Sound Notes Class 8 by Abhinav Sir

Sound - Class 8 CBSE Complete Chapter

1. Sound is Produced by a Vibrating Body

Definition: Sound is produced when an object vibrates. These vibrations create waves that travel through a medium and reach our ears.

Key Points:

Vibration: Rapid back and forth movement of an object
Sound Source: Any vibrating object that produces sound
Examples: Vibrating strings of guitar, vibrating drumhead, vibrating vocal cords

🎯 Real-life Examples:

• Guitar strings vibrate when plucked
• Drum membrane vibrates when struck
• Speaker cone vibrates to produce sound
• Tuning fork vibrates when struck

2. Sound Produced by Humans: Voice Box or Larynx

Larynx (Voice Box): The organ in the throat that produces sound in humans through the vibration of vocal cords.

Structure and Function:

Vocal Cords: Two stretched membranes in the larynx
Mechanism: Air from lungs passes through vocal cords, causing them to vibrate
Pitch Control: Muscles adjust the tension and length of vocal cords
Sound Modification: Tongue, teeth, and lips shape the sound

Important: Men typically have thicker and longer vocal cords than women, resulting in deeper voices.

3. Sound Needs a Material Medium for Propagation

Medium: A substance through which sound waves can travel. Sound cannot travel through vacuum.

Types of Media:

Medium	Speed of Sound (m/s)	Examples
Gases	343 (in air at 20°C)	Air, helium, carbon dioxide
Liquids	1482 (in water at 25°C)	Water, oil, alcohol
Solids	5000+ (in steel)	Wood, metal, glass

🧪 Bell Jar Experiment:

When an electric bell is placed in a bell jar and air is gradually removed, the sound becomes fainter and eventually disappears, proving sound needs a medium.

4. We Hear Sound Through Our Ears

Human Ear: The organ that detects sound waves and converts them into electrical signals for the brain to interpret.

Parts of the Ear:

Outer Ear: Pinna (collects sound), ear canal
Middle Ear: Eardrum (tympanum), three small bones
Inner Ear: Cochlea (converts vibrations to nerve signals)

Hearing Process:

Sound waves enter through pinna
Waves travel through ear canal
Eardrum vibrates
Vibrations amplified by middle ear bones
Cochlea converts to electrical signals
Brain interprets as sound

5. Amplitude, Time Period and Frequency of a Vibration

Amplitude

Amplitude: The maximum displacement of a vibrating object from its rest position.

• Determines the loudness of sound
• Greater amplitude = Louder sound
• Measured in meters (m)

Time Period

Time Period (T): The time taken to complete one vibration or oscillation.

Time Period (T) = 1/Frequency (f)
Unit: seconds (s)

Frequency

Frequency (f): The number of vibrations or oscillations per second.

Frequency (f) = 1/Time Period (T)
Unit: Hertz (Hz)

6. Loudness and Pitch

Loudness

Loudness: The characteristic of sound that depends on the amplitude of vibration.

Depends on amplitude
Measured in decibels (dB)
Higher amplitude = Greater loudness

Pitch

Pitch: The characteristic of sound that depends on the frequency of vibration.

Depends on frequency
Higher frequency = Higher pitch (shrill sound)
Lower frequency = Lower pitch (grave sound)

Sound Type	Frequency Range	Examples
Low Pitch	Low frequency	Lion's roar, drum beats
High Pitch	High frequency	Bird chirping, whistle

7. Audible and Inaudible Sounds

Audible Range: Sounds that human ears can hear (20 Hz to 20,000 Hz)

Types of Sounds:

Type	Frequency Range	Examples	Uses
Infrasonic	Below 20 Hz	Earthquake waves, elephant calls	Earthquake detection
Audible	20 Hz - 20,000 Hz	Human speech, music	Communication
Ultrasonic	Above 20,000 Hz	Bat calls, dolphin clicks	Medical imaging, SONAR

🦇 Applications of Ultrasound:

• Medical ultrasonography
• Cleaning delicate instruments
• SONAR (Sound Navigation and Ranging)
• Echolocation by bats and dolphins

8. Noise and Music

Music

Music: Pleasant sound produced by regular vibrations.

Produced by regular vibrations
Has a definite pattern
Pleasant to hear
Examples: Musical instruments, singing

Noise

Noise: Unpleasant sound produced by irregular vibrations.

Produced by irregular vibrations
Has no definite pattern
Unpleasant to hear
Examples: Traffic noise, machinery noise

9. Noise Pollution and its Harms

Noise Pollution: Unwanted or excessive sound that disrupts the environment and human activities.

Sources of Noise Pollution:

Transportation: Cars, trucks, airplanes, trains
Industrial: Factories, construction work
Social: Loudspeakers, festivals, parties
Household: TV, radio, kitchen appliances

Harmful Effects:

Health Effects:

Hearing loss and deafness
Sleep disturbance
Stress and anxiety
High blood pressure
Reduced concentration
Headaches and fatigue

10. Measures to Control Noise Pollution

Individual Measures:

Use headphones instead of loudspeakers
Keep TV and radio volume low
Use sound-absorbing materials
Plant trees around homes
Avoid honking unnecessarily

Government Measures:

Noise pollution laws and regulations
Industrial zoning
Sound barriers along highways
Limits on noise levels in different areas
Regular monitoring of noise levels

Area Type	Permissible Noise Level (Day)	Permissible Noise Level (Night)
Residential	55 dB	45 dB
Commercial	65 dB	55 dB
Industrial	75 dB	70 dB
Silence Zone	50 dB	40 dB

🎯 100+ PRACTICE QUESTIONS

A. Multiple Choice Questions (MCQs)

1. Sound is produced by:
a) Stationary objects
b) Vibrating objects
c) Moving objects
d) Heavy objects
Answer: b) Vibrating objects

2. The human voice is produced by the vibration of:
a) Tongue
b) Teeth
c) Vocal cords
d) Lips
Answer: c) Vocal cords

3. Sound travels fastest in:
a) Gases
b) Liquids
c) Solids
d) Vacuum
Answer: c) Solids

4. The audible range of human ear is:
a) 10 Hz to 10,000 Hz
b) 20 Hz to 20,000 Hz
c) 50 Hz to 50,000 Hz
d) 100 Hz to 100,000 Hz
Answer: b) 20 Hz to 20,000 Hz

5. Ultrasonic sounds have frequency:
a) Below 20 Hz
b) Between 20 Hz to 20,000 Hz
c) Above 20,000 Hz
d) Exactly 20,000 Hz
Answer: c) Above 20,000 Hz

6. The loudness of sound depends on:
a) Frequency
b) Amplitude
c) Time period
d) Wavelength
Answer: b) Amplitude

7. The pitch of sound depends on:
a) Amplitude
b) Intensity
c) Frequency
d) Loudness
Answer: c) Frequency

8. Sound cannot travel through:
a) Air
b) Water
c) Steel
d) Vacuum
Answer: d) Vacuum

9. The unit of frequency is:
a) Meter
b) Second
c) Hertz
d) Decibel
Answer: c) Hertz

10. Noise pollution can cause:
a) Hearing loss
b) Sleep disturbance
c) High blood pressure
d) All of the above
Answer: d) All of the above

11. The voice box is also called:
a) Pharynx
b) Larynx
c) Trachea
d) Epiglottis
Answer: b) Larynx

12. Which part of ear converts sound vibrations to electrical signals?
a) Eardrum
b) Pinna
c) Cochlea
d) Ear canal
Answer: c) Cochlea

13. Infrasonic sounds are produced by:
a) Birds
b) Bats
c) Elephants
d) Dolphins
Answer: c) Elephants

14. SONAR uses:
a) Infrasonic waves
b) Audible waves
c) Ultrasonic waves
d) Radio waves
Answer: c) Ultrasonic waves

15. The speed of sound in air at 20°C is approximately:
a) 330 m/s
b) 343 m/s
c) 350 m/s
d) 360 m/s
Answer: b) 343 m/s

B. Fill in the Blanks

Sound is produced by _______ objects. (vibrating)
The human voice is produced by the vibration of _______. (vocal cords)
Sound needs a _______ medium for propagation. (material)
The audible range for human beings is _______ Hz to _______ Hz. (20, 20,000)
Sounds with frequency below 20 Hz are called _______. (infrasonic)
Sounds with frequency above 20,000 Hz are called _______. (ultrasonic)
The loudness of sound depends on _______. (amplitude)
The pitch of sound depends on _______. (frequency)
The unit of frequency is _______. (Hertz)
Sound travels _______ in solids than in gases. (faster)
The time taken to complete one vibration is called _______. (time period)
Unwanted sound is called _______. (noise)
Pleasant sound is called _______. (music)
The outer part of ear is called _______. (pinna)
The eardrum is also known as _______. (tympanum)
SONAR stands for _______. (Sound Navigation and Ranging)
Bats use _______ for navigation. (ultrasonic waves/echolocation)
The voice box is located in the _______. (throat)
Sound cannot travel through _______. (vacuum)
The amplitude determines the _______ of sound. (loudness)
Higher frequency produces _______ pitched sound. (higher)
Lower frequency produces _______ pitched sound. (lower)
The maximum displacement from rest position is called _______. (amplitude)
One complete vibration is called one _______. (oscillation)
Sound intensity is measured in _______. (decibels)
Noise pollution causes _______ loss. (hearing)
Trees help in reducing _______ pollution. (noise)
The inner ear contains _______. (cochlea)
Frequency is the _______ of time period. (reciprocal)
In _______ zones, noise level should be minimum. (silence)

C. Hard Level Numerical Problems

Problem 1: A sound wave has a frequency of 500 Hz. Calculate its time period.
Solution:
Given: f = 500 Hz
We know: T = 1/f
T = 1/500 = 0.002 seconds
Answer: 0.002 s

Problem 2: If the time period of a vibrating object is 0.01 seconds, find its frequency.
Solution:
Given: T = 0.01 s
We know: f = 1/T
f = 1/0.01 = 100 Hz
Answer: 100 Hz

Problem 3: Sound travels at 343 m/s in air. How long will it take to travel 1715 meters?
Solution:
Given: Speed = 343 m/s, Distance = 1715 m
Time = Distance/Speed
Time = 1715/343 = 5 seconds
Answer: 5 seconds

Problem 4: A tuning fork vibrates 256 times in one second. What is its frequency and time period?
Solution:
Given: 256 vibrations in 1 second
Frequency = 256 Hz
Time period = 1/f = 1/256 = 0.0039 seconds
Answer: f = 256 Hz, T = 0.0039 s

Problem 5: An echo is heard after 4 seconds. If speed of sound is 340 m/s, find the distance of reflecting surface.
Solution:
Given: Time for echo = 4 s, Speed = 340 m/s
Total distance traveled by sound = Speed × Time = 340 × 4 = 1360 m
Distance of reflecting surface = 1360/2 = 680 m
Answer: 680 m

Problem 6: A sound wave travels 1500 m in 5 seconds in water. Calculate the speed of sound in water.
Solution:
Given: Distance = 1500 m, Time = 5 s
Speed = Distance/Time = 1500/5 = 300 m/s
Answer: 300 m/s

Problem 7: A vibrating body completes 2000 vibrations in 4 seconds. Calculate frequency and time period.
Solution:
Given: 2000 vibrations in 4 seconds
Frequency = 2000/4 = 500 Hz
Time period = 1/500 = 0.002 s
Answer: f = 500 Hz, T = 0.002 s

Problem 8: The frequency of a sound wave is 1000 Hz and its speed is 350 m/s. Find its wavelength.
Solution:
Given: f = 1000 Hz, v = 350 m/s
We know: v = f × λ, so λ = v/f
λ = 350/1000 = 0.35 m
Answer: 0.35 m

Problem 9: A bell rings every 2 seconds. What is the frequency of ringing?
Solution:
Given: Time period = 2 s
Frequency = 1/T = 1/2 = 0.5 Hz
Answer: 0.5 Hz

Problem 10: Sound takes 0.5 seconds to travel from one end of a steel rod to another. If speed of sound in steel is 5000 m/s, find the length of rod.
Solution:
Given: Time = 0.5 s, Speed = 5000 m/s
Distance = Speed × Time = 5000 × 0.5 = 2500 m
Answer: 2500 m

D. Short Answer Questions

Q: How is sound produced?
A: Sound is produced by vibrating objects. When an object vibrates, it creates waves in the surrounding medium.
Q: Why can't sound travel through vacuum?
A: Sound needs a material medium to travel. Since vacuum has no matter, sound waves cannot propagate through it.
Q: What is the difference between music and noise?
A: Music is pleasant sound with regular vibrations and definite pattern, while noise is unpleasant sound with irregular vibrations.
Q: How do we hear sound?
A: Sound waves enter the ear, vibrate the eardrum, get amplified by middle ear bones, and are converted to electrical signals by cochlea for brain interpretation.
Q: What are the harmful effects of noise pollution?
A: Hearing loss, sleep disturbance, stress, high blood pressure, reduced concentration, and health problems.
Q: How can we reduce noise pollution?
A: Use headphones, keep volume low, plant trees, avoid unnecessary honking, and follow noise regulations.
Q: What is the audible range of humans?
A: The audible range for humans is 20 Hz to 20,000 Hz.
Q: What are ultrasonic waves?
A: Ultrasonic waves are sound waves with frequency above 20,000 Hz, inaudible to human ears.
Q: What are infrasonic waves?
A: Infrasonic waves are sound waves with frequency below 20 Hz, inaudible to human ears.
Q: How does amplitude affect loudness?
A: Greater amplitude produces louder sound, while smaller amplitude produces softer sound.

E. Long Answer Questions

Q: Explain the structure and working of human ear.
A: The human ear has three parts: outer ear (pinna and ear canal), middle ear (eardrum and three bones), and inner ear (cochlea). Sound waves are collected by pinna, travel through ear canal, vibrate the eardrum, get amplified by middle ear bones, and converted to electrical signals by cochlea for brain interpretation.
Q: Describe how human voice is produced.
A: Human voice is produced in the larynx (voice box) by the vibration of vocal cords. When air from lungs passes through the stretched vocal cords, they vibrate and produce sound. The pitch can be changed by adjusting the tension and length of vocal cords using muscles.
Q: Explain noise pollution, its sources, effects and control measures.
A: Noise pollution is unwanted excessive sound. Sources include vehicles, industries, construction, loudspeakers. Effects include hearing loss, sleep disturbance, stress, high blood pressure. Control measures include sound barriers, regulations, plantation, proper urban planning.
Q: What are the characteristics of sound waves? Explain amplitude, frequency, and time period.
A: Sound waves have three main characteristics: Amplitude (maximum displacement, determines loudness), Frequency (vibrations per second, determines pitch, measured in Hz), and Time period (time for one complete vibration, reciprocal of frequency).
Q: Distinguish between audible, infrasonic, and ultrasonic sounds with examples and applications.
A: Audible sounds (20-20,000 Hz) can be heard by humans, like speech and music. Infrasonic sounds (<20 Hz) are produced by elephants and earthquakes, used in earthquake detection. Ultrasonic sounds (>20,000 Hz) are used by bats for echolocation and in medical imaging (SONAR).

F. Additional Practice Questions

True/False Questions:

Sound can travel through vacuum. (False)
Higher amplitude means louder sound. (True)
Bats use ultrasonic waves for navigation. (True)
Sound travels faster in gases than solids. (False)
The unit of frequency is decibel. (False)
Noise pollution can cause hearing problems. (True)
All vibrating objects produce audible sound. (False)
The voice box is called larynx. (True)
Time period and frequency are directly proportional. (False)
Sound waves are mechanical waves. (True)

Match the Following:

Column A	Column B	Answer
1. Amplitude	a. Hertz	1-d
2. Frequency	b. Voice box	2-a
3. Larynx	c. Above 20,000 Hz	3-b
4. Ultrasonic	d. Loudness	4-c
5. Cochlea	e. Inner ear	5-e

📋 COMPLETE ANSWER KEY

MCQs: 1-b, 2-c, 3-c, 4-b, 5-c, 6-b, 7-c, 8-d, 9-c, 10-d, 11-b, 12-c, 13-c, 14-c, 15-b

All numerical solutions and detailed answers are provided above with each question.

Remember: Practice these questions regularly and understand the concepts rather than memorizing answers.

🎯 Exam Tips:

Understand the relationship between frequency, time period, amplitude, loudness, and pitch
Remember the audible range and applications of ultrasonic and infrasonic waves
Practice numerical problems on speed, distance, time, frequency, and time period
Learn the structure of human ear and voice production mechanism
Understand noise pollution causes, effects, and control measures

Tuesday, July 29, 2025