📘 Chapter 12 – Sound

🧠 Easy and Detailed Notes

🔹 1. Sound and Its Production

• Sound is produced by vibrating objects.
• Vibrations → produce sound → travels in medium.

🔹 2. Medium for Sound

• Sound needs a medium (solid, liquid, gas) to travel.
• Cannot travel in vacuum.

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🔹 3. Characteristics of Sound

Property	Description
Wavelength (λ)	Distance between two compressions or rarefactions
Frequency (f)	Number of vibrations/sec (Hz)
Time period (T)	Time taken for one vibration
Amplitude (A)	Maximum displacement of particles
Speed (v)	Distance travelled per second

📌 Formulae

• v = f × λ
• T = 1/f

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🔹 4. Types of Waves

• Longitudinal Waves: Particles vibrate parallel (like sound).
• Transverse Waves: Particles vibrate perpendicular (like light).

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🔹 5. Speed of Sound

Medium	Speed (m/s)
Air	343
Water	1500
Steel	5000

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🔹 6. Reflection of Sound

• Obeys laws like light:
  i) Angle of incidence = Angle of reflection
  ii) Incident ray, reflected ray, normal lie in same plane

📌 Echo: Repetition of sound after reflection

• Minimum distance for echo: 17.2 m

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🔹 7. Applications

• Stethoscope
• SONAR
• Bats for navigation
• Megaphones

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🔹 8. Audible & Inaudible Sounds

• Audible: 20 Hz – 20,000 Hz
• Inaudible:
  i) Infrasonic < 20 Hz
  ii) Ultrasonic > 20,000 Hz

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🔹 9. SONAR

• Full form: Sound Navigation and Ranging
• Uses ultrasonic waves for measuring distance underwater.

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✅ MCQs

1. Sound needs ______ to travel.
   a) Light
   b) Medium
   c) Vacuum
   d) Radiation
2. Unit of frequency is:
   a) m
   b) Hz
   c) dB
   d) s
3. Echo is heard when distance is more than:
   a) 20 m
   b) 10 m
   c) 17.2 m
   d) 100 m
4. Audible sound range for humans:
   a) 0–1000 Hz
   b) 1000–10,000 Hz
   c) 20–20,000 Hz
   d) Above 20,000 Hz
5. In longitudinal wave, particles vibrate:
   a) Perpendicular
   b) Circular
   c) Parallel
   d) No direction
6. Speed of sound is fastest in:
   a) Air
   b) Water
   c) Steel
   d) Vacuum
7. Sound cannot travel through:
   a) Air
   b) Water
   c) Vacuum
   d) Metal
8. SONAR uses which wave?
   a) Infrared
   b) Infrasonic
   c) Ultrasonic
   d) Microwave
9. Frequency = 500 Hz, Time period = ?
   a) 0.002 s
   b) 2 s
   c) 0.5 s
   d) 0.005 s
10. Wavelength = 1 m, Frequency = 1000 Hz. Speed = ?
    a) 100 m/s
    b) 1000 m/s
    c) 10 m/s
    d) 500 m/s
11. Sound wave is:
    a) Transverse
    b) Longitudinal
    c) Rectilinear
    d) Circular
12. Which part of ear collects sound?
    a) Cochlea
    b) Eardrum
    c) Pinna
    d) Canal
13. Which device is used in sonar?
    a) Magnet
    b) Ultrasonic transmitter
    c) Infrasonic sensor
    d) None
14. Sound travels in air at:
    a) 343 m/s
    b) 5000 m/s
    c) 100 m/s
    d) 20 m/s
15. Loudness depends on:
    a) Frequency
    b) Amplitude
    c) Wavelength
    d) Time

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✅ Assertion & Reason

A: Sound travels faster in solids.
R: Solids have tightly packed particles.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Echo is possible only after 17.2 m.
R: Sound reflects only after large distances.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Sound travels in vacuum.
R: Sound needs no medium.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Sound is a longitudinal wave.
R: It travels as compressions and rarefactions.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Ultrasonic sound is heard by humans.
R: Its frequency is above 20,000 Hz.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

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✅ Case-Based Study

📘 Passage:
In submarines, SONAR is used to measure depth. A transmitter sends ultrasonic waves. The time taken for echo is used to calculate distance.

21. SONAR full form:
    a) Sound Near Range
    b) Sound Navigation and Ranging
    c) Solid Navigation and Radar
    d) None
22. Type of wave used in SONAR:
    a) Radio
    b) Infrasonic
    c) Ultrasonic
    d) Microwave
23. What is used to calculate depth?
    a) Time of echo
    b) Mass
    c) Wavelength
    d) Loudness
24. What type of wave is sound?
    a) Mechanical
    b) Electrical
    c) Optical
    d) None
25. Sound travels fast in:
    a) Air
    b) Water
    c) Vacuum
    d) Steel

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✅ Reason-Based Questions

1. Why can’t sound travel in vacuum?
   Because no particles are there to carry vibration.
2. Why do solids transmit sound faster?
   Because particles are closely packed.
3. Why does sound echo after 17.2 m?
   Because it takes 0.1 s for echo to return.
4. Why is frequency measured in Hertz?
   Because it counts number of vibrations per second.
5. Why are ultrasonic waves used in SONAR?
   Because they can travel long underwater.
6. Why is sound called mechanical wave?
   Because it needs medium to travel.
7. Why do animals hear different frequencies?
   Because their ears are sensitive to other ranges.
8. Why is loudness more for greater amplitude?
   Because amplitude ∝ energy ∝ loudness.
9. Why do we hear thunder after lightning?
   Because sound travels slower than light.
10. Why can’t humans hear infrasound?
    Because frequency is below 20 Hz.
11. Why is ear shaped like a funnel?
    To collect and direct sound waves.
12. Why are compressions formed in sound wave?
    Because particles come close while vibrating.
13. Why does sound travel slower in gases?
    Because particles are loosely packed.
14. Why is speed of sound higher in hot air?
    Because particles move faster.
15. Why does sound wave show reflection?
    Because it follows laws like light.

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✅ Final Answer Key

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

Assertion & Reason:
16–a, 17–a, 18–d, 19–a, 20–d

Case-Based:
21–b, 22–c, 23–a, 24–a, 25–d