Class 9 Physics Chapter 11 Sound: Complete MCQ Quiz with Answers & Explanations

The rationalized 2024-25 CBSE syllabus emphasizes conceptual understanding and real-world application. MCQs are the perfect tool for testing this because they require you to distinguish between correct and plausible-sounding wrong answers—a skill that demands genuine mastery, not rote learning. In the Sound chapter, MCQs test your ability to calculate wave speed using v = f × λ, identify conditions for echo formation, apply SONAR principles, and understand how the ear converts sound waves into nerve signals. Unlike long-answer questions, MCQs give instant feedback: a wrong choice tells you exactly which concept needs revision. The CBSE Board uses MCQs to assess your application of concepts in new scenarios. For example, a standard question asks 'What is the speed of sound?', but an exam-level MCQ might ask 'If a sound wave has frequency 1000 Hz and wavelength 0.34 m, what is its speed in air?' This shift rewards students who understand relationships, not those who memorise isolated facts. Spending 20 minutes daily on targeted MCQs helps you build pattern recognition and decision-making speed, critical for exam confidence.

**Q1.** Sound is produced by: (a) Uniform motion of an object (b) Vibrating objects (c) Stationary objects (d) Moving liquids **Ans:** (b) Vibrating objects **Reason:** Sound originates from mechanical vibrations that create pressure waves in a medium. **Q2.** A sound wave requires: (a) A medium (solid, liquid, or gas) (b) Only a vacuum (c) Only air (d) Electromagnetic field **Ans:** (a) A medium (solid, liquid, or gas) **Reason:** Sound is a mechanical wave and cannot propagate through vacuum; it needs particles to vibrate. **Q3.** The speed of sound in air at room temperature is approximately: (a) 100 m/s (b) 340 m/s (c) 1000 m/s (d) 3 × 10⁸ m/s **Ans:** (b) 340 m/s **Reason:** NCERT states ~343 m/s at 25°C; round off to 340 m/s for calculations unless specified otherwise. **Q4.** The number of vibrations per second is called: (a) Amplitude (b) Wavelength (c) Frequency (d) Period **Ans:** (c) Frequency **Reason:** Frequency (f) is measured in Hertz (Hz) and represents cycles per second; f = 1/T where T is period. **Q5.** The distance between two consecutive compressions in a sound wave is: (a) Amplitude (b) Wavelength (c) Pitch (d) Intensity **Ans:** (b) Wavelength **Reason:** In longitudinal waves, wavelength (λ) is the distance between adjacent compressions or rarefactions. **Q6.** Which of the following is a characteristic of sound waves? (a) They are transverse waves (b) They are longitudinal waves (c) They travel in straight lines only (d) They require no medium **Ans:** (b) They are longitudinal waves **Reason:** Sound waves cause particles to oscillate parallel to the direction of wave propagation. **Q7.** The loudness of sound is related to its: (a) Frequency (b) Wavelength (c) Amplitude (d) Pitch **Ans:** (c) Amplitude **Reason:** Louder sounds have greater amplitude (displacement of particles from rest position). **Q8.** The pitch of a sound depends on: (a) Amplitude (b) Frequency (c) Speed (d) Wavelength **Ans:** (b) Frequency **Reason:** Higher frequency sounds are perceived as higher pitch; pitch is a subjective sensation linked to frequency. **Q9.** An echo is heard when sound reflects from a surface at a minimum distance of: (a) 10 m (b) 15 m (c) 17 m (d) 20 m **Ans:** (c) 17 m **Reason:** For distinct echo, sound must travel to surface and back within ~0.1 s; using v = 340 m/s, minimum distance ≈ 17 m. **Q10.** The human ear's eardrum is part of: (a) Outer ear (b) Middle ear (c) Inner ear (d) Brain stem **Ans:** (a) Outer ear **Reason:** The tympanum (eardrum) is the boundary between outer and middle ear; it vibrates in response to sound waves.

**Q11.** If a sound wave has frequency 500 Hz and speed 340 m/s in air, its wavelength is: (a) 0.68 m (b) 0.82 m (c) 1.7 m (d) 2.4 m **Ans:** (a) 0.68 m **Reason:** λ = v/f = 340/500 = 0.68 m; remember the wave equation v = f × λ. **Q12.** Sound travels faster in: (a) Air than in water (b) Water than in air (c) Vacuum than in any medium (d) All media at equal speed **Ans:** (b) Water than in air **Reason:** Denser media allow faster sound propagation; sound speed in water ≈ 1500 m/s vs. 340 m/s in air. **Q13.** Reflection of sound obeys the law of reflection, which states: (a) Angle of incidence = angle of refraction (b) Angle of incidence = angle of reflection (c) All angles are equal (d) No angle relationship exists **Ans:** (b) Angle of incidence = angle of reflection **Reason:** Both measured from the normal to the reflecting surface; applies to sound as it does to light. **Q14.** Which of the following is NOT required for echo formation? (a) Reflecting surface (b) Sufficient distance (≥17 m) (c) Frequency > 20 kHz (d) Medium for sound propagation **Ans:** (c) Frequency > 20 kHz **Reason:** Echo formation is independent of frequency; ultrasound (>20 kHz) can also produce echoes but is not essential. **Q15.** SONAR technology works on the principle of: (a) Refraction of sound (b) Reflection and echoes of sound (c) Diffraction of sound (d) Doppler effect only **Ans:** (b) Reflection and echoes of sound **Reason:** SONAR (Sound Navigation And Ranging) emits ultrasonic waves and detects reflected echoes to locate underwater objects. **Q16.** The audible frequency range for humans is: (a) 0 Hz to 100 Hz (b) 20 Hz to 20,000 Hz (c) 100 Hz to 10,000 Hz (d) 1000 Hz to 100,000 Hz **Ans:** (b) 20 Hz to 20,000 Hz **Reason:** Frequencies below 20 Hz are infrasound; above 20 kHz are ultrasound; both inaudible to humans. **Q17.** In the middle ear, sound vibrations are amplified by: (a) Cochlea (b) Three tiny bones (ossicles) (c) Semicircular canals (d) Oval window **Ans:** (b) Three tiny bones (ossicles) **Reason:** The hammer, anvil, and stirrup mechanically amplify eardrum vibrations before transmitting to inner ear. **Q18.** The inner ear structure that converts sound vibrations into nerve signals is: (a) Tympanum (b) Eustachian tube (c) Cochlea (d) Malleus **Ans:** (c) Cochlea **Reason:** The cochlea contains hair cells that generate electrical signals sent to the brain via auditory nerve. **Q19.** If a dolphin emits a 120 kHz ultrasound that travels at 1500 m/s in water, the wavelength is: (a) 0.0125 m (b) 0.125 m (c) 1.25 m (d) 12.5 m **Ans:** (a) 0.0125 m **Reason:** λ = v/f = 1500/(120 × 10³) = 1500/120000 = 0.0125 m = 12.5 mm. **Q20.** A vibrating tuning fork placed on a table produces a louder sound than when held in air. This happens due to: (a) Increased frequency (b) Increased amplitude (c) Increased wavelength (d) Resonance with table vibrations **Ans:** (d) Resonance with table vibrations **Reason:** The table vibrates in sympathy with the tuning fork, acting as a larger resonating surface and radiating sound more effectively.

**Q21. Assertion (A):** Sound waves can travel through the vacuum of space. **Reason (R):** Sound is an electromagnetic wave. (a) Both A and R are true; R explains A (b) Both A and R are true; R does not explain A (c) A is true; R is false (d) Both A and R are false **Ans:** (d) Both A and R are false **Reason:** Sound is a mechanical wave requiring a medium; it cannot travel through vacuum. Sound is not electromagnetic. **Q22. Assertion (A):** The speed of sound increases with temperature. **Reason (R):** Higher temperature increases molecular motion and kinetic energy. (a) Both A and R are true; R explains A (b) Both A and R are true; R does not explain A (c) A is true; R is false (d) A is false; R is true **Ans:** (a) Both A and R are true; R explains A **Reason:** In air, v ∝ √T; faster molecular motion transmits sound disturbances more quickly. **Q23. Assertion (A):** Two sound waves with different frequencies can interfere to produce beats. **Reason (R):** Beats occur when the frequencies differ by a small amount (< 10 Hz). (a) Both A and R are true; R explains A (b) Both A and R are true; R does not explain A (c) A is true; R is false (d) Both A and R are false **Ans:** (b) Both A and R are true; R does not explain A **Reason:** Beats form whenever frequencies differ, but beat frequency = |f₁ − f₂|; small difference makes beats audible, not necessary for beat formation. **Q24. Assertion (A):** A bat uses ultrasound to navigate because ultrasound has a shorter wavelength than audible sound. **Reason (R):** Shorter wavelength allows detection of smaller objects. (a) Both A and R are true; R explains A (b) Both A and R are true; R does not explain A (c) A is true; R is false (d) Both A and R are false **Ans:** (a) Both A and R are true; R explains A **Reason:** λ = v/f; higher frequency (ultrasound) yields shorter λ, enabling precise echolocation of small prey. **Q25. Assertion (A):** When a person shouts in a canyon, they hear an echo after a delay. **Reason (R):** The sound must travel from the person to the canyon wall and back. (a) Both A and R are true; R explains A (b) Both A and R are true; R does not explain A (c) A is true; R is false (d) A is false; R is true **Ans:** (a) Both A and R are true; R explains A **Reason:** Total distance = 2d; delay t = 2d/v = 2d/340, where d ≥ 17 m for audible echo. **Q26. Assertion (A):** The loudness of sound decreases as distance from the source increases. **Reason (R):** Sound intensity is inversely proportional to the square of distance (I ∝ 1/r²). (a) Both A and R are true; R explains A (b) Both A and R are true; R does not explain A (c) A is true; R is false (d) Both A and R are false **Ans:** (a) Both A and R are true; R explains A **Reason:** Intensity decreases as wave spreads over larger spherical area (4πr²); loudness perception follows intensity. **Q27. Assertion (A):** The cochlea is the organ responsible for hearing in humans. **Reason (R):** The cochlea contains hair cells that convert mechanical vibrations into electrical nerve signals. (a) Both A and R are true; R explains A (b) Both A and R are true; R does not explain A (c) A is true; R is false (d) A is false; R is true **Ans:** (a) Both A and R are true; R explains A **Reason:** The spiral-shaped cochlea is the main hearing organ; its receptor cells (hair cells) transduce sound into neural signals. **Q28. Assertion (A):** Loudness is a subjective sensation, while intensity is an objective physical quantity. **Reason (R):** Loudness depends on frequency, while intensity depends only on amplitude. (a) Both A and R are true; R explains A (b) Both A and R are true; R does not explain A (c) A is true; R is false (d) Both A and R are false **Ans:** (c) A is true; R is false **Reason:** Loudness is subjective perception (measured in decibels subjectively); intensity is objective (I = Power/Area in W/m²). Intensity depends on amplitude; loudness also depends on frequency sensitivity of the ear. **Q29. Assertion (A):** A sound wave of frequency 15 kHz cannot be heard by humans but can be detected by dogs. **Reason (R):** Humans can hear frequencies up to 20 kHz, while dogs can hear only up to 15 kHz. (a) Both A and R are true; R explains A (b) Both A and R are true; R does not explain A (c) A is true; R is false (d) Both A and R are false **Ans:** (c) A is true; R is false **Reason:** 15 kHz is below the human upper limit of ~20 kHz; humans CAN hear it. Dogs hear 15 kHz easily, but this does not explain why humans cannot (the premise is false). Dogs can hear up to ~45 kHz. **Q30. Assertion (A):** SONAR equipment on a ship uses ultrasound to measure ocean depth. **Reason (R):** Ultrasound has a much longer wavelength than audible sound, allowing it to travel greater distances in water. (a) Both A and R are true; R explains A (b) Both A and R are true; R does not explain A (c) A is true; R is false (d) A is false; R is true **Ans:** (c) A is true; R is false **Reason:** SONAR does use ultrasound, but the advantage is shorter wavelength (not longer), which allows better resolution. Ultrasound travels well in water due to less absorption and high frequency (low attenuation).

**Trap 1: Confusing speed in different media.** Many students memorise '340 m/s' and apply it universally. Remember: sound travels at ~1500 m/s in water and ~5000 m/s in steel. Read the question context carefully. If it doesn't specify, assume air at room temperature. **Trap 2: Mixing up frequency, pitch, and loudness.** Students often select frequency when asked about pitch (correct) but then choose amplitude when asked about frequency change (wrong). Frequency determines pitch; amplitude determines loudness. They are independent. A high-amplitude, low-frequency sound (e.g., distant thunder) is loud and low-pitched. **Trap 3: Echo vs. Reverberation.** Echo requires a single, distinct reflection from a surface ≥17 m away. Reverberation is multiple rapid reflections blending together (heard in rooms). An MCQ asking 'What is heard when sound reflects from a close wall?' expects 'reverberation,' not 'echo.' Check the distance mentioned. **Trap 4: SONAR confusion with radar.** SONAR uses sound (hence works underwater); RADAR uses electromagnetic waves (works in vacuum and air). If a question mentions underwater detection, it's SONAR. If it mentions aircraft detection through clouds, it's RADAR. **Trap 5: Human ear anatomy.** The eardrum sits in the outer ear (at its boundary), not the middle ear. The three bones (hammer, anvil, stirrup) are in the middle ear. The cochlea is in the inner ear. A question asking 'Where does amplification occur?' expects 'middle ear' (bones), not 'inner ear' (cochlea does conversion, not amplification). **Trap 6: Ultrasound frequency.** Students often think ultrasound starts at 100 kHz. The correct threshold is 20 kHz. A frequency of 15 kHz IS audible to humans; 25 kHz is NOT. Many MCQs exploit this boundary. **Trap 7: Wave equation misapplication.** v = f × λ is often applied without checking units. If f is in Hz and v in m/s, λ comes out in metres—correct. But if you're given λ in cm, convert to m first. Example: f = 500 Hz, λ = 68 cm. λ in m = 0.68 m. v = 500 × 0.68 = 340 m/s. Sneaky MCQs give mixed units. **Trap 8: Assertion-reason logic.** An assertion can be true while the reason is false or irrelevant. Always check both independently, then evaluate causation. Example: 'Sound is loud because it has high frequency.' Both A and R are true, but R does not explain A (loudness depends on amplitude, not frequency)—answer is (b), not (a).

**Read the entire question before glancing at options.** Spend 5 seconds reading and forming a mental answer. Then check if your answer appears in the options. This prevents option-bias (getting swayed by plausible distractors). If your answer doesn't match exactly, re-read to ensure you haven't misunderstood. **Classify questions by difficulty as you scan.** Easy questions (definition-based: 'What is frequency?') take 15–20 seconds. Medium questions (calculation or scenario-based) take 40–50 seconds. Hard (assertion-reason) take 60–90 seconds. Scan the entire question set first, attempt easy ones in the first pass (builds confidence and points), medium ones in the second pass, and hard ones last (or skip if time runs short). **Use elimination aggressively.** With 4 options, even a 50% certainty lets you eliminate one obviously wrong option, raising your odds from 25% to 33%. For Sound questions: if an MCQ mentions frequency and you know the answer depends on amplitude, eliminate options mentioning frequency. If SONAR is mentioned, eliminate 'electromagnetic wave.' This costs only 10 seconds but raises accuracy significantly. **Calculation questions: verify units.** If the question asks for wavelength in metres and your calculation gives 0.68, check whether the option is 0.68 m, 68 cm, or 6.8 cm. Dimensional errors are the #1 reason students lose marks on medium MCQs. Write units as you calculate: λ = v/f = 340 m/s ÷ 500 Hz = (340/500) m = 0.68 m. ✓ **Assertion-reason: answer both parts separately first.** Bracket whether A is true/false and R is true/false. Then check causation. This 2-step approach prevents careless errors. Example: Q22, both parts are true, so you're choosing between (a) and (b). Re-read the reason: does it explain *why* speed increases? Yes—molecular motion. Answer is (a). This deliberate logic takes 70 seconds but ensures ~95% accuracy on hard MCQs. **For echo and SONAR calculations, use the standard formula.** Minimum distance for echo: d = v × t / 2, where t ≈ 0.1 s (limit for human perception). d = 340 × 0.1 / 2 = 17 m. Memorise this. If asked 'At what distance is echo heard?', instant answer without recalculating saves 15 seconds. SONAR depth: depth = (v × t) / 2, where t is time to echo return. Same logic. **In the exam, allocate time realistically.** For a 15-question section: Scan (1 min) → Easy questions (5 min) → Medium questions (6 min) → Hard/review (3 min). This leaves buffer time for re-checking suspicious answers. Never spend >3 minutes on a single MCQ; mark it and return if time permits. **Practice full-length timed quizzes.** Working through these 30 MCQs under timed conditions (45 minutes) trains your brain for exam pace. Use a stopwatch. Aim for 80%+ accuracy. If you score <70%, revisit the concept notes in your NCERT (Chapter 11, Sound) rather than memorising more MCQs.

**Day 1: Attempt all 10 Easy MCQs without notes.** Time yourself (15 minutes). Check answers. If you score <90%, re-read NCERT Section 11.1 (Production & Propagation of Sound) and retry. Aim for 100% before moving forward; these build foundational confidence. **Day 2: Attempt all 10 Medium MCQs, using rough paper for calculations.** Time yourself (30 minutes). Check answers. If you score <75%, identify which concept (wave equation, reflection, SONAR, ear anatomy) caused errors. Re-read the relevant NCERT section and re-attempt those specific MCQs. **Day 3: Attempt all 10 Hard MCQs, writing out assertion and reason separately before selecting answers.** Time yourself (45 minutes). Hard MCQs are worth 3-4x the study effort of easy ones; don't rush. If you score <60%, it's normal—these target students aiming for 90%+ in the exam. Revisit the reasoning section of each MCQ above. **Day 4: Full-length timed quiz (all 30 MCQs in 60 minutes).** Simulate exam conditions: no notes, no pausing. Check answers immediately. Calculate accuracy rate. Ideal targets: 90%+ on Easy, 75%+ on Medium, 55%+ on Hard. If overall is <75%, spend 2 more days revising weak areas and retaking the quiz. **Throughout: Maintain an error log.** Write down every wrong answer, the reason you chose it, and the correct reasoning. This log becomes your personal revision guide in the final week before the exam—re-read it 2–3 times rather than re-reading all NCERT sections. **Final tip: Use cbsetutor.ai for instant doubt-clearing.** After you've attempted these MCQs, if you're stuck on why a particular answer is correct, log in to our platform for video explanations of each MCQ and live support from experienced tutors. Adaptive quizzes on cbsetutor.ai also generate MCQs tailored to your weak areas, so you don't waste time on topics you've already mastered.