Sound Class 9 Notes PDF

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Sound Class 9 Notes - Description

Greetings to all, Today we are going to upload the Sound Class 9 Notes PDF to assist the students as well as teachers. Class 9 Science has conceptual chapters that prepare a strong foundation among the students. It will help them to identify their passion and continue to pursue it in the higher classes. The basic foundation related to science is very necessary at this stage. One of the tough chapters to study in this syllabus is sound. It is the 12th chapter in the subject that needs more attention from the students. They will know how sound waves originate and how they reproduce in a medium. This is the first time they will learn what an energy wave is and how it behaves in a medium. To make these new concepts clearer, the science teachers have created Class 9 Science Chapter 12 Sound notes to provide an easy medium to follow.

They have used simple language to explain all these concepts. They are aware of the common doubts and difficulties students face during studying this chapter. Therefore, Class 9 Ch 12 Science notes will be the most suitable bet to follow and prepare the chapter accordingly. These CBSE Solutions notes can be downloaded and used offline at your convenience.

1. Board CBSE
2. Textbook NCERT
3. Class Class 9
4. Subject  Notes
5. Chapter  Physics Chapter 12
6. Chapter Name Sound
7. Category CBSE Revision Notes

Sound Class 9 Notes PDF


  1. You’ve discovered that sound is a type of energy. Vibrations cause it to happen. Longitudinal waves are sound waves. Because they are elastic waves, they must be transmitted through a material medium. They are not capable of being communicated in a vacuum. They can move through solids, liquids, and gases. In solids, their velocity is greatest, whereas, in gases, it is lowest.
  2. In our daily lives, we hear a variety of sounds: pleasant sounds termed musical sounds, unpleasant noises called noise, loud sounds, high pitched sounds, and so on.
  3. In this chapter, we’ll look at the differences between pleasant and unpleasant sounds, as well as the elements that influence loudness, pitch, and other aspects of sound.

Sound as a Wave

  1. To us, a ringing bell, a thunderclap, laughter, and rock music are all very different noises. However, because all sounds are waves, they are all the same. Let’s look at how wave qualities can be used to apply to sound.
  2. Sound is a kind of energy that is sent as waves and received by our ears. Our vocal cords vibrate as we talk. When we play the guitar, the spring moves back and forth, producing sound. The vibrations of a tuning fork also make a sound. As a result of its vibrations, a body makes a sound. Sound waves can’t travel in a vacuum, hence they need to travel through a material medium.
  3. You can hear because sound waves cause your eardrums to vibrate when they reach your ears. The vibrations are then relayed to your brain by nerves. The messages are translated into sound by the brain.

Propagation of sound waves in the air from a tuning fork:

  1. A longitudinal wave is a wave motion in which the particles of the medium oscillate about their mean positions in the wave’s propagation direction.
  2. Longitudinal waves are the most common type of sound wave. Let’s look at how sound waves travel. Take a tuning fork and shake it while focusing on one of the prongs, say prong A. The tuning fork’s typical position and the initial state of air particles are depicted in diagram (a). As to prong, A advances to the right, air particles near it are compressed, generating a compression as depicted in fig (b). This compression proceeds forward as a disturbance due to vibrating air layers.
  3. The pressure on prong A’s right lessens as it returns to its previous position, generating a rarefaction. As a disturbance, this rarefaction travels forward like compression. As the tuning fork continues to vibrate, waves of alternated compressions and rarefactions propagate through the air, as shown in fig (d). Because sound waves travel in the same direction as air particles, they are classed as longitudinal waves. Longitudinal waves take the shape of compressions and rarefactions as they travel.

Sound Needs a Medium to Travel:
Some vibrating body is always the source of the sound. The vibrations of the source may be so small or so enormous that they are impossible to detect in some situations. Tuning forks, drums, bells, guitar strings, and other instruments produce this type of vibration. The vibrations of the vocal cords give rise to the human voice, and the vibrations of the air columns give rise to musical instrument sound. Sound travels in the form of a longitudinal wave that needs to be propagated through a material medium.
Experiment to show that sound waves (mechanical waves) require a material medium for their propagation:
A vacuum pump is attached to an electric bell hung inside an airtight glass bell jar. The sound is heard as the electric bell circuit is finished. After the air in the bell jar is gently withdrawn with a vacuum pump, the strength of the sound gradually decreases until no sound is heard when all of the air is removed. We would observe the hammer continually striking the gong. This clearly demonstrates that sound propagation requires the presence of a substance. Not only can sound travel through gases, but it can also travel through solids and liquids. Some materials, such as air, water, and iron, are good at transmitting sound energy from one location to another. Materials like blankets and thick curtains, on the other hand, absorb the majority of sound energy.
Basic Terms Connected to Waves:
Wavelength, Amplitude, Frequency, and Wave Velocity are the four key terms in the study of waves.
The distance between two consecutive spots on a wave that is in the same phase is known as the wavelength. (The same phrase denotes the same vibrational state.)
The largest displacement of a particle from its mean position is called amplitude. The number of periodic oscillations completed in one second is known as frequency. The frequency
f = 1/Tf = 1/T
, where ‘T’ is the time it takes for one oscillation to complete. The hertz HzHz is the unit of measurement. The wave velocity ‘v’ is the rate at which energy propagates through a medium.
Sound wave
The product of the wavelength and frequency gives us the wave velocity because the wavelength is the distance traveled during one oscillation and frequency is the number of oscillations per second.
Distance traveled in
1 s =1 s =
number of waves in one-second x wavelength
Wave velocity = Frequency ×× Wavelength
v = f (x)v = f (x)
Speed of Sound:
Although both occur at the same time, the flash of lightning caused by cloud interaction is noticed considerably before the thunder. This occurs because the speed of light is faster than the speed of sound. The qualities of the medium through which sound travels determine its speed. The medium’s elasticity, density, pressure, and temperature can all change. As sound goes from a solid to a gaseous state, its speed reduces. However, in any medium, the speed of sound increases as the temperature rises. The table shows the sound speed in various mediums at different temperatures.

Speed of sound in different media at
State Substance Speed in m/s
Solids Aluminium 6420
Nickel 6040
Steel 5960
Iron 5950
Brass 4700
Glass(Flint) 3980
Liquid Water (Sea) 1531
Water(distilled) 1498
Ethanol 1207
Methanol 1103
Gases Hydrogen 1284
Helium 965
Air 346
Oxygen 316
Sulphur dioxide 213

Reflection of Sound:

  1. When sound collides with a solid or liquid surface, it bounces back like light rays. The rules of reflection and refraction apply to sound waves as well. In order for sound waves to reflect, we need a huge surface or obstruction. The rolling of thunder, for example, is caused by consecutive reflections from clouds and terrain surfaces.
  2. The directions in which sound is incident and reflected make equal angles with the normal to the reflecting surface, and the three lie in the same plane, according to the rule of sound reflection.

Sound waves, like all waves, can be reflected. The enormous obstructions cause sound waves to be reflected. An echo is a sound that is heard as a result of a sound wave being reflected by a huge obstruction. The echo is normally undetectable because the reflected sound is integrated with the original sound. To hear an echo clearly, certain requirements must be met (as a separate sound). The experience of any sound lasts roughly
seconds in our ear. This is referred to as hearing persistence. The original sound and its echo cannot be separated if the echo is heard within this time span. The most critical criterion for hearing an echo is that the reflected sound should reach the ear only after the original sound has died off for at least
second. Because sound travels at 340 meters per second, the distance traveled by sound in
The second is 34 meters. This distance is twice the minimum distance between a sound source and a reflector. If the obstruction is at least 17 meters away, the reflected sound or echo can be clearly heard after
Furthermore, for any wave to be reflected, the size of the reflector must be big in comparison to the wavelength of the sound, which is on the order of 1 meter for ordinary sound. An echo can be produced by a large building, a mountainside, or a large rock formation, among other things. In addition, the reflected sound must have sufficient strength or volume to be heard. Furthermore, the echo and the original sound should not mix or overlap if the echo is to be differentiated from the original sound. The original sound, such as a clap or a shout, should be very short in duration for this.
As a result, the following conditions could be listed for echo formation:

  • The obstacle/reflector must be large in comparison to the incident sound wavelength (for reflection of sound to take place).
  • There should be at least 17 meters between the sound source and the reflector (so that the echo is heard distinctly after the original sound is over).
  • The sound’s intensity or loudness must be adequate for the reflected sound to be audible when it reaches the ear. The original sound should only last a few seconds.

Echoes’ Benefits and Drawbacks:

  • Echoes can be beneficial or annoying. If the walls and roof of a music hall are not appropriately built, echoes might disrupt a performance.
  • Echoes can be used to provide critical information if the walls are too hard or too flat to reflect sound waves. A ship’s sonar device (Sonar stands for sound navigation ranging) emits high-frequency sound waves to determine how close the ship is to the seabed. An ultrasound scanner, which is best known for producing images of an unborn baby, works in a similar way.
  • As they fly through the night, bats use echoes to navigate. It works in the same way as sonar and ultrasound scanners do. The bat emits a series of small, high-pitched squeaks that bounce off the objects along its route. The bat hears the echoes and changes its trajectory to avoid the obstacles. Many bats have big ears in order to capture as much reflected sound as possible.
  • It’s called echolocating when creatures like bats and dolphins use echoes. They utilize it to find their way about and hunt for prey. Some animals use echolocation to determine the size and location of items in their environment.
  • Bats use echolocation to guide them in flying at night. They fire off a series of tiny ‘clicks,’ which bounce off things and return to the bat. It creates a “sound” image of its surroundings.


  • A sound made in a large hall will persist due to light reflection until it is lowered to a level where it is no longer audible.
  • Reverberation is the persistence of audible sound caused by successive reflections from surrounding objects after the source has finished producing that sound.
  • Excessive reverberation should be avoided. The auditorium’s roof and walls are usually coated with sound-absorbing materials like compressed fiberboard, rough plaster, or drapes to lessen reverberation.
  • Practical Applications of Reflection of Sound

Some applications of the principle of reflection of sound are:

  • Megaphone
  • Hearing Board
  • Sound Boards

Megaphone: A megaphone is a tube that is formed like a horn. Sound waves are limited to the air in the tube due to consecutive reflections that prevent them from spreading out.
Hearing Board: Hearing aids are devices used by persons who have difficulty hearing. The sound waves that the hearing aid receives are reflected into a smaller region leading to the ear.
Sound Boards: Sound waves can be reflected by curved surfaces. In an auditorium, this reflection of sound waves is employed to distribute the waves evenly around the space. Sound Boards are used to reflect sound waves back to the source. The sound board’s focal point is where the speaker is positioned.
Musical Sound and Noise:

  • A pleasant continuous and uniform sound created by regular and periodic vibrations is referred to as a musical sound.
  • A guitar, piano, tuning fork, and other musical instruments, for example, generate a pleasing sound.
  • Noise is described as an irregular series of discordant and unpleasant to the ear disruptions.
  • By hearing the echo of their own sound, bats and dolphins may identify the presence of an obstruction. This is referred to as sound-ranging.

Range of Hearing:
A vibrating source emits sound waves, which are then transported via the air. Sound waves between 20 Hz and 20 kHz can be heard by the human ear. This range is referred to as the audible range. Ultrasonic waves are sound waves with frequencies above the audible range, and they are commonly referred to as ultrasound. Infrasonic waves are sound waves with frequencies lower than the hearing range.
Applications of Ultrasound:

  • It’s utilized for medical diagnosis and treatment, as well as surgical procedures.
  • Ultrasound is used by bats and porpoises to navigate and find food in the dark.
  • It’s used to spot a faulty foetus.
  • It’s employed in the treatment of muscular pain.
  • Ultrasonography (a procedure that uses ultrasonic waves to create 3-dimensional pictures) is used to pinpoint the exact location of an eye tumor.
  • Ultrasound is commonly used to clean spiral tubes, electronic components, and other similar objects.
  • Metal blocks are inspected with ultrasound to discover cracks and faults.

Oceanographic investigations are one of the most important applications of sound reflection. We employ a technique called SONAR for this purpose. The abbreviation SONAR stands for Sound Navigation and Ranging. The SONAR technology is used to detect and locate submerged submarines, sunken ships, and icebergs that are not visible from the surface. Sonar works by sending ultrasonic waves in all directions from the ship, which are then received after reflection.
Determination of the depth of an ocean:

  • With the use of SONAR, the depth of an ocean can be determined.
  • Ultrasonic waves are used in sonar to detect and locate items underwater.
  • Ultrasonic waves emitted by a ship’s transmitter are aimed towards the ocean floor.
  • These waves are reflected on the water floor.
  • Using the connection, we may compute the depth of the ocean by measuring the time interval t between the formation of the wave and the receiving of the echo.
  • s = 12vts = 12vt Where v is the velocity of ultrasonic waves.


  • Echolocation is a sensory perception mechanism used by certain animals to orient themselves to their surroundings, identify impediments, communicate with others, and locate food.
  • Bats use echolocation to navigate and find food in the dark.
  • A bat generates a succession of short, high-pitched ultrasonic waves from its mouth or nose, which move away from the animal and bounce off things along its path, resulting in an echo.
  • A bat can judge the size and shape of obstruction in its route, as well as the direction of the prey’s speed and its own.
  • Bats can detect insects as small as gnats and things as fine as human hair using their echolocation apparatus.
  • Dolphins, like bats, produce high-frequency sound waves and may identify obstructions in their path. Thus, dolphins can avoid fishing nets and discover fish even at night or in dirty water when visibility is limited.

Structure of Human Ear:
The Human Ear’s Structure The human ear is a very sensitive apparatus that allows us to hear. The outer ear, middle ear, and inner ear are the three basic sections of the ear. Each component of the ear is responsible for detecting and interpreting sound in its own way. Pinna is the name for the outer ear. Through the auditory canal, it collects and transports sound to the middle ear.
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