CBSE Class 9 Science, Sound: Chapter notes (Part-II)

Here you will get the CBSE Class 9 Science: Sound - Chapter Notes (Part-II). This part is in continuation with the ‘Sound - Chapter Notes (Part-I)’ In this part you will get a brief explanation of the remaining topics of the chapter.

Class 9 Science Chapter Notes Part-II
Class 9 Science Chapter Notes Part-II

In this article you will get the CBSE Class 9 Science, Sound: Chapter notes (Part-II). This part is a continuation of the part-I of Chapter Notes on Sound where we discussed about Sound, its production, propagation and other relevant topics.

CBSE Class 9 Science, Sound: Chapter notes (Part-I)

In this part all remaining topics have been covered.

Main topics covered in this part of CBSE Class 9 Science, Sound: Chapter Notes, are:

  • Echo
  • Reverberation
  • Infrasonic and Ultrasonic Waves
  • Applications of Ultrasound
  • Sonar
  • Working of Sonar
  • Human Ear: Structure and Working

These chapter notes are prepared by the subject experts to bring you a crisp and brief explanation of all important topics so as to make your learning easy and organized. At the end of the notes you can try the questions asked from the discussed set of topics. These questions will help you to track your preparation level and get a hold on the subject.

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Also Read: CBSE Class 9 Complete Study Material for 2020-2021

Keynotes for Chapter - Sound, are:


  • Phenomenon of hearing back our own sound is called an echo.
  • It is due to successive reflection of sound waves from the surfaces or obstacles of large size.
  • To hear an echo, there must be a time gap of 0.1 second in original sound and the reflected sound.

Necessary Conditions for the formation of an Echo
There are two conditions to experience the echo of sound -
(i) Time gap between the Sound must come back to the person after 0.1 second.
(ii) For above condition, the reflecting surface must be at a minimum distance of 17.2m. It also depends on temperature.

Also Read: CBSE Class 9 Science Syllabus (Latest)


  • Persistence of sound wave for a long time because of repeated (multiple) reflections of sound are called reverberation.
  • In big concert halls, due to the reverberation, sound may become blurred and distorted to be heard.
  • To avoid reverberation, soft sound absorbent materials, such as curtains, plant fibre, compressed fireboard, carpets, etc. are used in the auditorium.

Light Travels Faster Than Sound
Speed of light in air = 3 × 108 m/s
Man hears sound of thunder only after he sees the flash of lightning. This is because light travels at a very great speed as compared to the speed of sound, we see the flash before the sound.

Infrasonic and Ultrasonic Waves

Infrasonic Waves:

  • The waves having freqnecy less than 20 Hz are infrasonic waves.
  • A vibrating simple pendulum produces infrasonic sounds.
  • Elephants and whales produces infrasonic waves.
  • Earthquakes produces infrasonic waves

Ultrasonic Waves:

  • The waves having frequency more than 20,000 Hz are ultrasonic waves.
  • Bats and rats can produce ultrasonic sounds.

Applications of  Ultrasound
These are the sound waves having frequency more than 20,000 Hz. Due to the high frequency, ultrasound is associated with more energy and can penetrate upto a large extent. This characteristic of ultrasound makes it very useful for many purposes. Some of its uses are stated below:

  • They are used to detect any deformities (flaws, cracks) in metal blocks or sheets.
  • They are used to clean the hard to reach parts of machinery, like spiral tubes.
  • Bats use ultrasoung waves tp find their prey. Bats produce high- pitched ultrasonic squeaks. These squeaks are reflected by objects such as prey’s and are returned to the bat's ear. This allows a bat to know the distance of its prey.
  • Dolphins use ultrasound to find fish and to detect sharks that may attack them.
  • Ultrasound waves are commonly used for medical diagnosis and therapy, and also as a surgical tool. 
  • They are used to check the development of foetus during pregnancy to detect any abnormalities.
  • They find their application in breaking stones formed in the kidneys into fine grains.
  • Ecocardiography: These waves are used to reflect the action of heart and its images are formed. This technique is called echocardiography.
  • Ultrasonography: The technique of obtaining pictures of internal organs of the body by using echoes of ultrasound waves is called ultrasonography.


  • Sonar stands for Sound Navigation And Ranging.
  • It is a device which is used to find distance, direction and speed of underwater objects like, water hills, valleys, icebergs, submarines, sunken ships etc.

Working of SONAR


  • SONAR consists of a transmitter and a receptor or detector and is installed at the bottom of a ship.
  • The transmitter produces and transmits ultrasonic waves.These waves travel through sea water and after striking the objects on the bottom of sea, are reflected back and received and recorded by the detector.
  • The sonar device measures the time taken by ultrasound waves to travel from ship to bottom of sea and back to ship.
  • Half of this time gives the time taken by the ultrasound waves to travel from ship to sea bed.
  • Let the time interval between transmission and reception of ultrasound signal is t.

           Speed of sound through sea water is v

           Total distance travelled by waves = 2d.

           Then, 2d = v × t.

           This method of measuring distance is also known as ‘echo-ranging’.

Human Ear
Structure of Human Ear
The human ear can be divided into three main parts: Outer ear, middle ear and inner ear.

Structure of Human Ear

Outer Ear: The outer ear is outside the body and is also called pinna. It extends into the ear canal. Ear canal is filled with air.
Middle Ear: The middle ear is composed of the ear drum or tympanunn( it is an elastic membrane, circular in shape) and the bone ossicles. There are three bone ossicles, namely, the hammer, the anvil and the strirrup.
Inner Ear: The internal ear is composed of a cochlea and three semi-circular canals. The cochlea is filled with liquid. The cochlea makes the hearing apparatus and the auditory nerve from it goes to the brain.
Eardrum is the intersection of the outer and middle ear.
Oval window is the intersection of middle and inner ear.

Working of Human Ear: The outer ear called pinna collects the sound waves. The sound waves passes through the ear canal to a thin membrane called eardrum. The eardrum vibrates. The vibrations are amplified by the three bones of the middle ear called hammer, anvil and stirrup. The middle ear then transmits the sound waves to the inner ear. In the inner ear the sound waves are converted into electrical signals by the cochlea and sent to the brain through the auditory nerves. The brain then interprets the signals as sound.

Try the following questions:

Q1. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?

Q2. An echo is returned in 3 s. What is the distance of the reflecting surface from the source, given the speed of sound is 342 ms-1

Q3. Why are the ceilings of concert halls curved?

Q4. A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff?

Q5. An explosion takes place at the moon. After what time would it be heard at the earth?

Q6. How does the sound produced by a vibrating object in a medium reach your ear?

Q7. How do bats navigate even in the dark?

Q8. A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff?

Also Read: CBSE Class 9 Science Chapter Notes - All Chapters


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