Sound Class 9th Science NCERT
In the article below, we share Sound class 9th Science summary along with NCERT Solutions for class 9th Sound to help the students understand the lesson’s gist and go through the summary to understand the chapter better. This article will give you the proper understanding of the chapter Sound class 9th and some crucial diagrams to help you learn quickly.
Sound Class 9th: Introduction
You have learned that a sound is a form of energy. It is produced by vibrations. Sound waves are longitudinal waves. They are also elastic waves hence they need a material medium for their transmission. They cannot be transmitted in a vacuum. They travel in solids, liquids, and gases. Their velocity is maximum in solids and least in gases. We hear various kinds of sounds in our daily life, pleasant sounds called musical sounds, unpleasant sounds called noise, loud sound, high-pitched sounds, etc.
In this chapter, let us study the difference between pleasant and unpleasant sounds and the factors upon which the loudness, pitch, etc depend.
Sound Class 9th : Sound as a Wave
A ringing bell, a thunderclap, laughter, and rock music are sounds that seem very different to us. However, all sounds are alike because they are waves. Let us understand how wave properties can be applied to sound.
Sound is the form of energy propagated as waves that our ears receive. When we speak, our vocal cords vibrate. When we play the guitar, the spring on it makes to and fro motion and produces sound. A tuning fork also produces sound due to its vibrations. So, a body produces sound by its vibrations.
Sound waves cannot travel through a vacuum, i.e., they need a material medium to travel. You can hear because when sound waves reach your ears, the waves make your eardrums vibrate. Nerves then send to your brain the messages about the vibrations. The brain interprets the messages as sound.
Propagation of Sound
Propagation of sound waves in the air from a tuning fork
A wave motion, in which the particles of the medium oscillate about their mean positions in the direction of propagation of the wave, is called a longitudinal wave. Sound waves are classified as longitudinal waves. Let us now see how sound waves propagate.
- Take a tuning fork, vibrate it and concentrate on the motion of one of its prongs, say prong A. The normal position of the tuning fork and the initial condition of air particles is shown in fig
(a). As prong A moves towards the right, it compresses air particles near it, forming a
compression as shown in fig
- (b). Due to vibrating air layers, this compression moves forward as a disturbance. As prong A moves back to its original position, the pressure on its right decreases, thereby forming a rarefaction.
- This rarefaction moves forward like compression as a disturbance. As the tuning fork vibrates, waves consisting of alternated compressions and rarefactions spread in the air, as shown in fig
- (d). The direction of motion of the sound waves is the same as that of air particles; hence they are classified as longitudinal waves. The longitudinal waves travel in the form of compressions and rarefactions.
Sound Needs a Medium to Travel
The origin of the sound is always some vibrating body. In some cases, the source’s vibrations may be so small or very large that it may not be possible to detect them. This type of vibration is produced by a tuning fork, drum, bell, guitar string, etc. The human voice originates from the vibrations of the vocal cords and the sound from the musical instruments due to the air columns’ vibrations. Sound travels in the form of longitudinal waves, requiring a material medium for its propagation.
Experiment to show that sound waves (mechanical waves) require a material medium for their propagation
Electric bell suspended inside an airtight glass bell jar
An electric bell is suspended inside an airtight glass bell jar connected to a vacuum pump. As the electric bell circuit is completed, the sound is heard. Now, if the air is slowly removed from the bell jar by using a vacuum pump, the intensity of sound decreases, and finally, no sound is heard when all the air is drawn out. We would be seeing the hammer striking the gong repeatedly. This proves that sound requires a material for its propagation.
Sound can propagate not only through gases but also through solids and liquids. Some materials like air, water, iron, etc can easily transmit sound energy from one place to another. On the other hand, materials like blankets and thick curtains absorb most of the sound energy.
Basic Terms Connected to Waves
The four essential terms used in the study of waves are Wavelength, Amplitude, Frequency, and Wave velocity.
Wavelength is the distance between two consecutive points on the wave in the same phase. (The same phase means the same state of vibrations)
Amplitude is the maximum displacement of the particle from its mean position.
Frequency is the number of periodic oscillations completed in one second. The frequency f = 1/T, where ‘T’ is the time taken to complete one oscillation. The unit of this measure is hertz [Hz]. Wave velocity ‘v’ is the velocity with which the energy is propagated in a medium.
As the wavelength is the distance covered during one oscillation and frequency is the number of oscillations per second, the product of the wavelength and frequency would give us the wave velocity.
Distance traveled in 1 s = number of waves in one-second x wave length
Wave velocity = Frequency × Wavelength
or, v = f (x)
Speed of Sound
The flash of lightning due to the collision of clouds is seen much before the thunder, although both occur simultaneously. This happens because light’s velocity is more significant than sound’s. The speed of sound depends on the properties of the medium through which it travels.
The medium can vary in (1) elasticity, (2) density, (3) pressure, and (4) temperature. The speed of sound decreases as it moves from a solid to a gaseous state. But in any medium, the speed of sound increases with an increase in temperature. The speed of sound at a particular temperature in various media is listed in the table.
Speed of sound in different media at 250C
Reflection of Sound
When sound is incident on a solid or a liquid surface, it bounces off the surface like light rays. Sound waves also obey the laws of reflection and refraction. For sound waves to reflect, we need an extended surface or obstacle of enormous size. For example, the rolling of thunder is due to successive reflections from clouds and land surfaces.
According to the law of reflection of sound, the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface, and the three lie in the same plane.
Like all waves, sound waves can be reflected. Sound waves suffer reflection from the large
obstacles. As a result of reflection of a sound wave from a significant obstacle, the sound is heard, which is named an echo. Ordinarily, echo is not heard as the reflected sound merges with the original sound.
Certain conditions must be satisfied to hear an echo distinctly (as a separate sound). The sensation of any sound persists in our ear for about 0.1 seconds. This is known as the persistence of hearing. If the echo is heard within this time interval, the original sound and its echo cannot be distinguished.
So the most essential condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 seconds after the original
sound dies off. As the speed of sound is 340 m/s, the distance traveled by sound in 0.1 seconds is 34 m. This is twice the minimum distance between a source of sound and the reflector. So, if the obstacle is at a distance of 17 m at least, the reflected sound or the echo is heard after 0.1 seconds, distinctly.
Further, for the reflection of any wave to take place, the size of the reflector should be large compared to the wavelength of the sound, which for ordinary sound is of the order of 1 meter. A large building, a mountainside, a large rock formation, etc., are good sound reflectors for producing an echo.
Also, for the reflected sound to be heard, it must have enough intensity or loudness. Moreover, the two should not mix or overlap if the echo is distinguished from the original sound. For this, the original sound should be of a very short duration, like a clap or shout.
So, the following conditions could be listed for the formation of echo:
• The size of the obstacle/reflector must be large compared to the wavelength of the incident sound (for reflection of sound to take place).
• The distance between the source of sound and the reflector should be at least 17 m (so that the echo is heard distinctly after the original sound is over).
• The intensity or loudness of the sound should be sufficient for the reflected sound reaching the ear to be audible. The original sound should be of short duration.
Advantages and Disadvantages of Echoes
• Echoes can be helpful or a nuisance. In a concert hall, echoes can ruin a performance if the walls and ceiling are not correctly designed. If the walls are too hard or too flat, they make suitable reflecting surfaces for the sound waves.
• Echoes can be used to give vital information. A sonar (Sonar stands for sound navigation
ranging) device sends high-frequency sound waves from a ship to find out how close the
vessel is to the sea bed. An ultrasound scanner, mainly known for giving images of the
unborn baby, works roughly the same way.
• Bats use echoes to navigate as they fly in the night. This works on the same principle as
sonar and ultrasound scanners. The bat sends out tiny, high-pitched squeaks, which bounce off the objects in the bat’s flight path. The echoes reach the bat, which adjusts its course to avoid the obstacles. Many bats have very large ears to catch as much reflected sound as possible.
It is called echolocation when animals such as bats and dolphins use echoes. They use it to find their way around or to locate prey. Echolocation describes the way how some
animals detect the size and position of objects around them.
At night, bats use echolocation to guide them in flight. They send out tiny ‘clicks,’ which
bounce off objects and return to the bat. It builds up a ‘sound’ picture of its surroundings.
Due to the reflection of light, a sound created in a big hall will persist until it is reduced to a value where it is no longer audible.
This persistence of audible sound due to the successive reflections from the surrounding objects even after the source has stopped producing that sound is called reverberation.
There should not be excessive reverberation. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fiber-board, rough plaster, or draperies.
Practical Applications of Reflection of Sound
Some applications of the principle of reflection of sound are:
• Hearing Board
• Sound Boards
Megaphone: A megaphone is a horn-shaped tube. The sound waves are prevented from spreading out by successive reflections and are confined to the air in the tube.
Hearing aid: It is a device used by people who are hard of hearing. Here the sound waves received by the hearing aid are reflected into a narrower area leading to the ear.
Sound Boards: Curved surfaces can reflect sound waves. Sound waves are reflected in an auditorium to spread the waves uniformly throughout the hall. Reflection of sound waves is done by using Sound Boards. The speaker is located at the focus of the soundboard.
Musical Sound and Noise
A musical sound is a pleasant continuous, uniform sound produced by regular and periodic vibrations.
Example: The pleasant sound produced by a guitar, piano, tuning fork, etc.
Noise can be defined as an irregular succession of disturbances that are discordant and
unpleasant to the ear. Bats and dolphins can detect the presence of an obstacle by hearing the echo of the sound produced by them. This process is called sound ranging.
Range of Hearing
Sound waves are emitted from a vibrating source and transmitted through the air. The human ear can hear sound waves between 20 Hz and 20 kHz. This range is known as the audible range. The sound waves having frequencies above the audible range are known as ultrasonic waves, usually referred to as ultrasound. The sound waves having frequencies less than the audible range are called infrasonic waves.
Applications of Ultrasound
• It is used for medical diagnosis and therapy and as a surgical tool.
• Bats and porpoises use ultrasound for navigation and to locate food in darkness.
• It is used to detect a defective fetus.
• It is used as a tool in the treatment of muscular pain.
• Ultrasonography (is a technique of 3-dimensional photographs with the help of ultrasonic
waves) is used to locate the exact position of an eye tumor.
• Ultrasound is generally used to clean spiral tubes, electronic components, etc.
• Ultrasounds are used to detect cracks and flaws in metal blocks.
One of the most critical applications of the reflection of sound is oceanographic studies. For this purpose, we use a system called the SONAR. The SONAR is an abbreviated form of Sound Navigation and Ranging. The SONAR system is used to detect unseen underwater objects, such as submerged submarines, sunken ships, and icebergs, and locate them. In Sonar, ultrasonic waves are sent in all directions from the ship and are then received on their return after reflection.
Determination of the depth of an ocean
The depth of an ocean is determined with the help of SONAR. Sonar uses ultrasonic waves to detect and locate objects under water. Ultrasonic waves produced from the transmitter kept in a ship are directed towards the ocean floor. The ocean floor reflects these waves.
By measuring the time interval t between the generation of the wave and reception of the echo, we calculate the depth of the ocean by using the relation
S = v ×t 2, Where v is the velocity of ultrasonic waves.
• Echolocation is a method of sensory perception by which certain animals orient themselves
to their surroundings, detect obstacles, communicate with others, and find food.
• Bats use echolocation to navigate in the dark and find food.
• A bat emits short, high-pitched ultrasonic waves from its mouth or nose.
• These sound waves travel away from the animal.
• Then, they bounce off the objects in the animal’s path, creating an echo.
• A bat can determine the size and shape of the obstacle in its path, the direction of the prey, and the direction of motion.
• This echolocation system is so accurate that bats can detect insects, the size of gnats, and objects as fine as human hair.
• Like bats, dolphins also emit high-frequency sound waves and can detect obstacles in
• Thus, dolphins can avoid fishing nets and detect fish at night or even in muddy water, which it is impossible to see.
Structure of Human Ear
Ears are extremely sensitive devices with the help of which we can hear.
The ear consists of three essential parts – the outer ear, the middle ear, and the inner ear. Each part of the ear has a specific role in the task of detecting and interpreting sound. The outer ear is called the pinna. It collects and transmits the sound to the middle ear through the auditory canal.
At the end of the auditory canal is a thin membrane called the eardrum or tympanic
membrane. The eardrum moves inward and outward as the compression or rarefaction reaches it. In this way, the eardrum vibrates. These vibrations are amplified by the three bones, namely the hammer, anvil, and stirrup in the middle ear.
The middle ear transmits these vibrations to the inner ear. The cochlea converts the vibrations or pressure variations into electrical signals inside the inner ear. These electrical
signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.
Sound Class 9th: Mind Map
We are sharing the mind map for Sound Class 9th to enhance the retention capacity of the students for the lesson Sound Class 9th so that the concepts are retained for longer.
NCERT Solutions for class 9th Sound Science
Q1. How does the sound produce by a vibrating object in a medium reach your ear?
When a body vibrates, the air in its neighborhood is alternately compressed and rarefied. Compressed air has higher pressure than surrounding air. It, therefore, pushes the air particles near it, causing compression to move forward. A rarefaction or low pressure is created at the original place. These compressions and rarefaction cause particles in the air to vibrate about their mean position. The energy is carried forward in this vibration. This is how sound travels.
Q2. Explain how sound is produced by your school bell.
When the gong strikes the bell, vibrations are produced in the bell, which is transmitted
through the air to our ears. These vibrations produce the sensation of sound in our ears.
Q3. Why are sound waves called mechanical waves?
Sound waves are called mechanical waves because they need a material medium to travel.
Q4. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?
On the moon, sound cannot travel as there is no atmosphere. Sound cannot travel in a vacuum, so we will not be able to hear any sound.
Q5. Which does wave property determine
(a) loudness and (b) pitch?
a) Loudness is determined by the amplitude of the sound. The greater the amplitude
more will be the loudness.
b) Pitch is determined by frequency. Higher the frequency, greater will be the
Q6. Guess which sound has a higher pitch: guitar or car horn?
Q7. What are wavelength, frequency, time period and amplitude of a sound wave?
Wavelength: The distances between two consecutive compressions or rarefaction of a wave.
Its S.I unit is meter.
Frequency: One compression and one rarefaction constitutes one vibration. The number of vibration in a second is called frequency. Its unit is Hertz.
Amplitude: When waves are produced, the particles vibrate about their mean position. The maximum displacement from its mean position of a particle is called its amplitude. It is measured in meters.
Time period: The time taken by the wave to complete one oscillation i.e., the time between two consecutive compressions or rarefactions is called time period.
Speed = Wavelength x frequency
V = λ × ν
Q9. Distinguish between loudness and intensity of sound.
Loudness and intensity both depend upon the amplitude of the sound. But loudness is the physiological response of our ears to a particular frequency. Our ears are more sensitive to some frequencies as compared to others. Intensity is the amount of sound energy passing per second per unit area. It is proportional to the square of the amplitude.
Q10. In which of the three media, air, water, or iron, does sound travel the fastest at a particular temperature?
Sound travels faster in iron and slowest in air.
Q11. Why are the ceilings of concert halls curved?
The ceilings of concert halls are curved so that after reflections from the surface, sound can reach each and every part of the hall.
Q11. What is the audible range of the average human ear?
Audible range 20 Hz − 20,000 Hz
Q12. What is the range of frequencies associated with (a) Infrasound? (b) Ultrasound?
(a) Infra-sound less than 20 Hz
(b) Ultra-sound greater than 20 000 Hz.
We have shared all about Sound Class 9th science notes along with essential diagrams and NCERT Solutions for Class 9th sound to help the students learn the facts of the class 9th sound chapter and better understand the lesson.