Sound Notes


  • In this chapter sound for class like 9 science we will learn about concepts nature of sound, propagation of sound in various media, speed of sound and other concepts related to sound.
  • Every day we hear sounds from various sources such as birds, people around us, mobile ringing, begging of a mosquito radio, television etc.
  • It is a sensation felt by our ears.

Sound Definition

Sound is a form of energy which produces a sensation of hearing in our ears.

Production of sound-

To know about production of sound let us consider following activities
Take ruler and hold it flat on your table with about half of its length sticking out from the surface. Now bend it down and release it. It will more up and down rapidly and would produce sound at the same time. The sound will last as long as vibration or rapid up and down motion of scale continues.
Take a tuning for R. Hold it from its stem. Set this tuning fork vibrating by striking its pongs on a rubber pad. Now if you bring it near your ear you would hear a sound. If you touch one of the pongs of the vibrating tuning fork you would be able to experience the vibrations.
Note : Vibration means a rapid to and fro motion of an object.

From these activities we can conclude that sound is produced when something vibrates.
You can feel these vibrations by touching the source of sound in some cases. For example if you place your finger lightly on your throat near the vocal cards and then speak something then you will feel the vibration in your finger as long as you speak.
We can also produce sound by
(a) plucking – when strings of a guitar are plucked and released, they vibrate and produce sound.
(b) Blowing – When air is blown into the flute, pipe etc, it vibrates in the tube of the instrument thereby producing sound.
Apart from these we can also produce sound by scratching, rubbing or shaking different objects. In all these cases we set the objects to vibrate and produce sound.

Sound, travels as a wave

  • From the previous section we have already established that sound is produced by vibrating objects. Term vibration refers to the rapid to and fro motion of an object.
  • If we throw a piece of stone in a pond of still water then expanding circle of ripples or water waves are formed over the surface of water. These water waves moves on an outward direction on the surface of water.
  • This happens because when stone hits water surface it disturbs the particles of water surface. As a result water particles began to vibrate about their means positions.
  • These vibrating particles collide with the neighboring particles and make them vibrate.
  • This process continues and the disturbance travels through the water.
  • The disturbance travel in water due to the repeated periodic motion of the particles of water about their mean positions.

Definition of wave

“This movement of the disturbance through a medium due to the repeated periodic motion of the particles of the medium about their mean positions is known as a wave”
It is important to note here that
“Wave transfers energy not the matter.”

From above definition of wave and example of water wave we can conclude that
1. A wave is produced by the vibrations of the particles of the medium through which it passes. Here water waves are produced as a result of vibrations water particles due to disturbance created by stone.
2. When wave passes through the medium, the medium itself does not move along the direction of wave. Here only particles of medium vibrate about their fixed positions.

Sounds which we hear travels in the form of a wave. This means that sound waves comes to us through air and there is no actual movement of air from source of sound to our ears.

Propagation of sound

  • Now we will study how sound travels from one place to another.
  • When source of sound create a sound it vibrates.
  • When it vibrates it creates a disturbance in the medium around it.
  • As a result of these disturbance particles of the medium around the source of sound or in contact with it begin to vibrate.
  • These vibrating particles do not move from their equilibrium positions but simply vibrate about their equilibrium positions.
  • This vibrating particle of medium of contact with source of sound exerts a force on adjacent particle due to this the adjacent particle gets displaced from its position of rest or its equilibrium position.
  • After displacing the adjacent particle, the first particle comes back to its original position.
  • This process of disturbing adjacent particles of the medium continues till the sound reaches your ears.
  • Here you must note that the disturbance created by the source of sound in the medium is traveling through the medium. This does not involve the particles of medium to move from source of sound to your ears.
  • Now from the previous section we know that wave is the disturbance through a medium due to repeated periodic motion of particles of the medium about their mean position.
  • So, when sound propagates in a medium a wave of disturbance is created and it moves through the medium when the particles of the medium set neighboring particles to vibrate. This causes the sound wave to move forward.
  • Hence, we can say that sound travels through the medium in the form of a wave.

How sound waves move through air?

  • Air is the most common medium through which sound waves propagate. Now we shall see how sound travels through air.
  • To understand this let us consider a source of sound say a drum.
  • When we hit the drum its vibrating membrane moves back and forth.
  • This movement of membrane produces sound.
  • Figure given below shows the membrane of instrument at different instants of time and the condition of the air near it at those instants.

When membrane move forward
  • From this figure we can see that when the vibrating membrane moves towards right or moves forward, it pushes the air in front of it.
  • Due to this push, the particles of air in this layer moves closer to each other than normal.
  • This results in the increase in density of air and that layer gets compressed.
  • This compressed layer then pushes and compresses the layer next to it, which then compresses the next layer, and so on. This way the disturbance moves forward.
  • This type of disturbance is called compression pulse.
  • The particles of the medium do not travel with the compression pulse. These particles return to their normal positions soon after pushing the particles in front.

When membrane moves back
  • Let’s now look at what happens when the membrane moves back or towards the left.
  • Again from the figure we can clearly see that when the membrane moves back it drags back the layer of air near it.
  • This creates a decrease in the density of air in that region.
  • Now the air particles in the next layer on the right move in to fill this less dense or verified area.
  • As a result, its own density reduces. In the same way, the density of air in successive layers on the right decreases one after the other.
  • We call this type of disturbance a rarefaction pulse that moves to the right.
  • As the membrane of drum moves back and forth rapidly, a series of compression and rarefaction pulses are created in the air, one after another. Carry the disturbance and
  • These pulses make the sound wave propagate through the medium.

Density and pressure variation during sound propagation:
  • We already know that sound propagates through a medium we have alternate regions of low and high density in the medium.
  • Figure given below shows how the density of air varies when a sound wave passes through it.

  • The places marked by letter ‘C’ have high density and places marked by letter ‘R’ have low density in comparison to normal density of air.
  • Let us now mark a place say ‘P’ in above given figure.
  • You can clearly see that density of air is high or maximum at the instant.
  • But this density would not remain high at this place forever.
  • As the time passes the density gradually reduces to normal and then below normal till it reaches low or minimum value.
  • After reaching minimum value it again begin to increase, reaches normal and become maximum again.
  • This variation in density of air is periodic. We call it periodic because it is repeated after a fixed period of time.
  • “A periodic variation in the value of quantity between maximum value and minimum value is called oscillation.”
  • So, here in case of sound waves the occurrence of two consecutive maximum or minimum makes one oscillations.
  • Pressure is related to number of particles of the medium in a given volume.
  • More density of particles in the medium means more pressure and vice-vessa.
  • Thus, propagation of sound can also be visualized as propagation of pressure variations in the medium.
  • In case of pressure variations compression is the region of high pressure and rarefaction is the region of low pressure.

Sound needs a medium to travel

  • Sound cannot travel through vacuum.
  • This is because when sound travels from one place to another then energy is transferred from one particle to another particle of the medium.
  • This means that sound needs a material medium like solid, liquid or gas for its propagation.
  • Visit this link for demonstration that shows sound waves cannot travel through vacuum.

Longitudinal and transverse waves –

  • There are many types of waves like mechanical waves, electromagnetic waves, matter waves.
  • Here in this chapter we are only concerned with mechanical waves.

Mechanical Wave –
Mechanical wave is periodic disturbances which require material medium like solid, liquid and gas for its propagation.
Some examples of mechanical waves includes –
  • Sound waves
  • Water waves
  • Waves produced in stretched string
  • Waves produced in slinky or a long string

There are two types of mechanical waves
Transverse wave
In these waves the individual particles of the medium move in a direction perpendicular to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest

Longitudinal wave
In these waves the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest
Sound waves are longitudinal waves as in sound waves ,particles moves in a direction parallel to the direction of propagation of the disturbance


  • when the sound wave moves in the medium. The density as well as the pressure of the medium at a given time varies with distance, above and below the average value of density and pressure
  • Compressions are the regions where particles are crowded together. compressions are regions where density as well as pressure is high
  • Rarefactions are the regions of low pressure where particles are spread apart
  • A peak is called the crest and a valley is called the trough of a wave

  • The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called the wavelength
  • The wavelength is represented by letter $\lambda$
  • SI unit is meter

  • when sound is propagated through a medium, the density of the medium oscillates between a maximum value and a minimum value. The change in density from the maximum value to the minimum value, again to the maximum value, makes one complete oscillation. The number of such oscillations per unit time is the frequency of the sound wave
  • It is usually represented by $\nu$ .
  • Its SI unit is hertz (symbol, Hz).

Time period
  • time taken for one complete oscillation in the density of the medium is called the time period of the wave
  • It is represented by the symbol T.
  • Its SI unit is second (s).
  • Frequency and time period are related as follows:
    $\nu = \frac {1}{T}$

  • The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.
  • It is usually represented by the letter A
  • For sound its unit will be that of density or pressure

Speed of the sound wave
The speed of sound is defined as the distance which a point on a wave, such as a compression or a rarefaction, travels per unit time
$v= \frac {\lambda}{T}$
$v= \lambda \nu$


  • Based on the amplitude of a sound wave, we can determine the loudness of the sound. When the amplitude is high, it will produce a sound that is loud and when the amplitude is low, it will produce a sound that is soft.
  • Loudness is proportional to the square of the amplitude. This means that if the amplitude is doubled, the loudness increases four times.


  • Pitch denotes the shrillness or flatness of a sound. Sound can be high or low.
  • A woman’s voice generally has a high pitch than a man’s voice. This is because the frequency of a woman’s voice is higher.

Quality (Timbre):

The quality of a sound is that property by virtue of which two sounds of the same pitch and loudness produced by the two different musical instrument or people can be distinguished.

Reflection and absorption of sound:

Sound bounces off a solid or a liquid like a rubber ball bounces off a wall. Like light, sound gets reflected at the surface of a solid or liquid and follows the same laws of reflection

The echo:

Sound heard after reflection from a surface is called echo.
Just like heat or light, when sound falls on a surface, it is partly reflected and partly absorbed.
Soft surfaces are better absorbers of sound whereas hard surfaces are better reflectors of sound.

A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation

Uses of multiple reflection of sound

  1. Echo is used in SONAR (Sound Navigation and Ranging) to find the depth of seas or distance of submarines.
  2. In concert halls, echo is desirable to some extent, because it enhances the sound and produces a pleasing effect.
  3. Bats use the principle of echolocation to avoid hitting against obstacles in their path.
  4. Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound

Range of Hearing

  • Audible soundsare those that can be heard whileinaudible soundsare those that cannot be heard.
  • Human can hear sounds with frequency between 20Hz and 20,000Hz.
  • Low frequency sounds which cannot be hear arecalled infrasonics.
  • Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. Whales and elephants produce sound in the infrasound range. It is observed that some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before the main shock waves begin which possibly alert the animals
  • Objects that vibrate at frequencies of above 20,000Hz produce sound which also cannot be heard by us. Such sounds are calledultrasonics.
  • Ultrasound is produced by dolphins, bats and porpoises

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Also Read

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Reference Books for class 9 science

Given below are the links of some of the reference books for class 9 science.

These books by S.Chand Publications are detailed in their content and are must have books for class 9 students.
  1. Science for Ninth Class Part1 Physics
  2. Science for Ninth Class Part2 Chemistry
  3. Science for Ninth Class Part3 Biology
Some other books that might be worth considering are
  1. Oswaal CBSE Question Bank Class 9 Science Chapterwise & Topicwise We all know importance of practicing what we have studied. This book is perfect for practicing what you have learned and studied in the subject.
  2. Pearson Foundation Series (IIT-JEE/NEET) Physics, Chemistry, Maths & Biology for Class 9 (Main Books) | PCMB Combo These Foundation books would be helpful for students who want to prepare for JEE/NEET exams. Only buy them if you are up for some challenge and have time to study extra topics. These might be tough for you and you might need extra help in studying these books.
  3. Foundation Science Physics for Class - 9 by H.C. Verma This is one of my favorite Physics book for class 9. Most of the book is within the limits of CBSE syllabus. It might overwhelm you with its language but I feel if you can understand the content is authentic with plenty of problems to solve.

You can use above books for extra knowledge and practicing different questions.

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