oscillate about their mean positions in the direction of the wave’s propagation. The most common kind of Sound wave is a longitudinal wave. Let’s examine the path that Sound waves take. Shake a tuning fork while paying attention to one of the prongs, let’s say prong A. The diagram shows the typical position of the tuning fork and the initial state of air particles (a). Air molecules close to prong A are compressed as they move to the right, creating the compression shown in fig (b). The vibration of the air layers causes this compression to continue as a disturbance. As prong A returns to its original position, the pressure on its right decreases, creating a rarefaction.

Sound needs a medium to travel.

The source of sound is always some vibrating body. In certain circumstances, the source’s vibrations may be too small or too large to be detected. This vibration is produced by tuning forks, bells, drums, guitar strings, and other instruments. Vocal cord vibrations produce the human voice, and air column vibrations produce musical instrument sounds. A material medium must be used to propagate the longitudinal wave that represents sound as it travels.

An experiment to demonstrate the need for a physical medium for the propagation of sound waves (mechanical waves):

        A glass bell jar with an electric bell suspended inside is airtight.

An electric bell is suspended inside an airtight glass jar and connected to a vacuum pump. When the electric bell circuit is complete, the sound is audible. A vacuum pump is used to gently remove the air from the bell jar, and as the air is removed, the sound gradually fades away until it stops entirely. We would watch as the hammer kept banging the gong. This demonstrates unequivocally that a substance is necessary for sound to propagate. In addition to gases, solids and liquids can transmit sound. Some substances effectively transmit sound energy, including air, water, and iron. But materials like blankets and thick curtains can absorb most of the sound energy.

Characteristics of Sound wave

Sound travels through a medium by expanding and contracting parts of the medium from which it is travelling through. The four crucial terms in the study of waves are wavelength, amplitude, frequency, and wave velocity. 


The wavelength is the separation between two consecutive spots on a wave that is in phase. (The same phase indicates an identical vibrational state.) 


Amplitude is the measurement of a particle’s largest deviation from its mean position. 


Frequency is the number of periodic oscillations occurring in a second. The frequency f = 1/T, where ‘T’ denotes the duration of an oscillation. The unit of measurement is hertz [Hz]. 


The rate energy moves through a medium is known as the wave velocity, or “v.”. 

Sound wave

The product of the frequency and wavelength gives us the wave velocity because the wavelength is the distance travelled during one oscillation, and the frequency is the number of oscillations per second.

Distance traveled in 1 s = number of waves in one second x wavelength

Wave velocity = FrequencyWavelength 

or, v = f x 

Speed of Sound

Speed of sound refers to the rate of sound wave propagating through a medium. This concept can be understood through an example. Even though they both happen at the same time, the flash of lightning brought on by cloud interaction is seen much earlier than the sound of thunder is heard. The reason for this is that sound travels at a slower speed than light. The characteristics of the medium that sound travels through determine its speed. Elasticity, density, pressure, and temperature of the medium are all changeable. The speed of sound decreases as it transitions from a solid to a gaseous state. The speed of sound does, however, increase with temperature in any medium. The sound speed in various media at a particular temperature is displayed in the table.

Reflection of Sound

Sound reflects back like light rays when it strikes a solid or liquid surface. Sound waves are subject to the same reflection and refraction laws. We need a huge surface or obstruction in order for sound waves to reflect. For example, repeated reflections from the ground and clouds are the causes of the rolling of thunder. According to the rule of sound reflection, the directions in which sound is incident and reflected make equal angles with the normal to the reflecting surface, and all three lie in the same plane.


Like all waves, sound waves can also be reflected. Sound waves are reflected by the massive obstructions. An echo is the sound produced when a sound wave is reflected by a large obstruction. Because the reflected sound is integrated into the original sound, the echo is typically not audible. Certain conditions must be met in order to hear an echo clearly (as a separate sound). Any sound we hear in our ears lasts for about 0.1 seconds. This is known as hearing persistence. If the echo is audible during this time, the original sound and its echo cannot be distinguished from one another. The most important factor for an echo to be heard is that the reflected sound should only reach the ear after the original sound has subsided for at least 0.1 seconds. Sound moves 34 metres in 0.1 seconds as it moves at a speed of 340 metres per second. The minimum separation between a sound source and a reflector is twice that distance. After 0.1 seconds, the reflected sound or echo can be clearly heard if the obstruction is at least 17 metres away. In addition, the size of the reflector must be large in comparison to the wavelength of the wave, which for regular sound is in the order of 1 metre. Among other things, a large building, a mountainside, or a sizable rock formation can produce an echo. In addition, the reflected sound needs to be loud enough to be audible. If the echo is to be distinguished from the original sound, it should also not mix with or overlap with the original sound. For this, the original sound should be very brief, like a clap or a shout. The following requirements could be listed for echo formation as a result: 

  • For sound reflection to occur, the obstacle or reflector needs to be substantial compared to the incident sound wavelength.
  • The distance between the reflector and the sound source should be at least 17  metres (so that the echo is heard distinctly after the original sound is over).
  • The sound’s volume or intensity must be sufficient for the reflected sound to be audible when it enters the ear. The original sound must only last for a few seconds.

Echo: its Benefits and Drawbacks

Echo means a sound which is caused due to reflection of sound waves from the listener’s surface. Echoes can either be useful or inconvenient. To find out how close the ship is to the seabed, a ship’s sonar device (sonar is an acronym for sound navigation and ranging) sends out high-frequency sound waves. Similar principles apply to the operation of an ultrasound scanner, which is best known for producing images of an unborn child. Bats use echoes as a form of navigation as they fly at night. It functions in a similar manner to how sonar and ultrasound scanners do. The bat makes a series of brief, high-pitched squeaks, reverberating off the nearby objects. 


Reverberation is the continuation of audible sound after its source has stopped producing it due to successive reflections from surrounding objects. For example, The bat listens to echoes and modifies its course to avoid obstacles. Many bats have large ears to pick up as much reflected sound as possible. When animals like dolphins and bats use echoes, it’s referred to as echolocating. They make use of it to hunt for prey and navigate. Some animals use echolocation to understand the size and location of objects in their environment. Bats fly at night using echolocation as their navigation. They release a string of small “clicks” that bounce off objects and return to the bat. It produces an audible representation of its surroundings.

Practical Applications of Reflection of Sound 

Some application of principles 

  • Megaphone
  • Hearing Board
  • Sound Boards
  • Megaphone: A megaphone is a tube with a horn-shaped shape. Consecutive reflections prevent sound waves from spreading out, limiting them to the air inside the tube.
  • Hearing Board: People who have hearing loss use hearing aids as assistive technology. The sound waves that the hearing aid picks up are reflected into a more condensed area that surrounds the ear.
  • Soundboards: Curved surfaces can reflect sound waves. This sound wave reflection is used in auditoriums to disperse the waves evenly throughout the room. By reflecting sound waves back to their source, soundboards are used. The speaker’s position on the soundboard serves as its focal point.

Musical Sound and Noise

A musical sound is a pleasing continuous and uniform sound produced by regular and periodic vibrations. A tuning fork, guitar, piano, and other musical instruments, for instance, produce a pleasing sound.

Noise is defined as an irregular succession of disruptions which are unsettling and irritating to the ear.

Bats and dolphins can detect the presence of obstructions by hearing the reflection of their own sound. This process is known as sound ranging.

Range of Hearing

Sound waves are made by a vibrating source and then spread through the atmosphere by air.

Sound waves ranging from 20 Hz to 20 kHz are capable of being registered by the human ear. This is known as the audible range. The term “ultrasound” refers to waves that have frequencies higher than those of human hearing. Sound waves with frequencies below human hearing are known as infrasonic waves.

Applications of ultrasound

  • It is used in surgical procedures, medical diagnosis, and treatment.
  • Bats and porpoises use ultrasound to find food and navigate in the dark.
  • It is used to identify a defective fetus.
  • It is used to treat musculoskeletal pain.
  • An eye tumor can be located precisely using ultrasonography, a procedure that employs ultrasonic waves to produce 3-dimensional images.
  • Electronic components, spiral tubes, and other similar objects are frequently cleaned using ultrasound.
  • Ultrasound is used to inspect metal blocks for flaws and cracks.


One of the most vital uses of sound reflection is in oceanographic research. To achieve this, we use a technique known as SONAR. SONAR means Sound Navigation and Ranging. SONAR technology can be used to find submerged submarines, sunken ships, and icebergs that are not visible from the surface. Ultrasonic waves are sent from the ship in all directions, and after they reflect, they are received by the receiver.

Determination of the Depth of an Ocean

  • The depth of an ocean can be discovered using SONAR.
  • Sonar uses ultrasonic waves to find and detect objects underwater.
  • A ship’s transmitter sends out ultrasonic waves that are directed at the ocean floor.
  • On the ocean floor, these waves are reflected.
  • By measuring the time “t” between the wave’s formation and the echo’s reception and using the connection, we can determine the depth of the ocean.
  • s=12vt, where v denotes the ultrasonic wave’s speed.


  • Certain animals use echolocation, a sensory perception method, to orient themselves to their surroundings, recognise obstacles, interact with others, and find food.
  • At night, bats use echolocation for navigation and to locate food.
  • The mouth or nose of a bat emits a series of brief, high-pitched ultrasonic waves that travel away from the animal and bounce off objects in their path to produce an echo.
  • A bat can determine the size, shape, and direction of an obstruction in its path, as well as the relative speeds of its own movement and that of its prey.
  • With the help of their echolocation system, bats are able to find objects as small as gnats and as delicate as human hair.
  • Dolphins, like bats, can detect obstacles in their path by producing high-frequency sound waves. Therefore, dolphins can find fish and avoid fishing nets even at night or in dirty water when visibility is low.

Structure of the Human Ear

The human ear is a highly responsive instrument that gives us the ability to hear. The three basic parts of the ear are the middle ear, the inner ear, and the outer ear. The various parts of the ear have a unique role to play in the detection and interpretation of sound. The term “pinna” refers to the outer ear. It gathers and sends sound waves through the auditory canal to the middle ear.

                                              Structure of Human Ear

The auditory canal ends in a thin membrane called the eardrum, also referred to as the tympanic membrane. The compression, or rarefaction, pushes inward and outward as it reaches the eardrum. This causes a vibration in the eardrum. The hammer, anvil, and stirrup, three bones in the middle ear, amplify these vibrations.

The middle ear sends these vibrations to the inner ear. Within the inner ear, the cochlea converts pressure fluctuations and vibrations into electrical signals. These electrical signals are sent from the auditory nerve to the brain, which the auditory sense of the brain identifies as sound.

Access Revision Notes Class 9 Science Chapter 12 – Sound Notes

Class 9 Chapter 12 Sound Revision Notes Summary

The Chapter on sound is a great place to start with this conceptual platform. You will learn how sound waves are created and move through a medium. You will discover other defined terms used in phrase formulas and how to use them to solve problems. You can use the notes from Science Class 9 Chapter 12 to help with this. Let’s examine what using these revision notes while studying sound can do for you.

You will learn the definition of waves and their fundamental characteristics from the Class 9 Science Chapter 12 notes. They are rhythmically occurring medium disturbances. The speed, frequency, amplitude, and other physical characteristics of sound waves are determined by the movement of a particle in a medium. These concepts will be explained using easy examples so students can gather the difference between mediums transmitting sound waves.

The revision notes will first go over the various waves. They are of two kinds: transverse waves and longitudinal waves. Each type of wave has a unique set of characteristics that set it apart from the others. By carefully studying the Class 9 sound notes, you can find out how these waves behave. Examples are used to explain and illustrate these waves so students can understand them. 

In the next phase, you will discover how sound travels through a medium. When a source vibrates, sound waves are created. The medium in the nearest area is then affected by this vibration. The medium’s particles start vibrating after receiving energy. Due to harmonic vibration, compression and rarefaction of the particles in a medium happen. Sound waves travel through a medium in this way.

As we go on, the Class 9 Science Chapter 12 notes will also go over the various characteristics of sound waves. They differ from other energy waves like light and magnetism, among others. By studying these characteristics, you can better understand how sound waves differ from other waves. Additionally, it will explain why gases have the lowest sound speed, and solids have the highest. This section will familiarise you with various terms used to describe waves. They include volume, pitch, frequency, amplitude, time period, and wavelength. Find out how these physical factors affect the various aspects of sound. Remember what happens when physical quantities like wavelength, amplitude, and frequency rise or fall. These notes are based on the material given in NCERT books. The questions given in these notes are based on CBSE  past years’ question papers. These notes also contain important questions related to Sound.

Why Prefer Using Class 9 Chapter 12 Science Notes?

For this Chapter, CBSE revision notes are helpful for students so that they can quickly summarise the concepts. Sound is an essential Chapter in the Class 9 science CBSE syllabus. These revision notes will significantly reduce your preparation time. These notes contain CBSE extra questions that will help students test their understanding. Once you go through these notes, you can easily solve CBSE sample papers to test your understanding.


FAQs (Frequently Asked Questions)

1. How are sound waves different from light waves?

Sound waves need a medium, whereas light waves can travel in a vacuum. Therefore, sound waves are not stopped but instead propagated by solid media. Light waves will not pass through opaque solid mediums. Please refer to these Chapter 12 Class 9 notes to find more characteristics of sound waves.

2. What is a megaphone?

It is a horn-like tube where the sound waves are trapped in the air inside the tubes because of consecutive reflections, preventing them from spreading.

3. What is echolocation?

It means a sensory perception mechanism certain animals use to familiarise themselves with their surroundings, communicate with others, identify impediments, and locate food.

4. Define wavelength.

It is the separation between the closest wave points vibrating at the same speed. 

5. What frequency range can the typical human ear hear?

The average human ear can hear frequencies ranging from 20 Hz to 20,000 Hz.