Chapter 12: Class 9 Sound Notes

Best Class 9 Sound Notes

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Definition of Sound

What is Sound or Define Sound.

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

Production of Sound

Let us now talk about the production of a sound. How sound is produced?

Sound is produced by the vibration of object that produces sound or we can say sound is produced by vibrating objects.

Propagation of Sound

How sound is propagated or how the sound travels from one place to another?

Sound is a mechanical wave (longitudinal), so it needs a medium to travel from one place to another or from source to our ear.

When the source or the vibrating object vibrates, it sets the particles of the medium around it in motion or vibration, the particles of the medium move from its mean or equilibrium position and collide with the adjacent particles.

These adjacent particles also move from their mean position and collide with their adjacent particles. And this process keeps on going causing a disturbance in the medium.

Knowledge Booster: It is not the particles that move from the source to the destination (our ear) but the disturbance that is produced is travelled.

Particles of the medium just oscillates.


This disturbance is a series of compressions and rarefactions in which the sound waves travel or propagates through the medium from the sound source to the listener.

Define Compression and Rarefaction?

Compression (C)

When an object vibrates it compresses the particles around it and creates a high pressure region called Compression.

  • The Compression is also a high density region, as particles are very close to each other.

Rarefaction (R)

When the vibrating object moves backwards, it creates a region of low pressure called Rarefaction.

  • Rarefaction is a low density region, as particles move away from each other.

Sound travels fastest in solid than liquid and gas.


Sound cannot travel in vacuum.

Characteristics of a Sound Wave

A sound wave is characterized by its-

  • Wavelength
  • Amplitude
  • Frequency
  • Time Period
  • Speed


The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.


  • It is usually represented by the letter A.


It is defined as the number of vibrations or oscillations per second.

  • The SI Unit of frequency is Hertz (Hz).

Time Period

It is the time taken to complete one cycle or one vibration.

  • The SI Unit of time period is second (s).

What is the relation between time period and frequency?

The relation between time period and frequency is given by-

f = 1/T

ν = 1/T


Speed of sound is the distance travelled by it in 1 second.

The speed of sound can also be defined as the distance which a point on a wave, such as a compression or a rarefaction, travels per unit time.

Speed = distance/time

v = λ/T = λ×ν

Here λ is the wavelength of the sound wave and ν (nu) is the frequency

Reflection of Sound

Reflection of Sound is the bouncing back of sound waves from a surface.

When sound waves fall on any surface, it reflects back just like light. Large and Hard surfaces are good reflectors of light.

Sound waves also follow the same laws of reflection that governs the light.

Knowledge Booster: When we speak, the sound persists or remains in our brain for 0.1 second. So, the second sound can be heard only after 0.1 seconds.

Question: Which is a better reflector of sound, curtains or walls? Comment your answer in the below comment box.


It is a phenomena that takes place due to the reflection of Sound. So, Echo is an example of a reflected sound.

It is the repetition of same sound due to reflection of sound waves.

Note: The sound repeated in echo is distinct.

What is the formula for echo?

The formula to find echo is-

2 × Distance = Speed × Time

We do not hear echo all the time because it depends on –

  • temperature
  • minimum distance between source and reflector (17 m or 17.2 m)

The minimum distance should be 17 m to hear an echo if the surrounding has a normal temperature of 22° C.


Example- An echo is heard in 4 s. What is the distance of the reflecting surface from the source? (Take speed of sound 342 m/s)


  • Reverberation is the persistence of sound. It is the sensation of one prolonged sound.
  • The reflections must be within 0.05 seconds
  • Reverberation is due to the multiple reflection of sound. It is very common in halls and auditoriums.

Uses of Multiple Reflection of Sound

The below listed devices uses the technique of multiple reflection. The design of the devices is such that multiple reflection of sound takes place.

1. Megaphones, Horns – It is of conical shape that uses the technique of multiple reflection. This helps in magnification of the sound.

2. Stethoscope – Multiple reflection takes place in the tube of the Stethoscope. Doctors use this device to check the heart beat.

3. Curved Ceiling in Auditorium – The multiple reflection helps in reaching the sound from the podium to the audience.

Range of Hearing

What do you understand by Range of Hearing?

Range of hearing is the frequency of the sound that a human ear can listen, and the audible range of sound for human beings extends from about 20 Hz to 20000 Hz.

What are infrasound?

Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound.

Rhinoceros, Elephant, whales are some animals that communicate in this range.

What are Ultrasound?

The sounds having frequencies higher than 20 kHz are called ultrasonic sound or ultrasound.

Dolphins, Bats are some animals that produce ultra sound.

Define Sonic Boom?

Sonic Boom are very sharp and loud sound that is produced when a sound source moves with a speed higher than that of sound speed. It produces shock waves that carry large amount of energy.

Applications of Ultrasound

What are the applications of Ultrasound?

There are many applications of ultrasound. Let us see some of the applications of Ultrasound.

It is used in industries and for medical purposes.

  • Cleaning- for cleaning places that can’t be reached easily
  • used to scan internal organs for medical treatment
  • Used to find flaws in the materials
  • ultrasonic waves is used in SONAR


What is the Full form of SONAR?

SONAR – Sound Navigation And Ranging

What is the use of SONAR?

It is used to determine the depth of the sea and also to find threats like underwater hills, valleys, submarine, icebergs etc.

How does a SONAR work?

A sonar consists of a transmitter and a detector. The transmitter and the detector are attached at the bottom of the ship. The transmitter sends the ultrasonic sound waves and the detector detects or receives the sound waves after it is reflected back any object at the bottom of the sea.

The detector then converts the ultrasonic waves into electrical signals which are then interpreted.

The distance can be easily found using the formula 2×d = speed × time. Here ‘d’ is the distance between the ship and the object under the sea.

Question for You: Why in the above formula it is twice of distance?

Structure of Human Ear

The Human ear can be divided into 3 parts-

  1. Outer Ear (Pinna, Ear Canal, Eardrum)
  2. Middle Ear (Malleus, Incus, Stapes)
  3. Inner Ear (Cochlea, Auditory Nerves)

For more details about Human Ear – Click Here

Hope you like this Class 9 Sound Notes.

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