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I. Chapter Summary:

The chapter Sound explains how sound is produced, propagated through various media, and how humans perceive it. It introduces the mechanical nature of sound, longitudinal wave motion, characteristics of sound waves (amplitude, frequency, wavelength, time period, velocity), and their relationship through formulas. Concepts such as reflection of sound, echo, reverberation, and the use of SONAR are also discussed. It connects physics with technology and natural phenomena, making it one of the most relatable chapters.

II. Key Concepts Covered:

Concept	Explanation
Sound as Mechanical Wave	Requires a medium for propagation; cannot travel in vacuum
Longitudinal Waves	Particles vibrate in the direction of wave propagation (compression/rarefaction)
Frequency (f)	Number of vibrations per second (Hz)
Time Period (T)	Time for one complete vibration T=1fT = frac{1}{f}T=f1​
Wavelength (λ)	Distance between two compressions or rarefactions
Wave Speed (v)	v=fλv = f lambdav=fλ; depends on medium
Reflection of Sound	Bouncing of sound from surfaces (used in SONAR, stethoscope)
Echo	Reflection heard after a time gap (≥ 0.1 s, ≥ 17.2 m)
Reverberation	Persistence of sound due to multiple reflections
SONAR	Sound Navigation And Ranging – uses ultrasonic waves

III. Important Questions:

(A) Multiple Choice Questions (1 Mark):

1. Speed of sound in air is approximately:
   a) 300 m/s
   b) 343 m/s ✅
   c) 1500 m/s
   d) 1000 m/s
2. Which of the following is not a characteristic of sound?
   a) Loudness
   b) Pitch
   c) Transparency ✅
   d) Quality
3. Which of these is used to measure depth of sea?
   a) Radar
   b) Echo
   c) SONAR ✅
   d) Amplifier
4. Time taken by echo to return is 0.4 s. Distance of reflecting surface is:
   a) 68.6 m ✅
   b) 34.3 m
   c) 100 m
   d) 17.2 m

(B) Short Answer Questions (2/3 Marks):

1. What is an echo? State the condition for hearing an echo.
2. How does sound propagate in air?
3. Why is SONAR not used in air?
4. Define amplitude and frequency. Give SI units.
   – PYQ 2020

(C) Long Answer Questions (5 Marks):

1. Derive the relation v=fλv = flambdav=fλ. Explain each term.
2. What is reverberation? Write two ways to reduce it in auditorium design.
3. Explain the working of SONAR with a labelled diagram.
4. A submarine sends ultrasonic waves which return after 2 s. If speed of sound in water is 1500 m/s, calculate depth of the sea.
   – PYQ 2021

(D) HOTS (Higher Order Thinking Skills):

1. A person hears echo from a cliff after 2 seconds. Why do we not hear echo in a small room?
2. Why do astronauts use radio communication despite sound not traveling in space?

IV. Key Formulas/Concepts:

Formula	Description
v=fλv = f lambdav=fλ	Speed of sound = frequency × wavelength
T=1fT = frac{1}{f}T=f1​	Time period = inverse of frequency
Distance=v×t2text{Distance} = frac{v times t}{2}Distance=2v×t​	Echo formula (time includes to and fro)
Speed of sound in air≈343 m/stext{Speed of sound in air} approx 343 , text{m/s}Speed of sound in air≈343m/s	At 25°C

V. Deleted Portions (CBSE 2025–2026):

 No portions have been deleted from this chapter as per the rationalized NCERT textbooks.

VI. Chapter-Wise Marks Bifurcation (Estimated – CBSE 2025–2026):

Unit/Chapter	Estimated Marks	Types of Questions
Sound	8–10 marks	Numericals, derivations, definitions, reasoning-based

VII. Previous Year Questions (PYQs):

Year	Marks	Question
2019	3	Define SONAR and explain its principle.
2020	2	What is echo? Write the minimum distance required for it.
2021	5	Derive relation between wave speed, frequency, and wavelength.
2022	3	State laws of reflection of sound with one application.

VIII. Real-World Application Examples:

Concept	Application
Echo	Radar, sound mapping, communication in hills
SONAR	Submarine navigation, fish finding, depth measurement
Sound reflection	Acoustic design of theaters, stethoscopes
Ultrasonic waves	Medical diagnosis, industrial flaw detection

IX. Student Tips & Strategies:

 Time Management:

• Allocate time for numerical practice and concept revision.
• Use flashcards for formula revision.

 Exam Preparation:

• Practice unit conversions (e.g., ms to seconds).
• Prepare diagrams like SONAR and waveforms.

 Stress Management:

• Use real-life sounds to relate to pitch and loudness.
• Watch short simulations of wave motion and sound reflection.

X. Career Guidance & Exploration:

For Classes 9–10:

Stream	Career
Physics	Acoustical Engineer, Sound Designer
Technology	Audio Engineer, SONAR Technician
Medical	Ultrasonographer, Radiologist

Entry Platforms:

• NTSE, NSO, CBSE Olympiads
• School projects on sound mapping, DIY echo chambers

XI. Important Notes:

•  Learn unit names carefully (Hz, m/s, second).
•  Draw diagrams clearly and label (e.g., wave diagrams, SONAR).
•  Revise the difference between mechanical vs electromagnetic waves.
•  Echo questions are frequently asked—master conditions and numericals.