Class 9 Science Chapter 10 Sound MCQ with Answers – Complete CBSE Quiz

The revised CBSE assessment framework emphasizes objective-type questions because they test not just memory but conceptual depth and decision-making speed. In the Class 9 Science paper (Term I and II combined), MCQs now account for 40–50% of marks. Sound (Chapter 10) is frequently represented because it bridges physics concepts: mechanics (oscillation, waves), energy (amplitude, frequency), and real-world harm (noise pollution). MCQs force you to identify nuance: distinguishing frequency from pitch, amplitude from loudness, or a sound wave from a mechanical wave. Unlike descriptive answers, MCQs penalize half-knowledge—you cannot score partial credit by writing 'something close.' This makes targeted MCQ practice non-negotiable. A well-structured MCQ quiz trains you to spot trap options (plausible-sounding wrong answers) and recognize NCERT-exact language, both critical for scoring 90+ in Sound.

**Q1.** Sound is produced by a vibrating object. Which property of the vibrating object determines how fast the sound travels? (A) Amplitude (B) Frequency (C) Material of the medium (D) Shape of the object **Correct Answer:** (C) **Why:** Speed of sound depends on the medium (air ≈ 343 m/s, water ≈ 1480 m/s, steel ≈ 5000 m/s), not the vibrating source's properties. --- **Q2.** A tuning fork vibrates 256 times per second. What is its frequency? (A) 256 m/s (B) 256 Hz (C) 256 dB (D) 256 m **Correct Answer:** (B) **Why:** Frequency is measured in Hertz (Hz), defined as vibrations per second. --- **Q3.** Sound requires a medium to propagate. Which of the following is NOT a valid medium for sound? (A) Air (B) Water (C) Vacuum (D) Steel **Correct Answer:** (C) **Why:** Sound is a mechanical wave; it requires matter (particles) to vibrate. Vacuum has no particles, so sound cannot travel through it. --- **Q4.** The maximum displacement of a vibrating particle from its equilibrium position is called: (A) Frequency (B) Wavelength (C) Amplitude (D) Pitch **Correct Answer:** (C) **Why:** Amplitude is the measure of maximum displacement in oscillation. --- **Q5.** If a sound wave has a frequency of 20 Hz and travels at 340 m/s, what is its wavelength? (A) 17 m (B) 6800 m (C) 320 m (D) 0.059 m **Correct Answer:** (A) **Why:** Wavelength = Speed ÷ Frequency = 340 ÷ 20 = 17 m. --- **Q6.** Which of the following sound frequencies is in the infrasonic range (below human hearing)? (A) 50 Hz (B) 10 Hz (C) 15,000 Hz (D) 20,000 Hz **Correct Answer:** (B) **Why:** Human hearing range is 20 Hz to 20,000 Hz; 10 Hz is below this threshold, hence infrasonic. --- **Q7.** Pitch is the characteristic of sound that depends on: (A) Amplitude (B) Frequency (C) Speed (D) Medium **Correct Answer:** (B) **Why:** Pitch (how high or low a sound is) is directly determined by frequency. --- **Q8.** Two sound sources vibrate with different frequencies. Source A vibrates 100 times/s; Source B vibrates 200 times/s. Which sound has higher pitch? (A) Source A (B) Source B (C) Both have same pitch (D) Cannot determine **Correct Answer:** (B) **Why:** Higher frequency → higher pitch. Source B's 200 Hz > Source A's 100 Hz. --- **Q9.** Loudness of sound is related to: (A) Frequency (B) Wavelength (C) Amplitude (D) Pitch **Correct Answer:** (C) **Why:** Loudness increases with larger amplitude; amplitude determines energy in the wave. --- **Q10.** A drum is struck and produces sound. What is the initial cause of sound production? (A) Heat released by the strike (B) Vibration of the drum membrane (C) Air pressure change only (D) Light energy from friction **Correct Answer:** (B) **Why:** All sound begins with vibrating matter; the drum head's oscillation is the direct source.

**Q11.** A speaker plays a note at 440 Hz. If the speaker is moved to a location with a different medium (water instead of air), what happens to the sound's frequency? (A) Increases (B) Decreases (C) Remains 440 Hz (D) Becomes zero **Correct Answer:** (C) **Why:** Frequency is a property of the source and does not change when the medium changes; speed and wavelength change, but frequency remains constant. --- **Q12.** Two waves travel through the same medium. Wave X has a frequency of 100 Hz and Wave Y has a frequency of 50 Hz. What is the ratio of their wavelengths (λₓ : λᵧ)? (A) 1 : 2 (B) 2 : 1 (C) 1 : 1 (D) 4 : 1 **Correct Answer:** (A) **Why:** λ = v ÷ f. Since v is same in the same medium, λ is inversely proportional to f. λₓ ÷ λᵧ = fᵧ ÷ fₓ = 50 ÷ 100 = 1 : 2. --- **Q13.** A tuning fork vibrates with an amplitude of 2 mm. If the amplitude is increased to 4 mm, how does this affect the frequency? (A) Frequency doubles (B) Frequency halves (C) Frequency remains unchanged (D) Frequency becomes zero **Correct Answer:** (C) **Why:** Frequency is independent of amplitude; they are separate properties. Increasing amplitude increases loudness, not frequency. --- **Q14.** A sound wave travels from air into water. The speed of sound in air is 343 m/s and in water is 1480 m/s. If the frequency is 500 Hz, what is the wavelength change? (A) λ in water = 2.96 m; λ in air = 0.686 m (B) λ in water = 0.686 m; λ in air = 2.96 m (C) Both wavelengths are equal (D) Wavelength becomes zero in water **Correct Answer:** (A) **Why:** λ = v ÷ f. In water: λ = 1480 ÷ 500 = 2.96 m. In air: λ = 343 ÷ 500 = 0.686 m. Higher speed → longer wavelength (frequency stays 500 Hz). --- **Q15.** A musical instrument produces a sound wave of frequency 256 Hz in air. When the same instrument is played underwater, a diver hears: (A) A higher pitch (B) A lower pitch (C) The same pitch (D) No sound at all **Correct Answer:** (C) **Why:** Pitch depends on frequency, which originates from the vibrating source and does not change with medium. The diver perceives the same pitch. --- **Q16.** Two loudspeakers emit sound at the same frequency but with different amplitudes. Speaker 1 has amplitude A; Speaker 2 has amplitude 2A. The intensity ratio of their sounds is: (A) 1 : 2 (B) 1 : 4 (C) 2 : 1 (D) 4 : 1 **Correct Answer:** (B) **Why:** Intensity ∝ (Amplitude)². If amplitude doubles, intensity becomes 4 times. Intensity ratio = A² : (2A)² = 1 : 4. --- **Q17.** A vibrating object completes 50 oscillations in 2 seconds. Its frequency is: (A) 25 Hz (B) 50 Hz (C) 100 Hz (D) 0.04 Hz **Correct Answer:** (A) **Why:** Frequency = Number of oscillations ÷ Time = 50 ÷ 2 = 25 Hz. --- **Q18.** When a siren emits sound at 800 Hz and moves towards you, you hear a higher pitch. This is due to: (A) Increase in amplitude (B) Increase in frequency (Doppler effect) (C) Decrease in wavelength only (D) Change in medium **Correct Answer:** (B) **Why:** The Doppler effect causes the perceived frequency to increase when the source approaches, resulting in higher pitch. --- **Q19.** A sound wave has an amplitude of 3 units and a frequency of 200 Hz. If we double only the frequency (keeping amplitude constant), the new loudness is: (A) Doubled (B) Halved (C) Unchanged (D) Quadrupled **Correct Answer:** (C) **Why:** Loudness depends on amplitude, not frequency. Changing frequency alone does not affect perceived loudness. --- **Q20.** A echo occurs when: (A) Sound reflects from a distant surface and is heard again (B) Sound is absorbed by the medium (C) Sound refracts at the boundary (D) Sound diffracts around obstacles **Correct Answer:** (A) **Why:** Echo is the repetition of sound due to reflection from a surface (at least 17 m away in air for human perception of distinct echo).

**Q21. Assertion (A):** Ultrasonic frequencies (>20,000 Hz) cannot be heard by humans. **Reason (R):** Human ears can only detect vibrations with frequencies between 20 Hz and 20,000 Hz. (A) Both A and R are true; R correctly explains A (B) Both A and R are true; R does not correctly explain A (C) A is true; R is false (D) A is false; R is true **Correct Answer:** (A) **Why:** Both statements are correct and causally linked. The frequency range limit of human ears (20–20,000 Hz) is the reason ultrasonic waves are inaudible. --- **Q22. Assertion (A):** The speed of sound in steel is greater than in air. **Reason (R):** Sound travels faster through denser media because particles are more closely packed. (A) Both A and R are true; R correctly explains A (B) Both A and R are true; R does not correctly explain A (C) A is true; R is false (D) Both A and R are false **Correct Answer:** (A) **Why:** Steel (~5000 m/s) has faster sound than air (~343 m/s). Denser media allow vibrations to transfer more quickly between closely-spaced particles. --- **Q23. Assertion (A):** Noise pollution causes hearing damage but not other health effects. **Reason (R):** Sound is purely a mechanical wave with no biological impact beyond the ear. (A) Both A and R are true; R correctly explains A (B) Both A and R are true; R does not correctly explain A (C) A is false; R is false (D) A is true; R is false **Correct Answer:** (C) **Why:** Noise pollution causes not only hearing loss but also sleep disturbance, stress, hypertension, and cognitive impairment. Sound does affect the body beyond the ear (stress hormones, heart rate). --- **Q24. Assertion (A):** A sound wave with frequency 25 Hz is classified as infrasonic. **Reason (R):** Infrasonic waves have frequencies below the human hearing threshold of 20 Hz. (A) Both A and R are true; R correctly explains A (B) Both A and R are true; R does not correctly explain A (C) A is true; R is false (D) A is false; R is true **Correct Answer:** (A) **Why:** 25 Hz is above the 20 Hz lower limit, so it is NOT infrasonic—this is a trap. Actually, A is FALSE and R is TRUE. The correct answer is (D). [Corrected: 25 Hz is audible, not infrasonic. For A to be true, frequency must be <20 Hz.] **Revised Q24:** **Assertion (A):** A sound wave with frequency 15 Hz is classified as infrasonic. **Reason (R):** Infrasonic waves have frequencies below the human hearing threshold of 20 Hz. **Correct Answer:** (A) **Why:** Both true; 15 Hz < 20 Hz, so it is infrasonic; the reason correctly explains this. --- **Q25. Assertion (A):** Doubling the frequency of a sound wave doubles its speed in a given medium. **Reason (R):** Speed of sound depends on the properties of the medium, not on frequency. (A) Both A and R are true; R correctly explains A (B) Both A and R are true; R does not correctly explain A (C) A is false; R is true (D) Both A and R are false **Correct Answer:** (C) **Why:** A is false; doubling frequency does not double speed. R is true; speed is a constant for a given medium. R correctly refutes A. --- **Q26. Assertion (A):** When sound travels from air into water, its wavelength increases. **Reason (R):** The speed of sound in water (1480 m/s) is greater than in air (343 m/s). (A) Both A and R are true; R correctly explains A (B) Both A and R are true; R does not correctly explain A (C) A is true; R is false (D) A is false; R is true **Correct Answer:** (A) **Why:** Both true. Since λ = v ÷ f and f remains constant, higher v in water means longer λ. R explains A correctly. --- **Q27. Assertion (A):** Noise pollution limits (75 dB in residential areas per CBSE curriculum) protect only hearing but not general health. **Reason (R):** Exposure to prolonged high-intensity sound causes stress, sleep disruption, and elevated blood pressure. (A) Both A and R are true; R correctly explains A (B) A is false; R is true (C) Both A and R are false (D) A is true; R is false **Correct Answer:** (B) **Why:** A is false; noise pollution damages overall health, not just hearing. R is true and explains why A is wrong. --- **Q28. Assertion (A):** A tuning fork with higher amplitude produces a higher pitch than one with lower amplitude. **Reason (R):** Amplitude and frequency are independent properties of a vibrating object. (A) Both A and R are true; R correctly explains A (B) A is false; R is true (C) Both A and R are false (D) A is true; R is false **Correct Answer:** (B) **Why:** A is false; amplitude does not affect pitch. R is true and correctly explains why A is wrong. --- **Q29. Assertion (A):** Reverberation in a hall is a repeated reflection of sound from walls before dying out. **Reason (R):** For a distinct echo, the reflecting surface must be at least 17 m away in air. (A) Both A and R are true; R correctly explains A (B) Both A and R are true; R does not correctly explain A (C) A is true; R is false (D) A is false; R is true **Correct Answer:** (B) **Why:** Both are true, but they describe different phenomena. Reverberation is multiple overlapping reflections (not distinct); echo requires ≥17 m separation for human perception of separation. They are related but not causal. --- **Q30. Assertion (A):** A supersonic aircraft produces a sonic boom because its speed exceeds the speed of sound. **Reason (R):** When a source moves faster than sound, it creates a shock wave (cone-shaped) that reaches an observer as a loud bang. (A) Both A and R are true; R correctly explains A (B) Both A and R are true; R does not correctly explain A (C) A is true; R is false (D) Both A and R are false **Correct Answer:** (A) **Why:** Both true and causally linked. Supersonic speed (>Mach 1) compresses sound waves into a shock cone; when this cone reaches observers, they hear the sonic boom.

**Trap 1: Confusing Frequency with Pitch** Many students pick 'frequency' when asked 'what determines pitch?' and vice versa. **Fact:** Pitch is the *perception* of frequency. Frequency is the physical property (Hz); pitch is subjective. In Class 9, NCERT uses them almost interchangeably for simplicity, but MCQs distinguish them. If asked 'what characteristic of sound determines how high or low it sounds?', the answer is frequency (or pitch as perceived), not amplitude or wavelength. **Trap 2: Speed of Sound Depends on Frequency/Amplitude** Wrong option: 'Speed of sound in air changes if frequency increases.' **Fact:** Speed is constant in a given medium (343 m/s in 20°C air, regardless of frequency or amplitude). Only the medium changes speed. Students confuse this with Doppler effect (perceived frequency changes, but actual speed doesn't). **Trap 3: Amplitude Determines Pitch** Wrong option: 'Louder sounds have higher pitch.' **Fact:** Amplitude → Loudness; Frequency → Pitch. A bass drum (low frequency, high amplitude) is loud but has low pitch. A piccolo (high frequency, low amplitude) is quieter but has higher pitch. Don't mix them. **Trap 4: Sound Travels Fastest in Air** Wrong option: 'Sound travels fastest through air.' **Fact:** Sound travels fastest in solids (steel: 5000 m/s), then liquids (water: 1480 m/s), then gases (air: 343 m/s). Denser media = faster sound. Many students assume air (where they hear sound) is the fastest medium. **Trap 5: Wavelength = Amplitude** Wrong option: 'Wavelength is the maximum displacement of a particle.' **Fact:** Wavelength is the distance between two successive crests (or troughs); Amplitude is maximum displacement from equilibrium. Both are measured in meters but represent different spatial properties. **Trap 6: Infrasonic = Below 10 Hz** Wrong option: 'Infrasonic sounds have frequencies below 10 Hz.' **Fact:** Infrasonic = below 20 Hz (human hearing threshold). Ultrasonic = above 20,000 Hz. Students sometimes use arbitrary thresholds instead of NCERT-standard 20–20,000 Hz range. **Trap 7: Noise Pollution Affects Only Ears** Wrong option: 'Noise pollution causes only hearing loss.' **Fact:** Per CBSE curriculum, noise pollution causes sleep disturbance, stress, hypertension, cognitive impairment, and hearing damage. It's a multisystem health threat, not just auditory. **Trap 8: Sound Can Travel Through Vacuum** Wrong option: 'Sound travels through vacuum very slowly.' **Fact:** Sound cannot travel through vacuum at all; it requires a medium with particles to vibrate. This is a foundational concept often tested in hard MCQs. **Trap 9: Reflecting Frequency from Source** Wrong option: 'When a sound source moves towards you, the frequency emitted by the source increases.' **Fact:** The source's frequency stays constant; the *perceived* frequency increases (Doppler effect). The source doesn't 'emit' a higher frequency—the observer *receives* higher frequency due to wave compression.

**Total Time: 90 minutes for ~35 MCQs (typical Sound + other chapters exam)** **Step 1: Quick Scan (2 minutes)** Read all questions (not answers) in order. Mark 3 categories: • **Green (Easy):** Straightforward recall (Q1–10 level). Allocate 45 seconds each. • **Yellow (Medium):** Require calculation or concept linking (Q11–20 level). Allocate 90 seconds each. • **Red (Hard):** Assertion-reason or multi-concept (Q21–30 level). Allocate 2–3 minutes each. **Step 2: Attempt Green First (10–12 minutes)** Answer all easy MCQs without overthinking. These are confidence-builders and guaranteed marks. Use process of elimination: reject obviously wrong options (e.g., 'Sound travels through vacuum' → false immediately). **Step 3: Tackle Yellow (15–18 minutes)** For numerical MCQs, write down the formula first (λ = v ÷ f, Intensity ∝ A²). Check units. If options have vastly different magnitudes, your calculation is likely wrong—recalculate. For concept MCQs, re-read the question to ensure you're answering what's asked, not what you expect. **Step 4: Assertion-Reason Strategy (Red, 12–15 minutes)** Do NOT answer in the standard way (A & R both true → R explains A). Instead: 1. Evaluate A alone: True or false? 2. Evaluate R alone: True or false? 3. Check causality: Does R explain A? Use a shorthand: Write 'T/T/Y' (both true, reason explains) or 'T/F/—' (A true, R false, no causality). This prevents mixing up options. **Step 5: Flag & Review (3–5 minutes)** Mark any question you're unsure about (but don't re-solve yet). If you have 5+ minutes left, revisit only 2–3 flagged questions. For assertion-reason, a second look often catches trap options (e.g., both statements true, but no causal link). **Exam-Day Checklist:** ✓ Read the question stem *twice* before looking at options. ✓ Eliminate obviously wrong options first (e.g., units don't match, contradicts NCERT). ✓ Don't second-guess easy answers; move on. ✓ For Doppler / medium-change questions, remember: frequency doesn't change, but speed & wavelength do. ✓ Write calculations beside the question paper (visible to yourself only); cross them out after marking the answer. **Sample Calculation Shorthand (for Q14):** f = 500 Hz v_air = 343 m/s → λ = 343÷500 = 0.686 m v_water = 1480 m/s → λ = 1480÷500 = 2.96 m → Option (A) ✓ This approach ensures you maximize marks on questions you *can* solve, minimize time on tough ones, and avoid careless errors. Start a 3-day free trial at cbsetutor.ai to practice full-length Sound chapter tests with instant feedback.