Sound Class 9 CBSE Notes - Chapter 12

According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 11.

Particles in Waves

Introduction to Waves

A wave is a disturbance in a medium which moves from one point to another and carries energy without a net movement of particles. It may take the form of elastic deformation or a variation of pressure.

E.g. A rubber cork on the water that goes up and down when a rock falls into the water creates a ripple.

For more information on Particulars in Waves, watch the below video

To know more about Wave Particles Dual Nature, visit here.

Particle Motion of Mechanical Waves

(i) Transverse Waves

Particle motion is perpendicular to the direction of wave motion.  This type of wave is a mechanical wave.

E.g.Light and Mexican wave in a stadium.

(ii) Longitudinal waves

Particles travel parallel to the direction of wave motion by means of successive compressions or elongations. This is also a mechanical wave.

E.g. Sound waves in the air.

For more information on Types of Sound Waves, watch the below video

To know more about the Types of Waves, visit here.

Sound Properties

Introduction to Sound Waves

Sound needs a medium to propagate. The matter or material through which sound propagates is called a medium. When particles vibrate about their mean positions, it pushes a region of compressed air, creating a region of high pressure, followed by a region of low pressure as the particle retreats to its mean position. The sound wave propagates by compressions and rarefactions of particles in a medium. Sound propagation can be visualised as the propagation of pressure variations in the medium.

Characteristics of Sound Waves

Watch the below video to know more about the Characteristics of Sound Waves

To know more about the Characteristics of Sound Waves, visit here.


The distance between two successive crests or troughs (or) successive compressions and rarefactions is called wavelength (λ).  The SI unit of wavelength is metre (m).


Time period

The time taken by two consecutive compressions or rarefactions to cross a fixed point is called a Time period (T). The SI unit of time in seconds (s).


The number of compressions or rarefactions per unit time is called frequency (𝛎).
The SI unit of frequency is Hertz. The  SI unit is Hertz (s−1)

\(\begin{array}{l}v=\frac{1}{T}\end{array} \)

Speed (v), wavelength (λ) and frequency (𝛎) are related as v=λ𝛎


The magnitude of disturbance in a medium on either side of the mean value is called an amplitude (A).
As shown in the figure below, the unit of amplitude will be the density or pressure. Distance between mean position and crest (maximum displacement).

  Amplitude (A)
                      Amplitude (A)


The number of compressions or rarefactions per unit time. Directly proportional to frequency.

Representation of low and high pitch


The volume or loudness of a sound depends on the amplitude. The force with which an object is made to vibrate gives the loudness.

Higher force → higher amplitude → louder sound

The amount of sound energy flowing per unit time through a unit area is called the intensity of sound.

     The Intensity of Sound
             The Intensity of Sound

Note and Tone

A sound of a single frequency is called a tone. A sound produced with a mixture of several frequencies is called a note.

Quality of Sound

The richness or timber of sound is called quality. Sound with the same pitch and loudness can be distinguished based on the quality. Music is pleasant to the ears, while noise is not. However, they both can have the same loudness and pitch.

Speed of Sound

Sound travels through different media at different speeds. The speed of sound depends on the properties of the medium: pressure, density and temperature.

Speed of Sound: Solids > Liquids > Gases

Speed of sound in air = 331 m/s at 00C and 344 m/s at 22 C

When a source emits sound with speed greater than the speed of sound in air, it creates a sonic boom which produces shockwaves with lots of energy. They produce a very loud noise which is enough to shatter glass and damage buildings.

For more information on Sound Properties, watch the below video

To know more about the Properties of Sound, visit here.

Reflection of  Sound Waves

Like light, sound also follows laws of reflection, it bounces off the surface of solid and liquid.


The phenomenon where a sound produced is heard again due to reflection is called an echo.
E.g. Clapping or shouting near a tall building or a mountain.

To hear a distinct echo sound, the time interval between the original and reflected sound must be at least 0.1s, as sound persists in our brain for about 0.1s. The minimum distance for obstruction or reflective surface to hear an echo should be 17.2 m. Multiple echoes can be heard due to multiple reflections.

For more information on Reflection of Sound Waves, watch the below video

To know more about Reflection of Sound Waves, visit here.

Sonar and Radar

SONAR – Sound Navigation And Ranging.

It is a technique that uses sound or ultrasonic waves to measure distance. The human range of hearing is 20Hz- 20kHz.

What Are Ultrasonic Sounds?

Ultrasonic sounds are high-frequency sounds having a frequency greater than 20kHz (inaudible range).

Applications of Ultrasound

(i) Scanning images of human organs
(ii) Detecting cracks in metal blocks
(iii) Cleaning parts that are hard to reach
(iv) Navigating, communicating or detecting objects on or under the surface of the water (SONAR).

Sonar consists of a transmitter and detector mounted on a boat or ship. The transmitter sends ultrasonic sound waves to the seabed, which get reflected back and picked up by the detector. Knowing the speed of sound in water, distance can be measured using:   2d=v×t. This method is called echolocation or echo ranging.

To know more about the Uses of Radar, visit here.


The persistence of sound because of multiple reflections is called reverberation. Examples: Auditorium and a big hall.
Excessive reverberation is undesirable, and to reduce this, halls and auditoriums have sound-absorbing materials on the walls and roofs.  E.g. Fibreboard and rough plaster.

To know more about Reverberation, visit here.

Doppler’s Effect

If either the source of sound or the observer is moving, then there will be a change in frequency and wavelength for the observer. The frequency will be higher when the observer moves towards the source, and it decreases when the observer moves away from the source.

Example: If one is standing on a street corner and an ambulance approaches with its siren blaring, the sound of the siren steadily gains in pitch as it comes closer and then, as it passes, the pitch suddenly lowers.

To know more about Doppler’s Effect, visit here.

Human Ear

The ear is a sensitive organ of the human body. It is mainly involved with detecting, transmitting and transducing sound and maintaining a sense of balance is another important function of the human ear. The human ear includes:

  • The outer ear, or the visible part of the ear, is called the pinna.
  • Pinna collects sound from the surroundings.
  • Sound passes through a tube called an auditory canal.
  • Eardrum (tympanic membrane) vibrates in response to incident sound waves.
  • Vibrations are amplified and transmitted further by three bones hammer, anvil and stirrup in the middle ear to the inner ear.
  • In the inner ear, the cochlea converts pressure signals into electrical signals.
  • Electrical signals are transmitted by the auditory nerve to the brain for interpretation.
Human Ear
              Human Ear

For more information on the Structure of the Human Ear, watch the below video

To know more about the Human Ear, visit here.

Frequently Asked Questions on CBSE Class 9 Science Notes Chapter 12 Sound


What is a ‘wave’?

A wave can be described as a disturbance that travels through a medium, transporting energy from one location to another location without transporting matter.


What is the definition of ‘amplitude’?

The amplitude of a sound wave is the measure of the height of the wave.


What is an ‘echo’?

An echo is a sound that is repeated because the sound waves are reflected back.


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