Class 9 Science Chapter 10 Sound Previous Year Questions: 13+ Real Exam Questions with Solutions

Most Class 9 students read the NCERT chapter, highlight definitions, and hope to remember them on exam day. This approach fails because exam questions test *application*, not recitation. When you solve past papers, you: (1) Identify which concepts appear most often—frequency and pitch linkage, sound propagation speed in different media, decibel scale for noise pollution; (2) Train yourself to answer under time pressure, exactly as you'll face in the real exam; (3) Spot your weak spots immediately—for instance, calculating the time taken for sound to travel given speed and distance; (4) Learn how examiners phrase multi-part questions, so you know what they're really asking. Studies show students who attempt 10+ past papers score 15–20% higher than those who only read textbooks. In the 2024–25 rationalized syllabus, production mechanisms, wave properties (frequency f, amplitude A, speed v = f × λ), and environmental impacts remain core. By working through this chapter's PYQs, you'll build the muscle memory to tackle any sound question confidently—whether it's about longitudinal waves, Doppler effect fundamentals, or decibel intensity levels.

One-mark questions test quick recall and basic definitions. These five patterns appear reliably across papers: **Q1: What is the SI unit of frequency?** A: Hertz (Hz). Frequency is the number of oscillations per second. A sound wave with frequency 20 Hz completes 20 cycles in 1 second. **Q2: A sound wave has a frequency of 256 Hz and wavelength 1.3 m. Calculate its speed.** A: Speed v = f × λ = 256 × 1.3 = 332.8 m/s. This is typical speed of sound in air at room temperature. **Q3: Define amplitude in the context of sound waves.** A: Amplitude is the maximum displacement of a vibrating particle from its mean position. It determines the loudness of sound—greater amplitude means louder sound. **Q4: Which of the following best describes noise pollution? (a) Low-frequency sound only (b) Unwanted or excessive sound causing discomfort (c) Sound above 20 kHz (d) Ultrasound** A: (b) Noise pollution is unwanted sound that causes physical discomfort, hearing loss, or environmental harm. Decibel levels above 80 dB are generally considered polluting. **Q5: Sound travels fastest in which medium: air, water, or steel?** A: Steel. Speed of sound ≈ 343 m/s in air, ≈ 1480 m/s in water, ≈ 5960 m/s in steel. Sound propagates faster in denser, more elastic media.

Three-mark questions demand explanations and calculations. These five cover the core topics: **Q1: Explain how sound is produced and how it propagates through a medium. Why can't sound travel in a vacuum?** A: Sound is produced when objects vibrate. Vibrating particles push neighboring particles, creating a chain of compressions and rarefactions that travel outward—this is a longitudinal wave. Sound propagates through the elastic oscillations of medium particles. In a vacuum, there are no particles to vibrate or transfer energy; therefore, sound cannot propagate. Sound requires a material medium (solid, liquid, or gas). For example, an alarm clock in a vacuum chamber makes no sound even though it vibrates. **Q2: Distinguish between frequency and pitch. How are they related?** A: Frequency is the number of complete oscillations per second, measured in Hertz (Hz). Pitch is the perceptual quality—how high or low a sound seems to our ear. They are directly related: higher frequency produces higher pitch, and vice versa. A tuning fork vibrating at 512 Hz produces a higher pitch than one at 256 Hz. Pitch is subjective (depends on listener), but frequency is objective (measurable). **Q3: A sound wave travels through air at 340 m/s. If the frequency is 85 Hz, find the wavelength. Also, state the relationship between speed, frequency, and wavelength.** A: Wavelength λ = v ÷ f = 340 ÷ 85 = 4 m. The relationship is v = f × λ. This shows that for a constant speed, if frequency increases, wavelength decreases proportionally (inverse relationship). **Q4: Define amplitude. Explain its effect on loudness. What is the decibel scale?** A: Amplitude is the maximum displacement of vibrating particles from equilibrium. Loudness increases with amplitude—a louder sound has greater particle displacement. The decibel (dB) scale measures loudness intensity logarithmically: 0 dB is the threshold of hearing, 10 dB is normal breathing, 60 dB is conversation, 80–90 dB causes discomfort, and above 120 dB risks hearing damage. The scale is logarithmic because human hearing responds to intensity changes on a logarithmic basis. **Q5: List three main sources of noise pollution in urban areas and suggest one control measure for each.** A: (1) Traffic noise—control by reducing vehicle speed, using sound barriers, and improving road surfaces. (2) Industrial machinery—control through proper maintenance, sound insulation in factories, and enforceable emission standards. (3) Construction activities—control by restricting work hours, using mufflers on equipment, and maintaining distance from residential zones. These measures reduce both decibel levels and exposure time.

Five-mark questions integrate multiple concepts and require detailed reasoning: **Q1: Explain the production and propagation of sound in terms of compressions and rarefactions. Draw a simple diagram showing these regions in a longitudinal wave.** Solution: When an object vibrates (e.g., a tuning fork), it pushes and pulls air molecules. When the fork moves forward, it squeezes molecules together, creating a compression (high-pressure region). When it moves backward, it leaves behind a low-pressure region called a rarefaction. These alternate regions travel outward from the source, forming a longitudinal wave. Each molecule oscillates back and forth parallel to the wave's direction of travel—particles don't move away with the wave, only vibrate locally. The diagram shows alternating dark regions (compressions) and light regions (rarefactions) along a straight line. The distance between consecutive compressions (or rarefactions) is the wavelength λ. The speed at which this pattern travels is the speed of sound in that medium (≈340 m/s in air at 25°C). This mechanism explains why sound is a mechanical wave—it requires a material medium and particle motion to propagate. **Q2: A source emits sound at frequency 500 Hz. The speed of sound in air is 340 m/s. (a) Calculate the wavelength. (b) If the same sound travels through water (speed ≈ 1400 m/s), what is its wavelength and frequency in water? (c) Explain why frequency remains constant but wavelength changes.** Solution: (a) λ_air = v ÷ f = 340 ÷ 500 = 0.68 m. (b) In water: λ_water = 1400 ÷ 500 = 2.8 m. Frequency remains 500 Hz. (c) Frequency is a property of the source (the vibrating object)—it depends only on how fast the source oscillates, not on the medium. When sound enters a new medium, the speed changes due to different density and elasticity, but the source continues vibrating at the same rate. Since v = f × λ, and v increases in water while f stays constant, λ must increase proportionally. This shows that wavelength is medium-dependent, but frequency is not. **Q3: Noise pollution affects urban populations. (a) Define the decibel scale. (b) Name four sources of noise pollution in a city and their typical decibel levels. (c) Discuss three health effects of prolonged exposure to high noise levels. (d) Suggest two structural and two legislative control measures.** Solution: (a) The decibel (dB) scale is a logarithmic measure of sound intensity relative to the threshold of human hearing (0 dB = 10⁻¹² W/m²). Each 10 dB increase represents a tenfold increase in intensity. (b) Sources and levels: (i) Traffic noise, 70–80 dB; (ii) Aircraft, 140 dB; (iii) Industrial machinery, 90–100 dB; (iv) Construction drills, 100–110 dB. (c) Health effects: (i) Hearing loss (temporary or permanent) from prolonged exposure above 85 dB; (ii) Sleep disruption and fatigue; (iii) Stress, anxiety, and elevated blood pressure affecting cardiovascular health. (d) Structural measures: (i) Sound-absorbing walls and barriers near highways; (ii) Noise-insulating windows and doors in homes. Legislative measures: (i) Strict emission standards for vehicles and factories; (ii) Restricted working hours for construction and industrial activity near residential areas. Start a 3-day free trial at cbsetutor.ai to access video explanations and interactive practice for all sound concepts.

The 2024–25 rationalized CBSE syllabus maintains core sound concepts but emphasizes competency-based assessment over rote memorization. Key shifts evident in recent sample papers and exam trends: (1) **Increased application-focus**: Questions now ask students to apply speed = frequency × wavelength to real-world scenarios—calculating echo times in halls, predicting wavelength changes across media, analyzing noise pollution's quantitative impact on communities. (2) **Graph interpretation**: Recent papers feature speed vs. medium density graphs, frequency vs. pitch perception curves, and decibel vs. exposure time risk charts. Students must read, analyze, and draw conclusions from visual data. (3) **Case-study integration**: Multi-part questions blend physics with environmental or health contexts—for example, linking industrial noise (105 dB) to OSHA exposure limits and suggesting control measures. (4) **Reduced definition-only questions**: Straight definitional 1-marks are fewer; instead, they're embedded in concept-linking scenarios. (5) **Numerical problems with units rigor**: Examiners penalize missing or incorrect units; v = 340 m/s, not just 340. Expect questions requiring unit conversions (e.g., frequency in kHz to Hz) and dimensional analysis checks. (6) **Practical/exploratory emphasis**: Though not in all papers, some boards include questions on simple sound experiments—measuring echo distance, observing resonance, or analyzing voice frequency using online tools. Preparing with this framework means solving problems end-to-end with units, sketching diagrams, and explaining *why* an answer makes sense, not just computing a number.

Exam day strategy differs from practice. Here's a proven approach: **Before the exam (last week):** Solve at least 8–10 complete previous-year papers in one sitting under timed conditions (match your board's allotted time). Mark your errors, note weak areas (e.g., speed-frequency-wavelength conversions, decibel scales), and revise those topics. **In the exam hall (first 5 minutes):** Skim all questions. Identify question types: pure concept (1-mark), calculation-based (3-mark), application/case-study (5-mark). Estimate time: 1 minute per mark is standard, so allocate ~5 min for 5-mark, ~3 min for 3-mark. **Tackling 1-mark questions:** Answer quickly without showing work. If uncertain, flag and return at the end. **Tackling 3-mark questions:** Write the formula/principle first, then substitute values, then final answer with units. Show at least 2–3 steps to earn part-marks if calculation is wrong. **Tackling 5-mark questions:** Spend 1–2 minutes planning (jot key points), then write in structured paragraphs or numbered sub-answers. Always include diagrams for wave production/propagation questions—they earn marks even if explanation is incomplete. **Numerical checks:** Before writing final answer, ask: Is the magnitude reasonable? (Sound speed ≈ 300–400 m/s in air, not 3000.) Are units consistent? **Last 5 minutes:** Review: Did I answer all parts of multi-part questions? Did I label diagram axes? Is handwriting legible? This strategy, combined with past-paper practice, typically raises scores by 8–12% in final exams.

Keep these on a revision card for last-minute checks: **Speed, Frequency, Wavelength:** v = f × λ (where v is speed in m/s, f is frequency in Hz, λ is wavelength in m). **Speed of sound (at 25°C):** Air ≈ 340 m/s; Water ≈ 1480 m/s; Steel ≈ 5960 m/s. Denser media → faster sound. **Human hearing range:** 20 Hz to 20,000 Hz (infrasound <20 Hz, ultrasound >20,000 Hz). **Decibel scale (reference: 10⁻¹² W/m²):** 0 dB = threshold of hearing; 30 dB = whisper; 60 dB = conversation; 85 dB = hearing damage risk; 120+ dB = pain threshold. **Echo time:** If sound travels distance d at speed v, time t = 2d/v (factor 2 because it travels to object and back). **Loudness vs. amplitude:** Directly proportional—double amplitude ≈ perceived as louder. **Pitch vs. frequency:** Higher frequency = higher pitch. Frequency is objective (measurable), pitch is subjective (perceptual). These formulas appear in 80%+ of numerical questions across all exam boards. Practice until substitution is automatic—it saves exam time and reduces arithmetic errors.