SOUND, CBSE CLASS IX, PHYSICS NOTES PART II

  

CBSE CLASS IX, SOUND, SCIENCE (PHYSICS) NOTES-(PART II)

                               SOUND

According to the CBSE Syllabus 2025-26

CBSE Class 9 Science Chapter 12 Sound Notes

Remaining Topics of this Chapter

10. Velocity

11. Speed of sound in various medium

12. Sonic Boom

13. Reflection of Sound

14. Echo

15. Reverberation

16. Range of Hearing

17. Hearing Aid

18. Application of Ultrasound

19. SONAR

20. Structure of the Human Ear

a. Working of the Human Ear

QUALITY OF SOUND

The richness or timber of sound is called quality. Sounds with the same pitch and loudness can be distinguished based on their quality. Music is pleasant to the ears, while noise is not. However, they both can have the same loudness and pitch.

 SPEED OF SOUND IN VARIOUS MEDIUMS

Sound travels through different media at different speeds. The speed of sound depends on the properties of the medium, pressure, density, and temperature.

Speed of Sound: Solids > Liquids > Gases

When a source emits sound at a speed greater than the speed of sound in air, it creates a sonic boom, which produces shockwaves with a lot of energy. These shockwaves generate a very loud noise, enough to shatter glass and damage buildings.

a. The speed of sound depends on the nature of the material through which it travels. It is slowest in gases, faster in liquids, and fastest in solids.

b. The speed of sound increases as the temperature rises.

c. The speed of sound increases with higher humidity in the air.

d. The speed of light is faster than that of sound.

e. In air, the speed of sound is 344 m/s at 22ºC.

Speed of sound in air = 331 m/s at 0ºC and 344 m/s at 22ºC.

SONIC BOOM

Some aircraft, bullets, rockets, etc. have ‘supersonic speed’.
• Supersonic refers to the speed of an object that is greater than the speed of sound, and it produces thunderous sound waves called ‘shock waves’ in air.
a. Sonic boom is an explosive noise caused by shock waves.
b. It emits tremendous sound energy, which can shatter the glass panes of windows.

REFLECTION OF SOUND

Like light, sound also bounces back when it falls on a hard surface. It is called the reflection of sound.
• The laws of reflection of light are obeyed during the reflection of sound.
(i) The incident sound wave, the reflected sound wave, and the normal at the point of incidence lie in the same plane.
(ii) The angle of reflection of sound is always equal to the angle of incidence of sound.

ECHO

The repetition of sound caused by the reflection of sound waves is called an echo. E.g., Clapping or shouting near a tall building or a mountain.
a. We can hear an echo when there is a time gap of 0.1 second between the original sound and the echo (reflected sound).
b.  Echo is produced when sound is reflected from a hard surface (i.e., brick wall, mountain, etc.) as a soft surface tends to absorb sound.
Minimum distance to hear an echo
Speed = Distance/Time
Here, the Speed of sound in air = 344 ms^-1 at 22ºC
Time = 0.1 second
344 = Distance/0.1 sec
⇒ Distance = 344 × 0.1 = 34.4 m
So, the distance between the reflecting surface and the audience = 34.4/2 = 17.2 (at 22ºC).
c. Rolling of thunder is due to multiple reflections of the sound of thunder from a number of reflecting surfaces, such as clouds and the earth.

APPLICATIONS OF REFLECTION OF SOUND

(i) Megaphone, loudspeakers, bulb horns and trumpets, shehnai, etc., are designed to send sound in a particular direction without spreading all around. 
> All these instruments have a funnel tube that reflects sound waves repeatedly towards the audience. In this, the amplitude of sound waves adds up to increase the loudness of sound.
(ii) Stethoscope: It is a medical instrument used for listening to the sounds produced in the human body, mainly in heart and lungs. The sound of the heartbeats reaches the doctor’s ears through the multiple reflections of the sound waves in the rubber tube of the stethoscope.
(iii) Sound Board: In big halls or auditoriums, sound is absorbed by walls, ceiling, seats, etc. So a curved board (sound board) is placed behind the speakers so that his speech can be heard easily by the audience. The soundboard works on the multiple reflection of sound.
 (iv) The ceiling of concert halls is made curved, so that sound, after reflection from the ceiling, reaches all the parts of the hall.

RANGE OF HEARING

 (i) The range of hearing in hearing is 20 Hz to 20000 Hz.
> Children younger than 5 years and dogs can hear up to 25 KHz.
(ii) The sounds of frequencies lower than 20 Hz are known as ‘infrasonic sounds’.
> A vibrating simple pendulum produces infrasonic sounds.
> Rhinoceroses communicate with each other using frequencies as low as 5 Hz.
> Elephants and whales produce infrasonic waves.
> Earthquakes produce infrasonic waves (before shock waves)
which some animals can hear and get disturbed.
(iii) The sounds of frequencies higher than 20 kHz are known as ‘ultrasonic waves’.
> Dogs, dolphins, bats, and rats can hear ultrasonic sounds.
> Bats and rats can produce ultrasonic sounds.

 HEARING AID

(a) It is battery operated electronic device used by persons who are hard of hearing.
(b) The microphone converts sound into electrical signals, and then those are amplified by an amplifier. Amplified signals are sent to the speaker of the hearing aid. The speaker converts the amplified signal to sound and sends it to the ear for clear hearing.

SONAR (Sound Navigation And Ranging)

(a) The word ‘SONAR’ stands for ‘Sound Navigation And Ranging’.

(b) SONAR is a device that is used to find the distance, direction, and speed of underwater objects.

(c) SONAR consists of a transmitter and a receiver, or a detector, and is installed at the bottom of a ship.

(d) It is a technique that uses sound or ultrasonic waves to measure distance. The human range of hearing is 20Hz- 20kHz.                                                        

(e) These waves travel through water and, after striking the objects on the bottom of the sea, are reflected back and received by the detector.                            

(f) These reflected waves are converted into electric signals by the detector.                                     

(g) The sonar device measures the time taken by ultrasound waves to travel from the ship to the bottom of the sea and back to the ship.                     

(h) Half of this time gives the time taken by the ultrasound waves from the ship to the bottom.

Let the time interval between transmission and reception of the ultrasound signal be t.
Speed of sound through sea water is v
Total distance travelled by waves = 2d.
Then, 2d = v × t.
This is called echo ranging.

(i) The sonar is used to find the depth of the sea, to locate underwater hills, valleys, submarines, icebergs, and sunken ships, etc.
(j) Bats fly in the dark night by emitting high-pitched ultrasound waves, which are reflected from the obstacle or prey and returned to the bat's ear.
(k) The nature of reflection tells the bat where the obstacle or prey is and what it is like.

 What Are Ultrasonic Sounds?

Ultrasonic sounds are high-frequency sounds having a frequency greater than 20kHz (inaudible range).

APPLICATIONS OF ULTRASOUND

(i) Scanning images of human organs.                                              

(ii) It is used to detect cracks in metal blocks in industries without damaging them.

(iii) It is used in industries to clean ‘hard to reach’ parts of objects such as spiral tubes, odd-shaped machines, etc.

(iv) It is used to investigate the internal organs of the human body, such as the liver, gall bladder, kidneys, uterus, and heart.

(v) Ecocardiography: These waves are used to reflect the action of the heart, and their images are formed. This technique is called echocardiography

(vi) Ultrasonography: The technique of obtaining pictures of internal organs of the body by using echoes of ultrasound waves is called ultrasonography.

(vii) Ultrasound is used to split tiny stones in the kidneys into fine grains

(viii) Navigating, communicating, or detecting objects on or under the surface of the water (SONAR).

Sonar consists of a transmitter and detector mounted on a boat or ship. The transmitter sends ultrasonic sound waves to the seabed, which get reflected back and picked up by the detector. Knowing the speed of sound in water, distance can be measured using:   2d=v×t. This method is called echolocation or echo ranging.

REVERBERATION

The persistence of sound because of multiple reflections is called reverberation. Examples: Auditorium and a big hall.

Excessive reverberation is undesirable, and to reduce this, halls and auditoriums have sound-absorbing materials on the walls and roofs.  E.g., Fibreboard and rough plaster.

→ If reverberation is too long, sound becomes blurred, distorted, and confusing due to the overlapping of different sounds.

Methods to reduce reverberation in big halls or auditoriums

a. Panels made of felt or compressed fibreboard are put on walls and ceiling to absorb sound.
b. Heavy curtains are put on doors and windows.
c. Carpets are put on the floor.
d. Seats are made of material having sound-absorbing properties.

Doppler’s Effect

If either the source of sound or the observer is moving, then there will be a change in frequency and wavelength for the observer. The frequency will be higher when the observer moves towards the source, and it decreases when the observer moves away from the source.
Example: If one is standing on a street corner and an ambulance approaches with its siren blaring, the sound of the siren steadily gains in pitch as it comes closer and then, as it passes, the pitch suddenly lowers.

HUMAN EAR

The ear is a sensitive organ of the human body. It is mainly involved with detecting, transmitting and transducing sound and maintaining a sense of balance is another important function of the human ear. The human ear includes:

WORKING OF THE HUMAN EAR

·         Pinna → Ear canal → Ear drum → Hammer → Anvil → Stirrup → Oval window → Cochlea → Auditory nerve 

    a. The outer ear, or the visible part of the ear, is called the pinna. 
b. Pinna collects sound from the surroundings. 
c. Sound passes through a tube called the auditory canal. 
d. The eardrum (tympanic membrane) vibrates in response to incident sound waves. 
e. Vibrations are amplified and transmitted further by three bones hammer, anvil and stirrup in the middle ear to the inner ear. 
f. In the inner ear, the cochlea converts pressure signals into electrical signals. 
g. These electric signals are sent to the brain via auditory nerve and the brain interprets them as sound. 
g. Electrical signals are transmitted by the auditory nerve to the brain for interpretation.

STRUCTURE OF HUMAN EAR

a. The ear consists of three parts: the outer ear, the middle ear, and the inner ear.                          

b. The ears are the sense organs that help us hear sound.                           

c. The outer ear is called the pinna. It collects the sound from the surroundings.   

d. This sound passes through the auditory canal.                                               

e. At the end of the auditory canal is a thin elastic membrane called ear eardrum or tympanic membrane.

f. The middle ear consists of three bones: the hammer, anvil, and the stapes, linked with one another. The free end of the hammer touches ear eardrum, and that of the stirrup is linked with the membrane of the oval window of the inner ear.                                                         
g. The lower part of the middle ear has a narrow ‘Eustachian tube’.                                                  
h. The inner ear has a coiled tube called the cochlea, which is connected with the oval window. The cochlea is filled with a liquid containing nerve cells.                                                                 
i. The other side of the cochlea is connected to the auditory nerve, which goes to the brain.