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

CBSE Class 9 Science notes for chapter 12 - Sound are available here. This is the first part of the notes in which you will get a crisp and brief explanation of important topics like production and propagation of sound, medium, waves, characteristics of sound, range of sound, reflection of sound and its applications.

Class 9 Chapter Notes on Sound
Class 9 Chapter Notes on Sound

Here, you will get the CBSE Class 9 Science, Sound: Chapter notes (Part-I). These chapter notes are prepared by the subject experts and cover every important topic from the chapter. You get a crisp and brief explanation of all these 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.

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

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

  • Sound-Production and Propagation
  • Medium
  • Wave and its Types
  • Characteristics of Sound
  • Audible Frequency
  • Reflection of Sound and its Laws
  • Applications of Reflection of Sound

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

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Keynotes for Chapter - Sound, are:


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

Production of Sound
Sound is produced due to the vibration of objects.
Vibration is a periodic back-and-forth motion of the particles of an elastic body or medium about a central position. It is also named as oscillation.
For example:

  • Stretched strings of a guitar vibrate to produce sound.
  • When membrane of a table is struck, it vibrates to produce sound.

CBSE Class 9 Science Syllabus (Latest)

Propagation of Sound

  • The travelling of sound is called propagation of sound.
  • Sound is propagated by the to and fro motion of particles of the medium.
  • When an object vibrates, the particles around the medium vibrate. The particle in contact with the vibrating object is first displaced from its equilibrium position. Each particle disturbs the other particle in contact. Thus, the disturbance is carried from the source to the listener.

Propagation of sound

Only the disturbance produced by the vibrating body travels through the medium but the particles do not move forward themselves.


  • The matter or substance through which sound is transmitted is called a medium.
  • A medium is necessary for the propagation of sound waves.
  • The medium can be solid, liquid or gas.
  • Sound cannot travel in vacuum.

Wave that requires medium to propagate is called Mechanical wave.


Wave is a phenomenon or disturbance in which energy is transferred from one point to another without any direct contact between them. For example: Heat, light and sound is considered as a wave.

Types of Waves
On the basis of direction of propagation, waves are categorized into two parts:

(i) Longitudinal waves: These are the waves in which the particles of the medium vibrate along the direction of propagation of the wave. For example: sound wave.
(ii) Transverse waves: In this type of wave the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave. For example: waves produced in a stretched string.
Another type of waves which do not require any medium for propagation are called electromagnetic waves. These waves can travel through vacuum also. For example, light waves, X-rays.

Compressions and Rarefactions:

  • Compression is the part of wave in which particles of the medium are closer to one another forming the region of high pressure and density.

          Compressions are represented by the upper portion of the curve called crest.

  • Rarefaction is a part of the wave where particles spread out to form a region of low pressure and density.

          Rarefactions are represented by the lower portion of the curve called trough.

Compression and Rarefaction

Characteristics of a sound wave

Sound waves have following characteristics:
(1) Amplitude
(2) wavelength
(3) Frequency
(4) Time period



  • The maximum displacement of each particle from its mean position is called amplitude.
  • It is denoted by A.
  • Its SI unit is metre (m).


  • The distance between two nearest (adjacent) crests or troughs of a wave is called its wavelength.
  • It is denoted by the Greek letter lamda (λ).
  • Its SI unit is metre.


  • The number of vibrations per second is called frequency.
  • The SI unit of frequency is hertz (Hz).
  • The symbol of frequency is ν (nu).

Time period

  • The time taken to complete one vibration is called time period.
  • It is denoted by T.
  • Its SI unit is second (s).
  • The frequency of a wave is the reciprocal of the time period.

               i.e., v = 1/T


  • The distance travelled by a wave in one second is called velocity of the wave or speed of the wave.
  • Its S.I. unit is metres per second (m/s).
  • Velocity = Distance travelled/Time taken

⟹      v = λ/T  , where λ = wavelength of the wave travelled in one time time period T)

⟹      v = λv      (As 1/T = v)

Thus, Velocity = Wavelength × Frequency

v = λv is called the wave equation.

Pitch and loudness of Sound

Pitch: It represents shrillness or flatness of sound.

It depends on the frequency of vibration. Higher the frequency of sound wave, the higher will be the pitch of sound and vice-versa.

Loudness: It is a measure of the sound energy reaching the ear per second.

It depends on the amplitude of the sound wave.

It is measured in decibel ‘dB’.

Music And Noise
It is the sound that is pleasant to hear. For example: Sound coming out of musical instruments)
Noise: It is the sound that is unpleasant to hear. For example: Sound produced by vehicles.

Tone and Note
A pure sound of single frequency is called tone.
Note: An impure sound produced by mixture of many frequencies is called a note. For example: A musical note has tones of various frequencies.

Audible Frequency

The audible range of human ear is 20 Hz and 20,000HZ, i.e., the human ears can hear only those waves whose frequency lies between 20 Hz and 20,000HZ.

Reflection of sound  

When sound waves strike a surface, they return back into the same medium. This phenomenon is called reflection.

Laws of reflection

There are two basic laws of reflection of sound:

(i) Angle of incidence is equal the angle of reflection.

(ii) The incident wave, the reflected wave and the normal all lie in the same plane.

Applications of  Reflection of Sound

Many instruments work on the basis of multiple reflections of sound:

  • Megaphone, loudspeakers, bulb horns, musical instruments like trumpets, shehnais etc.  are designed to send sound by multiple reflections in a particular direction due to which the amplitude of sound waves adds up to increase loudness of sound.
  • Stethoscope which is used by doctors to listen to the sounds from the human body, also works on principle of multiple reflections of sound. Here, sound of heartbeat reaches the doctor's ears by multiple reflections.
  • In big halls or auditoriums sound is absorbed by walls, ceiling, seats, etc. So a curved board (sound board) is placed behind the speaker. Then the voice of speaker suffers multiple reflections to increase its loudness so that his speech  can be heard easily by audiences.

Try the following questions:

Q1. Is sound wave longitudinal or transverse.

Q2. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a darkroom?

Q3. An airplane produces a sound wave with frequency of 5 KHz and wavelength 30 m. In how much time would the sound wave cover the distance of 4 Km?

Q4. A source is producing 15 waves in 3.0 s. The distance between a crest and a trough is 10.0 cm. Find : (a) the frequency, (b) the wavelength, and (c) the velocity of the wave.

Q5. A source is producing 1500 sounds waves in 3 seconds. If the distance covered by a compression and an adjacent rarefaction be 68 cm, find (a) frequency (b) wavelength and (c) velocity of sound wave.

Q6. A sound wave travels at a speed of 340m/s. If its wavelength is 2 cm, what is the frequency of the wave? Will it be in the audible range?

Q7. Can we hear the ringing of a mobile phone placed in a vacuum chamber?

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

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