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Sound

CBSE Class 9 Science • Chapter 11 • Detailed Master Notes

Chapter Overview:

Sound is a form of energy that produces a sensation of hearing. In this chapter, we will explore:

How sound is produced (Vibrations).
How it propagates through a medium (Compressions & Rarefactions).
Characteristics of sound waves (Frequency, Amplitude, Speed).
Reflection of sound (Echo, Reverberation).
Practical applications (SONAR, Structure of Ear).

1. Production of Sound

Sound is produced due to the vibration of objects. Vibration means the rapid to-and-fro motion of an object.

Examples of Sound Production:

Human Voice: Produced by vibrations of vocal cords in the larynx.
Guitar: Vibrations of strings.
Drum: Vibrations of the stretched membrane.
Flute: Vibrations of the air column.

Note: The energy required to make an object vibrate is provided by some outside source (like our hand hitting a drum).

2. Propagation of Sound

Sound waves are Mechanical Waves because they require a material medium (Solid, Liquid, or Gas) for their propagation. Sound cannot travel through a vacuum.

Mechanism of Propagation:

When an object vibrates, it sets the particles of the medium around it in vibration. These particles do not travel all the way from the source to the ear. instead:

A vibrating object pushes air particles forward, creating a region of high pressure called Compression (C).
When the object moves backward, it creates a region of low pressure called Rarefaction (R).
This series of Compressions and Rarefactions travels through the air as a Sound Wave.

Sound as Longitudinal Waves:

In sound waves, the individual particles of the medium move in a direction parallel to the direction of effective disturbance. Hence, sound waves are longitudinal.

Comparison: Light waves are Transverse waves (particles oscillate perpendicular to direction of propagation).

Experiment: Bell Jar Experiment

This experiment proves sound needs a medium.

An electric bell is placed inside an airtight glass jar connected to a vacuum pump.
As air is pumped out, the sound becomes fainter.
When perfect vacuum is created, no sound is heard, even though the hammer is still striking the gong.

3. Characteristics of Sound Wave

A sound wave can be described by its Frequency, Amplitude, and Speed.

1. Wavelength ($\lambda$):

The distance between two consecutive compressions (C) or two consecutive rarefactions (R).
SI Unit: Meter ($m$).

2. Frequency ($\nu$ - nu):

The number of complete oscillations (waves) passing through a point in one unit of time.
SI Unit: Hertz ($Hz$).

3. Time Period ($T$):

The time taken by two consecutive compressions or rarefactions to cross a fixed point.
Relation: $T = \frac{1}{\nu}$. SI Unit: Second ($s$).

4. Amplitude ($A$):

The maximum magnitude of displacement of particles from their mean position. It determines the Loudness of the sound.

Pitch vs Loudness:

Pitch: Depends on Frequency. (High frequency = High pitch/Shrill sound e.g., Women's voice).
Loudness: Depends on Amplitude. (High amplitude = Loud sound). Loudness $\propto A^2$.

Relationship between Speed, Frequency, and Wavelength:

Speed = Distance / Time

Since usually distance $\lambda$ is covered in time $T$:

$$ v = \frac{\lambda}{T} = \lambda \times \nu $$

Speed ($v$) = Wavelength ($\lambda$) $\times$ Frequency ($\nu$)

4. Speed of Sound in Different Media

Speed depends on the properties of the medium (Temperature, Density, Elasticity).

Speed is maximum in Solids > Liquids > Gases.
Effect of Temperature: Speed of sound increases with increase in temperature. (In air: $331 m/s$ at $0^\circ C$ and $344 m/s$ at $22^\circ C$).
Sonic Boom: When an object travels faster than the speed of sound, it produces shock waves carrying immense energy, called a sonic boom.

5. Reflection of Sound

Sound bounces off a solid or liquid surface following the Laws of Reflection:

Angle of incidence = Angle of reflection ($\angle i = \angle r$).
Incident wave, reflected wave, and normal all lie in the same plane.

(a) Echo

It is the repetition of sound caused by reflection of sound waves.

Condition for hearing distinct Echo:

The sensation of sound persists in our brain for about 0.1 seconds.
To hear a distinct echo, the reflected sound must reach the ear after 0.1s.
Speed of sound in air $\approx 344 m/s$.
Total distance traveled (Source to Wall + Wall to Ear) = $2d$.
$2d = v \times t \Rightarrow 2d = 344 \times 0.1 = 34.4 m$.
$d = 17.2 m$.

Therefore, the minimum distance between the source and the reflector must be 17.2 meters.

(b) Reverberation

The persistence of sound due to repeated reflection (even after the source stops) is called reverberation. It is often undesirable in concert halls.

How to reduce Reverberation:

Covering roofs and walls with sound-absorbent materials (Compressed fibreboard, rough plaster).
Using heavy curtains.
Upholstered seats.

6. Range of Hearing

Types of Sound	Frequency Range	Details
Infrasonic	< 20 Hz	Produced by earthquakes, volcanoes, elephants, whales. (Felt as vibrations).
Audible Sound	20 Hz - 20,000 Hz (20 kHz)	Humans can hear this range only. Sensitivity decreases with age.
Ultrasonic	> 20,000 Hz	Produced by bats, dolphins, porpoises. Used in medical scans (Ultrasound).

7. Applications of Ultrasound

Cleaning: Parts located in hard-to-reach places (spiral tubes) are cleaned using ultrasonic solutions.
Detection of Defects: To detect cracks and flaws in metal blocks.
Echocardiography: To image the heart.
Ultrasonography (USG): Used for examining fetus during pregnancy and other internal organs (kidney stones).
SONAR: (Sound Navigation And Ranging).

SONAR (Sound Navigation And Ranging)

A technique used to determine depth of sea or locate underwater hills/submarines using ultrasonic waves.

Working:

A transmitter sends ultrasonic waves.
Waves hit the seabed and get reflected.
A detector receives the waves and converts them to electrical signals.

$$ 2d = v \times t $$

Where $d$ = depth, $v$ = speed of sound in water, $t$ = time taken.

8. Structure of Human Ear

The ear allows us to convert pressure variations in air with audible frequencies into electric signals.

Outer Ear (Pinna): Collects the sound from surroundings and passes it through the auditory canal.
Middle Ear:
- Eardrum (Tympanic membrane): Vibrates when compactions/rarefactions reach it.
- Three Bones (Hammer, Anvil, Stirrup): Amplify these vibrations several times.
Inner Ear (Cochlea): A coiled tube that converts amplified vibrations into electrical signals.
Auditory Nerve: Transmits these electrical signals to the brain.

Practice Zone

Q1: A person claps his hands near a cliff and hears the echo after 4 seconds. What is the distance of the cliff from the person if the speed of sound, $v = 346 m/s$?

Ans: Time taken for echo return $t = 4 s$. Speed $v = 346 m/s$.
Distance travelled $= 2d = v \times t$.
$2d = 346 \times 4 = 1384 m$.
$d = 1384 / 2 = 692 m$.

Q2: A ship sends out ultrasound that returns from the seabed and is detected after 3.42 s. If the speed of ultrasound through seawater is 1531 m/s, what is the distance of the seabed from the ship?

Ans: $t = 3.42 s$, $v = 1531 m/s$.
$d = \frac{v \times t}{2} = \frac{1531 \times 3.42}{2} = \frac{5236.02}{2} = 2618.01 m$.

Q3: Why are the ceilings of concert halls curved?

Ans: Ceilings are curved so that sound after reflection reaches all corners of the hall evenly. This prevents dead spots and ensures clearer audibility.