Class 9 Science: Sound

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Class 9 Science - Sound

All the chapters that are taught in Class 9 Science subjects impart fundamental concepts of Science in the most simplified manner, with loads of examples and exercises to make it interesting for the students. One such chapter that holds importance in Class 9, as well as advanced levels of Science, is sound under Unit 3: Motion, Force, and Work. In this Class 9 chapter 12, students would learn the definition of Sound, how it is produced, its characteristics, what are waves, how sound travels through air, the Laws of Reflection of Sound, and other relevant topics. Therefore, if you are a class 9 student and want radical solutions for science NCERT class 9 subject, especially for the chapter Sound, read this whole blog on sound notes for class 9 with a clear mind.

Meaning of Sound

A form of energy that creates a sensation of hearing in our ears and makes us hear something is called sound. Sound can neither be created nor destroyed. A person can produce a sound by converting a form of energy into sound energy. For example, the sound produced by a ringing phone is possible when electrical energy is transformed into sound energy.


Production, Propagation, and Medium of Sound

When an object vibrates or a form of energy of some outside source, such as wind, hand, etc., assists in vibrating the object, a sound is produced or made. For instance, when the two vocal cords in a living being’s throat vibrate, a sound is produced, which is known as the voice of the living being. A melodious sound from a flute is produced when its air column vibrates because of the air that passes through it.

Propagation of sound means traveling or movement of sound. And, the substance that helps sound travel is called the medium. The particles that are present in and around the medium causes motion in each other (and not forward or backward) that results in traveling of sound. Since sound cannot travel in a vacuum, it requires a medium that can be in the form of solid, liquid, or even gas.  

Sound Waves and its Types

A disturbance that is produced in a sound’s medium when its particles vibrate is known as a wave. The perfect examples of wave are Heat, Light, and sound. Based on the direction of propagation, there are two types of waves, i.e. Longitudinal Waves and Transverse Waves.

Characteristics of Sound

As given in the solutions for science NCERT class 9, there are 5 characteristics of sound, namely Frequency, Wavelength, Amplitude, Time Period, and Velocity. 

Characteristics of sound

Amplitude is denoted by A, and its SI unit is meter (m)

The Greek letter “Lambda” (ƛ) symbolizes Wavelength whose SI unit is meter (m)

Frequency is denoted by v (nu)

The letter “T” denotes Time Period, the reciprocal of which is the Frequency of a wave
which is, v = 1/T

The flatness or shrillness present in a sound is called Pitch that is dependent on the frequency of

Loudness means measuring sound energy that reaches our ears every second. The measuring unit of Loudness is “decibel” (dB)

Difference between Sound and Noise

Speed of Sound in Different Mediums

The density of the medium through which sound travels decides the speed of the sound. A sound travels at its fastest speed through solids, faster when traveling through liquids, and slowest when using gases as its medium to travel. With a rise in temperature, the speed of the sound increases too. Also, when there is an increase in the humidity of the air, there is an increase in the speed of sound as well.  

Laws of Reflection of Sound

The striking or falling of sound waves upon a hard surface causes them to return back through the same is a phenomenon known as a reflection of sound. The laws of reflection of sound that obeys the laws of reflection of light are as follows:

  1. The angle of reflection is equivalent to the angle of incidence, and
  2. The three sound waves at the point of incidence, i.e. normal, incident, and reflected lie or fall in the same place.

There are numerous instruments that work following multiple reflections of sound, such as loudspeakers; musical instruments, like shehnais, trumpets, etc.; stethoscope; large in size auditoriums, and halls.

Reflection of Sound
Reflection of Sound


Another topic which is a part of class 9 Science Sound is Echo. When a person is able to hear back his/her own sound, then such a situation is referred to as the echoing of sound. Sounds echo when it successively gets reflected from large and hard surfaces or obstacles. There are two primary conditions in order to hear a sound echo:

  1. The time gap between the actual sound and the reflected sound must be of 0.1 seconds, and
  2. To satisfy the above condition, the minimum distance between you and the reflecting surface must be of 17.2 meters at 22℃. This minimum distance is also dependent on the room temperature.


The persistence or repetition of sound waves for a prolonged period of time due to continuous (non-stop) reflections of sound is denoted as reverberation. If reverberation stays over a longer period of time, the sound gets impacted, which makes it confusing, blurred, and distorted because of the overlapping of various sounds. Therefore, to avoid reverberation, materials that absorb soft sound, like plant fibre, curtains, carpets, compressed fiberboard, etc., are used in such places viz auditoriums.

Range of Hearing

The audible range of hearing of a human ear is 20 Hz and 20,000 Hz. This means that a human can clearly hear those sound waves, which have a frequency of 20 Hz to 20,000 Hz. Sound frequencies that are lower than 20 Hz are called “infrasonic waves,” and those above 20 Hz are called “ultrasonic sound waves.” The examples given in sound notes for class 9 explains that an elephant, whales, earthquakes (before shock waves), the vibration of the pendulum produces infrasonic sound waves. Such sounds can only be heard by some animals. Whereas, only bats and rats can produce ultrasonic sounds, which are audible by them, dogs, dolphins, and porpoises. 

Ultrasound and its Application

Sound waves of frequencies higher than 20,000 Hz are defined as ultrasound. Such high-frequency sound waves that are produced in ultrasound have more energy that can penetrate to a very large extent. This special attribute of ultrasound creates its utility for various purposes, some of which are as follows:

  1. Ultrasound is widely used in detecting deformities, like cracks in metal sheets or blocks.
  2. Bats make an ultrasound to search their prey. The sound of their ultrasonic squeaks strikes the prey’s body or any other object that reflects the sound waves and returns to the bat’s ear. This way, bats come to know about the distance between him and his prey.
  3. Ultrasound waves are a common surgical tool and a technique in medical therapy and diagnosis. Like, during the pregnancy period, it is through ultrasound that gynaecologists are able to check the development of the fetus or to break down the stones into small grains that are developed in the kidneys. 
  4. Ultrasonography  
  5. Echocardiography     


Sound Navigation And Ranging, abbreviated as SONAR is a device that finds its use or application when looking for distance, speed, and direction of underwater objects, such as icebergs, water hills, submarines, valleys, sunken ships, etc. a SONAR device that is installed at the bottom/base of a ship comes with a transmitter and a receptor inside it. The work of the transmitter is to produce and send ultrasonic waves, which travel through water and get reflected back after it strikes the objects that are present on the floor of the sea. These reflections of ultrasonic waves are then received by the receptor. The detector converts these reflected waves into electric signals. The work of the SONAR device is to calculate the time that ultrasonic waves take to travel from ship to the seafloor and vice-versa, i.e. back to the ship.

So, 2d = v x t,
where t = time interval between transmission of the ultrasound and its reception

v = sound speed through seawater
2d = the total distance that the waves travel
The above formula is called echo ranging.      

Placement of the SONAR device
Placement of the SONAR device

Structure of Human Ear

A human ear is made up of three parts viz outer ear, middle year, and the inner ear. We know that the human ear is that sense organ of a human body that helps them hear a sound. The outer ear part of the human ear is called pinna that does the work of collecting sound from its surroundings. The outer ear passes this sound through the auditory canal, at the end of which you are a thin elastic membrane termed as a tympanic membrane or eardrum. 

The second part, i.e. the middle ear comprises three bones that are hammer, anvil, and stirrup, which are all linked up with one another. The hammer bone has a free end that meets the eardrum and that part of the stirrup bone, which is connected with the membrane of the oval window present in the inner ear. There is a narrow “Eustachian Tube” at the lower segment of the middle ear.

The cochlea, a coiled tube, is situated in the inner ear, linked with the oval window. This cochlea is full of liquid that consists of nerve cells. The auditory nerve, which straight goes to the brain, is connected with the other side of the cochlea. 

The Structure of a Human Ear
The Structure of a Human Ear

The flow of sound in a human ear is as follows:

Sound —> Pinna —> Ear Canal —> Eardrum —> Hammer —> Anvil —> Stirrup —> Oval Window —> Cochlea —> Auditory —> Nerve Brain

Important NCERT Questions and Answers

How does the sound produced by a vibrating object in a medium reach your ear?

The most prevalent medium for sound propagation is air. When vibrating items, such as tuning fork prongs, travel forward, they push the molecules of air in front of them. This, in turn, compresses the air, resulting in a zone of high pressure and density known as compression. The air compresses as it moves ahead. When the tuning fork prongs travel backward, they generate an area of low pressure in the air, which is known as rarefaction. This area has low pressure, a low density, and a higher volume. As the tuning fork continues to vibrate, the compression and rarefaction zones in the air alternate. These zones alternate at the same location. The force of a ringing bell spreads outward. This energy that enters the ears causes the eardrums to vibrate, allowing us to perceive sound.

Why are sound waves called mechanical waves?

To make an item vibrate, some mechanical energy is necessary. Sound energy cannot be created by itself. The mechanical energy of a vibrating item passes via a medium before reaching the ear. As a result, sound waves are referred to as mechanical waves.

Which wave property determines (a) loudness, (b) Pitch?

(a) The loudness is determined by the amplitude of the wave; the greater the amplitude of a wave, the greater the loudness generated.
(b) The frequency of the wave determines the pitch. The higher the frequency of a wave, the higher its pitch and the shriller the sound.

Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m s-1 in a given medium.

Given Frequency = 220 Hz
Speed of sound = 440 m s-1
We know that the speed of sound = Frequency x Wavelength = 220 x Wavelength
Wavelength = Speed/Frequency = V/ϑ  = 440/220
Hence, Wavelength = 2m

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

Sound travels faster in iron than in water or air.

What is the audible range of the average human ear?

An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.

Important Class 9 Sound Questions

  1. Explain the application of SONAR. 
  2. Define longitudinal waves with examples. 
  3. With the help of an activity, prove that sound needs a medium to travel. 
  4. How does a human ear function? Explain with the help of a well labled diagram. 
  5. Two strings of wire are taken to produce notes of the same pitch and loudness. However, their quality differs to a great extend. What is the reason behind it? 
  6. What is a wave and how many types of wave are there? Explain with examples. 
  7. How can frequency and amplitude affect a musical sound? 
  8. What is an echo and how is it produced? 
  9. What is ultrasound? How is it used? 
  10. What is the difference between loudness and intensity of the sound? 
  11. Why are the ceilings of concert halls round? 
  12. Define wavelength and its symbol. 
  13. A source is producing 1500 sounds waves in 3 seconds. If the distance covered by compression and an adjacent rarefaction be 68 cm, find (a) frequency (b) wavelength and (c) velocity of sound wave. 
  14. What are the different characteristics of sound? 
  15. What is resonance?
  16. What is the range of frequencies associated with (a) Infrasound? (b) Ultrasound?
  17. Cite an experiment to show that sound needs a material medium for its propagation.
  18. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?
  19.  A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 ms-1.
  20. Two children are a± opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in the air and in aluminium to reach the second child.

Thus, the above comprehensive yet precise sound notes for class 9 will serve as the best article to revise the whole chapter and get a stronghold over it. Seeing the importance of this concept, the marks distribution is high for this chapter. If you are looking for more such solutions for Science NCERT class 9 chapters, please visit Leverage Edu.

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