CBSE NCERT Notes for Class 9 Science Chapter 12 Sound

NCERT Solutions for Class 9 Science Chapter 12- Sound

NCERT Solutions Class 9 Science Chapter 12 – Free PDF Download
According to the Revised term-wise CBSE Syllabus 2021-22, this chapter has been removed.

We do know that matter is composed of particles organized in a certain manner. While the particles in gases are well separated and may move freely, the particles in solids are closely packed, with little space for movement.

CBSE Class 9 Science notes will assist students in studying the topic thoroughly and clearly.

These CBSE Class 9 Science notes were written by subject experts who made the study material very basic, both in terms of language and format.

Production of Sound

Vibrating objects generate sound. Vibration is a term that refers to an object’s fast back and forth motion. Vibrations in the vocal cords generate the sound of the human voice.

By hitting the tuning fork, plucking, scraping, rubbing, blowing, or shaking various objects, we may generate sound. They all generate sound as a result of vibrations.

Propagation of Sound

When an object vibrates, it causes the medium’s particles (solid, liquid, or gas) to vibrate as well. The particles do not go all the way to the ear from the vibrating object.

The equilibrium point of a medium particle in contact with the vibrating object is initially displaced. It then applies a force on the particle close to it. As a consequence, the adjoining particle is displaced from its resting position. The initial particle returns to its original place after displacing the adjacent particle.

This procedure is repeated throughout the medium until the sound reaches our ear.

The source of sound generates a disturbance in the medium through which the sound travels. The medium’s particles do not move ahead, but the disturbance does.

This is sound propagating across a medium, and hence sound may be seen as a wave. Because sound waves move through a medium, they are referred to as mechanical waves.

Formation Of Compression And Rarefaction In Air

When a vibrating object goes ahead in the air, it compresses and pushes the air in front of it, causing a compression that begins to move away from the vibrating object. Rarefaction occurs when a vibrating object moves backward.

Compression is the portion of a longitudinal wave in which the medium’s particles are closer together than they are typically, and it is also the region of greatest pressure. It is marked by C in the following diagram.

Rarefaction is the zone of a longitudinal wave in which the medium’s particles are closer together than they are ordinarily, and it is a region of low pressure. It is indicated by R in the following diagram.

As the object travels back and forth quickly, the air undergoes a succession of compressions and rarefactions.

Thus, sound propagation may be seen as the propagation of density or pressure variations in the media, since pressure is proportional to the number of particles in a medium. Increased particle density in a medium results in increased pressure and vice versa.

Sound Needs A Medium To Travel

A medium is the material through which sound travels. It may take the form of a solid, a liquid, or a gas. A sound wave must propagate through a material medium such as air, water, or steel. In a vacuum, sound waves cannot travel.

Types of Waves

There are mainly two types of waves.

Longitudinal Waves : In longitudinal waves, the medium’s individual particles travel in a direction parallel to the disturbance’s propagation direction. The particles do not travel from one location to another; instead, they oscillate back and forth between their resting places.

This is precisely how a sound wave propagates, and that is why they are referred to as longitudinal waves. In all three media, solids, liquids, and gases, longitudinal waves may be generated. The longitudinal waves are the waves that travel along a spring when it is pushed and pulled at one end.

Compressions (C) are found in springs when coils are closer together than usual. Rarefactions (R) are noticed when coils arc more apart than typical. A slinky is a long, flexible spring that may be compressed or expanded easily.

Transverse Waves : Transverse waves cause the medium’s individual particles to move in a direction perpendicular to the direction of wave propagation. For example, light is a transverse wave (but not a mechanical wave, since it does not propagate through a medium).

Transverse waves exist exclusively in solids and liquids; they do not exist in gases.

Transverse waves are created when one end of a long spring or rope is quickly moved up and down while the other end remains fixed.


When a sound wave travels through air, the density of the air constantly changes.

As seen below, a sound wave in air has been represented using a density-distance graph.

Terms to Describe Sound Waves

Sound waves are classified according to their frequency.

Wavelength : The wavelength is the distance between two consecutive compressions (C) or two successive rarefactions (R).

The wavelength of a sound wave is the shortest distance over which it may repeat.

In other words, it is the length of a compression combined by the length of an adjacent rarefaction. The Greek letter lambda A is used to denote it. The metric system uses the metre as its SI unit (m).

Frequency ; The frequency of a wave is defined as the number of complete waves (or oscillations) generated in one second. It is the frequency of vibrations per second.

Alternatively, the number of compressions or rarefactions that pass through a place in a unit period.

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A wave’s frequency is constant and does not alter when it travels through various things. It is symbolised by the letter v. (Greek letter, nu). Its SI unit is the hertz (symbol, Hz), which was named after Heinrich Rudolf Hertz, the inventor of the photoelectric effect.

Time Period : The time required for two successive compressions or rarefactions to cross a fixed point is referred to as the wave’s time period.

In other terms, the time necessary to generate a full wave (or series of oscillations) is referred to as the wave’s time period. It is symbolised by the letter T. It is the second SI unit (s). A wave’s time duration is proportional to its frequency.

Amplitude : The maximum displacement of the medium’s particles from their initial mean locations caused by the passage of a wave is referred to as the wave’s amplitude.

It is used to define the wave’s size. Typically, it is indicated by the letter A.

The metric system uses the metre as its SI unit (m). A wave’s amplitude is equal to the amplitude of the vibrating body that generated the wave.

Speed : The distance travelled by a wave in one second is referred to as the wave’s speed or velocity. The speed of sound stays equal for all frequencies under identical physical circumstances. It is symbolised by the letter v. It is denoted by the SI unit metre per second (m/s or ms -1).

The relationship between a wave’s speed, frequency, and wavelength:

Speed v = Distance / Time

Characteristics of Sound

Three qualities define a sound. These are the levels of volume, pitch, and quality (or timbre).

Loudness: It is a unit of measurement for the amount of sound energy that reaches the ear every second. The more sound energy that reaches our ear per second, the louder the sound appears.

When sound waves have a small amplitude, they are weak or quiet; when they have a large amplitude, they are loud. The waveforms of a loud and a gentle sound of the same frequency are shown in the figure above.

Because the amplitude of a sound wave is equal to the amplitude of the vibrations created by its source, the loudness of sound is proportional to the amplitude of the vibrations produced by its source.  Due to the increased energy involved with louder sound, it may travel a larger range.

As a sound wave propagates further from its source, its amplitude and loudness decrease. The decibel scale is used to express the loudness of a sound (dB). It is determined by our ears’ sensitivity or responsiveness.

Pitch Or Shrillness : It is the feature of sound that allows us to differentiate between sounds of similar intensity. We can tell the difference between a man’s and a woman’s voice of the same volume without seeing them.

The pitch of a sound is determined by the vibration’s frequency. The greater the frequency of a sound, the higher its pitch will be. In other words, the quicker the source vibrates, the higher the frequency and hence the higher the pitch, as seen in the image.

Thus, a higher pitch sound corresponds to a larger number of compressions and rarefactions per unit time travelling through a given location.

A sound with a low pitch has a low frequency, whereas a sound with a high pitch has a high frequency. Objects of various sizes and conditions vibrate at variable frequency, resulting in noises of different pitches.

Quality Or Timbre : The character of sound, or timbre, helps us to differentiate one sound from another with the same pitch and volume. A tone is a sound with a single frequency.

The sound created by the combination of various frequencies is referred to as a note, and it is also pleasant to listen to. Noise is unpleasant to the ear. The music is pleasing to the ear and of excellent quality.


The sound speed in air is about 344 ms-1 at 22°C and 331 ms-1 at 0°C, whereas the light speed in air is 300000000 ms-1 or 3108ms-1. Thus, the speed of light is far faster than the speed of sound.

This is why, during the rainy season, the flash of lightning is seen first and the sound of thunder is heard afterwards, due to the fact that both units are in clouds.


The medium through which sound travels might be solid, liquid, or gas. The speed of sound is determined by the qualities of the medium through which it travels and the medium’s temperature.

When we go from a solid to a gaseous state, the speed of sound falls. When the medium’s temperature rises, the speed of sound increases as well.


When an object travels faster than the speed of sound, it is said to have supersonic speed. Numerous things, including some aeroplanes, bullets, and rockets, move at supersonic speeds. When a source of sound travels faster than the speed of sound, it generates shock waves in the air that carry a great deal of energy.

The shock waves’ enormous air pressure changes generate a loud burst of sound known as a sonic boom.

It generates intolerably loud noise that causes ear pain.

The shock waves generated by a supersonic aircraft are powerful enough to break glass and potentially cause structural damage.

Reflection of Sound Wave

The act of sound bouncing back when it comes into contact with a hard surface is referred to as reflection of sound. It may be reflected off any smooth or rough surface. Sound, like light, is reflected according to the same laws of reflection:

  1. At the point of incidence, the incident sound wave (AO), the reflected sound wave (OB), and the normal (ON) all lie in the same plane.
  2. Sound’s angle of incidence is equal to its angle of reflection.


When a person speaks in a huge empty hall, the original sound is first heard, followed by the reflected sound of that shout. Thus, an echo is the repetition of sound induced by the reflection of sound waves.

Our brain retains the sensation of sound for around 0.1 second. Thus, for a distinct echo to be audible, the time delay between the original and reflected sounds must be at least 0.1 s.

This distance will alter as the temperature changes.

Due to successive multiple reflections, echos may be heard several times. Thunder rolls as a result of consecutive reflections of sound from a variety of reflecting surfaces, including clouds and ground.

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Reverberation is the persistence of a sound in a huge room as a result of repeated reflections off the walls, ceiling, and floor of the wall. This happens when the original and reflected sounds merge. Reverberation happens when reflection occurs at a distance of less than 17 metres.

In a performance hall where music is being performed, a quick reverberation time is ideal since it increases the sound level. However, excessive reverberation is very undesirable since it blurs, distorts, and confuses the sound as a result of the overlapping of multiple sounds.

To minimise reverberation, the auditorium’s roof and walls are often coated with sound-absorbing materials such as compressed fibre board, rough plaster, or drapes.

Uses Of Multiple Reflection Of Sound

Sound is reflected in the function of devices such as megaphones, horns, stethoscopes, and sound boards. These gadgets use numerous sound wave reflections.

  • Megaphone And Horn : A megaphone is a huge cone-shaped instrument that is used to amplify and guide the sound of the one speaking into it. When a person talks into the small end of the megaphone tube, the sound waves generated are stopped from spreading due to successive reflections from the broader end of the megaphone tube, allowing his voice to be heard across a greater distance.
  • Stethoscope : It is a medical device that doctors use to listen to the sounds of heart activity and lungs in the human body. The sound of the heart (or lungs) is sent to the doctor’s ears through repeated reflections of sound waves via the stethoscope tube.
  • Sound Board : It is a concave board (curved board) put behind the stage in large venues to ensure that sound travels equally over the width of the auditorium after reflecting off the sound board. Generally, concert halls, conference halls, and movie halls have curved ceilings to ensure that sound reaches all corners of the room after reflection.

Range of Hearing

The audible range is the average frequency range to which the human ear is sensitive. For humans, the hearing range of sound is between 20 Hz and 20,000 Hz (20 kilohertz).

Children under the age of five, as well as some animals such as dogs, have a hearing

range of up to 25000 Hz. Individuals’ ears become less sensitive to higher and lower frequencies as they age.

Infrasonic Sound : Infrasonic noises or infrasound are those with frequencies less than 20 Hz that are not audible to humans. Earthquakes and some animals such as whales, elephants, and rhinoceroses release infrasonic sounds with a frequency of 5 Hz.

It has been noticed that certain animals get upset and begin running in random directions immediately before earthquakes strike. This is because earthquakes generate low frequency infrasound prior to the main shock waves, which may notify animals and force them to run.

Ultrasonic Sound: Ultrasonic sounds or ultrasounds are those with frequencies greater than 20000 Hz that are audible to humans. Dogs have the ability to detect ultrasonic sounds with a frequency of up to 50,000 Hz. This is why police departments use canines for detective work. Ultrasonic sounds are produced by bats, dolphins, and porpoises.

Ultrasound and Its Applications

Ultrasounds are radio waves with a very high frequency. They go in a straight path and do not curve around bends. They have a high degree of penetration into matter. Ultrasound is employed in business and in hospitals for medical reasons because of these qualities.

The following section discusses some of the most significant uses of ultrasonography.

In Cleaning Minute Parts Of Machines: Ultrasound is used to clean sections that are difficult to reach, such as spiral tubes, irregularly shaped machinery, and electronic components. The objects to be cleaned are immersed in a cleaning solution, which is then penetrated by ultrasonic  waves.

Due to the high frequency of the ultrasound waves, they stir up the solution, causing dust, oil, and dirt particles to vibrate excessively, get detached from the item, and fall into the solution. Thus, the things are fully cleansed.

In Internal Investigation Of Human Body: Ultrasound is used to examine the human body’s internal organs, including the liver, gallbladder, pancreas, kidneys, uterus, and heart.

Ultrasound waves may enter the human body and are reflected differently by various kinds of tissues in areas with a variation in tissue density. By translating ultrasound into electrical impulses, ultrasonography allows us to view the human body and provide pictures of the inner organs.

Following that, these photographs or images are shown on a monitor or reproduced on film. This is referred to as ultrasonography. Ultrasonography is used to examine the baby during pregnancy in order to discover any growth abnormalities and to assist in correcting the problems.

The ultrasonic scanner is a device that assists the physician in detecting abnormalities such as gall bladder and kidney stones, turnouts in various organs, and a variety of other conditions. By examining the heart from the inside, ultrasound may also be used to diagnose heart diseases.

This is called echocardiography. Ultrasound may be used to shatter small kidney stones into fine grains that are then washed out with urine. In this manner, the patient is relieved of discomfort.

In Industries: In industry, ultrasound is used to identify faults (cracks, etc.) in metal blocks without causing damage to them.

Metal blocks are used to build large buildings such as bridges, machineries, and scientific equipment. If there are cracks and flaws in the metal blocks that are not apparent from the exterior, this diminishes the structure’s strength.

Ultrasound may be used to detect these. This is because an interior crack (or hole) prevents ultrasound from passing through it.

It reflects ultrasonic waves. Ultrasound waves are permitted to pass through one face of a metal block (to be tested) and are detected by detectors mounted on the other face of the metal block.

If even the smallest defect exists, the ultrasonic waves are reflected back, revealing the existence of the flaw or defect, as seen in the image.

Normal sound waves cannot be utilised to identify faults in metal blocks because they would bend around the faulty location’s corners and so enter the detector.

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Bats hunt for food and fly at night by producing and detecting ultrasonic vibrations. Echolocation is a technique used by certain animals such as bats, tortoises, and dolphins to identify items by hearing the echoes of their ultrasonic squeaks.

While flying, bats emit high-frequency or high-pitched ultrasonic squeaks and listen for echoes produced by their squeaks being reflected by obstacles or prey in their route.

Bats can assess the distance to an obstacle or prey based on the time it takes for the echo to be heard and may either avoid the impediment by adjusting their route or capture the prey.

Certain moths, on the other hand, can hear the bat’s high-frequency ultrasonic squeaks and may identify where the bat is flying nearby, allowing them to evade capture.


SONAR is an initials for Sound Navigation And Ranging. Sonar is a device used to determine the depth of a sea or to locate underwater objects such as shoals of fish, shipwrecks, or enemy submarines. It measures the distance, direction, and speed of submerged objects using ultrasonic waves.

Sonar Consists of Three Parts

  • a transmitter
  • a detector
  • a receiver (for detecting ultrasonic waves): as shown in figure.

Ultrasonic waves are generated and transmitted by the transmitter.

These waves go down the sea’s surface water toward the sea’s bottom. When an ultrasonic sound pulse touches the bottom of the water, it is reflected back and detected by the detector as an echo.

The detector transforms ultrasonic waves to electrical signals that may be interpreted appropriately. Calculating the distance between the object that reflected the sound wave requires knowledge of the sound wave’s speed in water and the time gap between transmission and receipt of the ultrasound.

This will provide us with the sea’s depth. Let the time interval between ultrasound transmission and reception be and the sound speed through seawater be v. If the ultrasound travels a total distance of 2d, then 2d = v x t.

This method is referred to as echo-ranging. Sonar is used to determine the sea’s depth and to find underwater hills, valleys, submarines, icebergs, and sunken ships, among other things.

Human Ear

The ears are the auditory sense organs that enable humans to hear sound. It enables humans to translate audible pressure fluctuations in the air to electrical signals that pass to the brain through the auditory nerve.


The ear is divided into three compartments: the external ear, the middle ear, and the inner ear.

Outer Ear: The outer ear is the portion of the ear that is visible outside the head. It is composed of a large section called the pinna and a 3 cm long channel called the ear canal. At the end of the ear canal, a thin, elastic, round membrane called the eardrum, also known as the tympanum or tympanic membrane, is present.

Middle Ear : The Middle Ear is composed of three small bones—a hammer, an anvil, and a stirrup—that are related to one another. One end of the hammer is in contact with the eardrum, while the free end of the stirrup is in contact with the oval-window of the inner ear.

The lower portion of the middle ear is connected to the throat by a small tube called the eustachian tube. It maintains the same air pressure within and outside the middle ear.

Inner Ear : The inner ear has a coiled tube called the cochlea. The cochlea is joined to the middle ear on one side by an elastic membrane. The cochlea is filled with fluids and includes nerve cells that are sensitive to sound. The opposite side of the cochlea is linked to the auditory nerve that travels to the brain.


Pinna catches sound waves from its environment. These gathered sound waves are conducted via the ear canal (auditory canal) and hit the eardrum. Because sound waves are longitudinal waves, they include compressions (regions of higher pressures) and rarefactions (low pressure regions).

When the medium is compressed to the eardrum, the pressure on the outside of the membrane (eardrum) rises, causing the eardrum to go inward.

Similarly, when a sound wave rarefies and hits the eardrum, the pressure outside the membrane (eardrum) lowers and travels outward. Thus, when sound waves hit the eardrum, it begins quickly vibrating back and forth.

These vibrations are magnified numerous times in the middle ear by the three bones (hammer, anvil, and stirrup) and then transmitted to the cochlea through CO, the liquid. As a result, the liquid in the cochlea starts to vibrate, and the cochlea converts the pressure fluctuations into electrical signals.

The auditory nerve transmits these electrical impulses to the brain. They are interpreted as sound by the brain, and we experience hearing.

NCERT questions & answers from Chemical Reactions and Equations

Why are sound waves called mechanical waves? (CBSE 2012)

Answer: Particles in a medium move to produce sound waves. Mechanical waves accurately describe the nature of sound.

Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Answer: The propagation of sound requires a material medium such as air. Lack of atmosphere on the moon means that communication via sound is impossible. As a result, noise is inaudible on the Moon.

Guess which sound has a higher pitch : guitar or car horn?

Answer: When compared to a car horn, the sound frequency of a guitar is much higher, so it is the clear winner.

What are wavelength, frequency, time period and amplitude of a sound wave? (CBSE 2012)


Wavelength (or length of a wave): The length of time between two compressions (areas of high pressure) or two rarefactions (areas of low pressure) is the wavelength of a sound wave.. It is denoted by λ (read as lambda).

In S.I., unit of wavelength is metre (m).

Frequency: The frequency of a sound wave is defined as the rate at which one cycle of compression or rarefaction passes through a given point in time.. It is denoted by μ (read as Neu). In S.I., unit of frequency is hertz (Hz).

1 hertz = one oscillation completed by a vibrating body or a vibrating particle in one second.

Time period: The interval between two compressions or rarefactions that occurs at a constant temperature. Amplitude. The greatest deviation from a body’s mean position or equilibrium position that can occur during a vibration.

In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature ?

Answer: Sound travels the fastest in iron.

Why are the ceilings of concert halls curved ? (CBSE 2011, 2012)

Answer: This will allow for the sound to be reflected off the ceiling and spread to the far reaches of the hall.

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