🔊 CBSE · Class 9 · Science · Chapter 12

Chapter 12:
Sound

Complete chapter resources for CBSE Class 9 Science — topic breakdown, key formulas, sample questions, previous year board questions, and instant AI question paper generation.

4Topics
5–7Board marks
8Sample questions
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Key Formulas — Chapter 12
  • Wave speed: v = f × λ (m/s)
  • Time period: T = 1 / f (seconds)
  • Echo distance: d = (v × t) / 2
  • SONAR depth: depth = (v × t) / 2
  • Speed in air (~25°C): v ≈ 346 m/s
  • Audible range: 20 Hz – 20,000 Hz
Chapter Overview

What this chapter covers

Sound is a form of energy that travels as a mechanical longitudinal wave, requiring a material medium — solid, liquid, or gas — to propagate. Chapter 12 begins with the production of sound (vibrating objects) and its propagation through different media, introducing key wave parameters: frequency (f), wavelength (λ), amplitude (A), and wave speed (v). The fundamental relationship v = fλ connects these quantities and forms the basis of most numerical problems in this chapter.

The chapter covers three characteristics of sound — loudness (related to amplitude), pitch (related to frequency), and quality or timbre — followed by the important phenomenon of reflection of sound. Key concepts here include echo (minimum distance 17.2 m from a reflecting surface) and reverberation, and their practical applications such as megaphones, concert hall design, and stethoscopes. Students also study range of hearing: infrasound (< 20 Hz), audible range (20 Hz – 20,000 Hz), and ultrasound (> 20,000 Hz).

The latter portion focuses on applications of ultrasound (medical imaging, SONAR, industrial cleaning) and a detailed description of the human ear — its parts (pinna, eardrum, ossicles, cochlea) and how it converts pressure vibrations into electrical nerve signals. Board questions on the human ear (labelled diagram + function) and SONAR numericals carry the highest marks in this chapter.

Topics

What's inside Chapter 12

As per NCERT Class 9 Science (CBSE syllabus)

Topic 1
Production & Propagation of Sound
How vibrating objects produce sound. Longitudinal wave nature — compressions and rarefactions. Speed of sound in solids, liquids, and gases. Wave parameters: frequency, wavelength, amplitude, and time period. The relation v = fλ.
Topic 2
Characteristics of Sound
Loudness (amplitude-dependent, measured in decibels), pitch (frequency-dependent), and quality/timbre (waveform shape). Distinction between musical sound and noise. Sonic boom and the speed of sound barrier.
Topic 3
Reflection of Sound & Applications
Laws of reflection of sound. Echo: minimum distance = 17.2 m, persistence of hearing = 0.1 s. Reverberation and methods to reduce it. Practical applications: megaphone, hearing aid, stethoscope, concert halls. Infrasound, audible range, and ultrasound.
Topic 4
Ultrasound, SONAR & Human Ear
Properties and applications of ultrasound: echocardiography, prenatal scans, industrial cleaning. SONAR principle and depth calculation (d = v×t/2). Structure and working of the human ear — pinna, ear canal, eardrum, ossicles, cochlea, auditory nerve.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Ch 11 · Work and Energy
Sound as a form of energy; energy transfer through waves
Ch 8 · Motion
Speed, distance, and time — foundation for wave speed numericals
Chapter 12 Sound
Leads to
Class 10 · Light (Ch 10)
Wave behaviour: reflection, refraction — parallels with sound waves
Class 11 Physics · Waves
Transverse vs longitudinal waves, Doppler effect, standing waves
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 9 Science school and board papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Fill in the blank 1 1–2 Speed of sound, audible range, or defining echo/reverberation
Very Short Answer 2 1 Distinguish loudness/pitch, infrasound/ultrasound, echo/reverberation
Short Answer 3 1 SONAR explanation with diagram or numerical on echo distance
Long Answer / Diagram 5 1 Human ear labelled diagram + working, or applications of ultrasound
Total (approximate) 5–7 4–5 Weightage varies across school terms and paper sets
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 9 Science Chapter 12. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
The audible range of frequencies for the human ear is: (a) 20 Hz to 2000 Hz (b) 20 Hz to 20,000 Hz (c) 200 Hz to 20,000 Hz (d) 2 Hz to 20,000 Hz
Q2 Easy 2 marks Short Answer
Distinguish between loudness and pitch of a sound. On what physical property of sound does each depend?
Q3 Medium 2 marks Short Answer
What is the minimum distance from a reflecting surface at which a person must stand to hear an echo? State the assumption you make about the speed of sound.
Q4 Medium 3 marks Numerical
A ship sends a SONAR pulse and receives the echo after 4 seconds. If the speed of sound in seawater is 1500 m/s, find the depth of the ocean at that point. Show your working clearly.
Q5 Medium 3 marks Short Answer
Explain the principle of SONAR. List any two applications of SONAR in everyday life or industry.
Q6 Hard 4 marks Numerical
A sound wave has a frequency of 440 Hz and a wavelength of 0.75 m. (i) Calculate the speed of the wave. (ii) How long does it take for the wave to travel 1 km? (iii) If the frequency is doubled but the speed remains the same, what happens to the wavelength?
Q7 Hard 5 marks Long Answer
With the help of a labelled diagram, describe the structure of the human ear. Explain how the ear converts sound vibrations into nerve impulses that the brain can interpret.
Q8 Hard 5 marks Long Answer
A girl standing between two parallel cliffs fires a pistol. She hears the first echo after 2 s and the second echo after 3 s. If the speed of sound in air is 340 m/s: (i) Find the distance of each cliff from the girl. (ii) Find the distance between the two cliffs. (iii) Will the girl hear a third echo? Justify your answer with a calculation.
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Previous Year Questions

From CBSE board examinations

Representative questions from past Class 9 Science papers — Sound chapter.

Board 20223 marks
What is SONAR? Explain its working principle. A SONAR device on a submarine sends out an ultrasonic signal and receives the echo 5 seconds later. Calculate the distance of the object from the submarine if the speed of sound in seawater is 1450 m/s. (CBSE 2022)
Board 20235 marks
Draw a labelled diagram of the human ear. Write the function of each of the following parts: (i) Ear drum (ii) Cochlea (iii) Eustachian tube. (CBSE 2023)
Board 20202 marks
Explain why the ceilings of concert halls are made curved. What material is used on the walls and why? (CBSE 2020)
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FAQ

Questions teachers ask

How many marks does Sound carry in the CBSE Class 9 Science exam? +
Sound typically carries 5–7 marks in CBSE Class 9 Science unit tests and annual exams. Expect one 1-mark MCQ or fill-in-the-blank, one 2-mark short answer (often on reflection of sound or characteristics), and one 3–5 mark question on SONAR, the human ear, or numerical problems involving the speed of sound.
What is the speed of sound in air at room temperature and is it given in the CBSE exam? +
The speed of sound in air at room temperature (approximately 25 °C) is about 346 m/s. CBSE Class 9 board and school exams typically state the value in the question (often 340 m/s or 344 m/s) when it is needed for a numerical. Students should know the approximate value and the formula v = nλ (or v = fλ) to apply it confidently.
What is the condition for an echo and how is it calculated? +
An echo is heard when the reflecting surface is at least 17.2 m away from the source (assuming speed of sound = 344 m/s at 22 °C). The minimum time gap for persistence of hearing is 0.1 s, so the sound must travel to the wall and back in more than 0.1 s. The formula is: distance = (speed × time) / 2 = (344 × 0.1) / 2 = 17.2 m. Numerical problems based on this appear frequently in CBSE exams.
What is SONAR and which topics related to it are high-priority for CBSE Class 9 exams? +
SONAR (Sound Navigation And Ranging) uses ultrasonic waves to detect underwater objects. High-priority board topics include: the principle (pulse emitted → echo received → distance calculated using d = v × t / 2), its applications (submarine detection, ocean depth, fish shoal location), and its distinction from RADAR (which uses radio waves). A 3-mark question asking students to explain SONAR with a diagram or calculation appears regularly.
How do I generate a custom question paper for Sound (Class 9 Science) using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 9 Science, select Chapter 12 (Sound), set your preferred question-type mix (MCQ, short answer, word problem) and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.