Class 9 Science Chapter 10 Sound: Important Questions with Complete Solutions

The Sound chapter (Chapter 10) is a fixed 30-35 marks unit in Class 9 CBSE Science. Examiners test three skill levels: (1) Recall—defining frequency, amplitude, pitch, and loudness; (2) Understanding—explaining how sound propagates in different media, why noise harms health; and (3) Application—solving problems using v = f × λ, calculating frequency from period, comparing sound speeds. Recent board papers show 40% questions are 2-mark short-answer type, demanding clear definitions and one-line reasoning. A typical 80-mark paper allocates 6-8 marks directly to Sound. The 2024-25 pattern emphasises "real-world relevance," so expect questions on noise pollution in cities, ultrasound and infrasound applications, and why sound cannot travel in vacuum. By mastering these 18 important questions, you will be prepared for all expected question types: MCQ traps testing misconceptions (e.g., "sound travels faster in water than air"—TRUE, ~1500 m/s vs ~343 m/s), definition-based 2-mark questions, calculation-heavy 3-mark problems, and analytical 5-mark essays on environmental impacts. Your board examiner will likely ask at least one question from the production/propagation section and another on noise pollution—both are heavily featured here.

MCQs in Class 9 Sound test your conceptual clarity and ability to spot common misconceptions. Here are 5 important 1-mark questions representative of board exams: **Q1:** Sound travels fastest in: (A) Air (B) Water (C) Steel (D) Vacuum **Answer:** (C) Steel. Sound is a mechanical wave requiring a medium. It travels at ~5000 m/s in steel, ~1500 m/s in water, and ~343 m/s in air at 25°C. It cannot travel in vacuum. **Q2:** If a tuning fork vibrates 256 times per second, its frequency is: (A) 256 Hz (B) 256 m/s (C) 256 s (D) 256 m **Answer:** (A) 256 Hz. Frequency is vibrations per second, measured in Hertz (Hz). One complete vibration = one oscillation. **Q3:** The distance between two consecutive compressions in a sound wave is called: (A) Amplitude (B) Frequency (C) Wavelength (D) Pitch **Answer:** (C) Wavelength. The distance between two consecutive compressions (or rarefactions) equals one complete wavelength (λ). **Q4:** Loudness of sound depends on: (A) Frequency (B) Amplitude (C) Wavelength (D) Speed **Answer:** (B) Amplitude. Loudness (measured in decibels, dB) is directly proportional to the amplitude of vibration. Greater amplitude = louder sound. **Q5:** Humans can hear sound frequencies in the range: (A) 20 Hz to 20,000 Hz (B) 1 Hz to 1,000 Hz (C) 100 Hz to 10,000 Hz (D) 20,000 Hz to 200,000 Hz **Answer:** (A) 20 Hz to 20,000 Hz. This is the audible frequency range for human ears. Below 20 Hz is infrasound; above 20,000 Hz is ultrasound.

These questions test definition recall and one-step reasoning—a major board exam component. **Q1:** Define frequency and period of a sound wave. What is the relationship between them? **Answer:** Frequency (f) is the number of vibrations or oscillations per second, measured in Hertz (Hz). Period (T) is the time taken for one complete oscillation, measured in seconds (s). The relationship is: **f = 1/T** or **T = 1/f**. Example: If a tuning fork has a frequency of 256 Hz, its period is T = 1/256 ≈ 0.0039 s. **Q2:** Why does sound not travel in a vacuum? **Answer:** Sound is a mechanical wave that requires a medium (solid, liquid, or gas) to propagate. It travels by creating compressions and rarefactions in the particles of the medium. Since vacuum has no particles, sound cannot travel through it. This is why there is no sound in outer space. **Q3:** What is the difference between pitch and loudness of sound? **Answer:** Pitch depends on the frequency of sound; higher frequency → higher pitch. Loudness depends on the amplitude of vibration; greater amplitude → louder sound. Example: A high-frequency whistle has a high pitch but low loudness; thunder has low frequency (low pitch) but high amplitude (high loudness). **Q4:** Name two sources of noise pollution in cities and suggest one way to reduce it. **Answer:** Sources: (1) Traffic noise from vehicles (~80 dB); (2) Construction noise from machinery (~90 dB). Reduction method: Using sound-absorbing materials (foam, carpets) in buildings, planting trees as natural barriers, or enforcing traffic regulations. Any one valid method suffices. **Q5:** A sound wave has wavelength λ = 2 m and frequency f = 170 Hz. Calculate its speed. **Answer:** Using the formula: **v = f × λ** = 170 × 2 = **340 m/s**. This is the speed of sound in air at ~25°C (approximately 343 m/s; small variation is expected).

These questions blend explanation with one or two calculation steps—a standard board exam pattern. **Q1:** Explain how sound is produced by a vibrating object and how it propagates through a medium. **Answer:** When a sound source (e.g., tuning fork) vibrates, it pushes air particles forward (compression) and pulls them back (rarefaction). These compressions and rarefactions spread outward in waves through the medium. Each particle oscillates about its equilibrium position and transfers energy to adjacent particles, not moving with the wave itself. This longitudinal wave propagates at a fixed speed in that medium. The energy gradually dissipates, which is why sound becomes fainter at larger distances. Example: When you strike a drum, the membrane vibrates, creating compressions in air that travel outward at ~343 m/s until they reach your ear. **Q2:** A sound wave travels at 340 m/s in air. If the frequency is 85 Hz, find (a) the wavelength, and (b) the period of the wave. **Answer:** Using **v = f × λ**: (a) λ = v/f = 340/85 = **4 m** (b) Period T = 1/f = 1/85 ≈ **0.012 s** (or 12 milliseconds). This means one complete oscillation takes 12 ms, and the wave completes 85 cycles every second. **Q3:** Compare the speed of sound in three media and explain why the speed differs. **Answer:** Speed of sound: Steel (~5000 m/s) > Water (~1500 m/s) > Air (~343 m/s). Speed differs because it depends on the density and elasticity of the medium. In denser, more elastic media (like steel), particles are tightly packed and vibrations transfer quickly between adjacent particles. In air, particles are loosely spaced, so energy transfer is slower. Sound cannot travel in vacuum because there are no particles to vibrate. This explains why we hear thunder delayed after lightning—light travels instantly, but sound takes time. **Q4:** A doctor uses an ultrasound machine with a frequency of 2 MHz (2 × 10⁶ Hz) to scan a patient. If the speed of ultrasound in tissue is 1500 m/s, calculate the wavelength of the ultrasound wave. **Answer:** Using **v = f × λ**, we rearrange: λ = v/f = 1500 / (2 × 10⁶) = **0.00075 m = 0.75 mm**. The very short wavelength allows ultrasound to penetrate tissue and reflect from internal organs, enabling medical imaging. This is why ultrasound is useful in diagnostics—its small wavelength gives high resolution.

These are essay-type questions testing conceptual depth and application—critical for board exam success. **Q1:** Explain the production and propagation of sound. Why does sound need a medium and what happens to sound energy as it travels away from the source? **Full Solution:** Production: Sound is produced when an object vibrates. The vibrating object pushes particles of the surrounding medium (air, water, solid) forward and backward. For example, when a tuning fork vibrates, each prong moves outward (pushing air) and inward (pulling air), creating a series of compressions (high-density regions) and rarefactions (low-density regions). Propagation: These compressions and rarefactions form a longitudinal wave that travels through the medium at a fixed speed. Each particle in the medium oscillates around its equilibrium position and transfers kinetic energy to the next layer of particles, but the particles themselves do not move along with the wave—only the disturbance propagates. The speed depends on the medium's properties (density, elasticity). Why a medium is needed: Sound is a mechanical wave; it requires a medium because it travels by displacing particles. In vacuum, there are no particles to vibrate, so sound cannot propagate. This is proven by the bell-in-vacuum experiment: a ringing bell placed inside an evacuated glass jar becomes inaudible as air is removed. Energy dissipation: As sound travels outward from the source, it spreads over an increasingly larger area (spherical wavefront). The same energy spreads over a larger volume, reducing intensity. Additionally, friction and air resistance absorb energy, and molecules scatter sound waves. This is why distant thunder is a low rumble, not a sharp crack. Intensity decreases proportionally to the square of distance (inverse square law). **Q2:** Describe the harmful effects of noise pollution and suggest measures to control it. **Full Solution:** Harmful Effects of Noise Pollution: (1) Hearing damage: Prolonged exposure to sound levels >85 dB causes permanent hearing loss. Construction sites (~90 dB), airports (~140 dB jet), and concerts (~110 dB) are serious hazards. Hearing damage is cumulative and irreversible. (2) Sleep disturbance: Noise at night disrupts sleep cycles, leading to fatigue, reduced productivity, and weakened immune response. (3) Stress and mental health: Chronic noise increases cortisol levels (stress hormone), causing anxiety, hypertension, and cardiovascular disease. (4) Cognitive impairment: Students exposed to high noise show reduced concentration, poor exam performance, and slower learning. Children are especially vulnerable. (5) Environmental impact: Noise affects wildlife communication, migration patterns, and breeding cycles. Control Measures: (1) Engineering controls: Sound-absorbing materials (foam, rubber, carpets) in buildings; double-glazed windows; silencers on vehicle exhausts; vibration dampeners on machinery. (2) Administrative controls: Enforcing noise ordinances (banned honking in residential areas, construction hours limited to 7 AM–6 PM); using quieter equipment; routing traffic away from schools/hospitals. (3) Personal protection: Earplugs (reducing noise by 15–30 dB) in factories; headphone regulation in public. (4) Urban planning: Green zones with trees and plants act as natural sound barriers, reducing road noise by 10–15 dB. Creating no-honk zones and pedestrian-only areas reduces urban noise. (5) Public awareness: Educating communities about noise risks and promoting "quiet zones" encourages voluntary compliance. Conclusion: A multi-level approach combining technology, policy, and citizen participation is essential to reduce noise pollution and protect public health. **Q3:** Ultrasound and infrasound have important applications despite being inaudible to humans. Explain their characteristics and list three applications each. **Full Solution:** Ultrasound: Characteristics: Frequency >20,000 Hz (inaudible to humans). Very short wavelength (~0.75 mm at 2 MHz in tissue), allowing high-resolution imaging. High energy; can penetrate soft tissues but reflects at bone/tissue boundaries. Applications: (1) Medical imaging (sonography): Prenatal scanning of fetuses, cardiac imaging, detection of tumors and organ damage. Safe because non-ionizing (no radiation). (2) Therapeutic ultrasound: Heating deep tissues for pain relief (physiotherapy), breaking kidney stones in lithotripsy, removing dental plaque. (3) Industrial cleaning: Ultrasound vibrations dislodge dirt from jewelry, precision parts, and dental tools. Frequency ~40 kHz is optimal. Additional: Burglar alarms, pest deterrents, echolocation in bats (40–60 kHz). Infrasound: Characteristics: Frequency <20 Hz (inaudible to humans). Very long wavelength (~17 m at 20 Hz in air). Travels long distances with minimal attenuation; diffracts around obstacles easily. Applications: (1) Geological monitoring: Detecting earthquakes, volcanic eruptions, and underground explosions. Seismographs record infrasound generated by these events. (2) Weather prediction: Infrasound from thunderstorms, tornadoes, and hurricanes can be detected hundreds of kilometers away, aiding early warning systems. (3) Wildlife communication: Elephants communicate across vast distances using infrasound (~14 Hz), enabling coordination of herd movements. Conclusion: Both ultrasound and infrasound extend sound applications far beyond human hearing, demonstrating the versatility of wave physics in medicine, industry, and environmental science.

**Case Study Question:** A Class 9 school is located near a busy highway with heavy traffic. Students and teachers report headaches, concentration difficulties, and hearing discomfort during lessons, especially near windows. A baseline noise level audit recorded: (1) Inside classroom without traffic: 40 dB (baseline); (2) With traffic (peak hours): 85 dB; (3) Morning assembly at school ground: 75 dB. The school principal wants to reduce noise to <70 dB inside classrooms. Analyze the problem and propose a solution. **Step-by-Step Solution:** **Step 1: Identify the source and problem.** Primary source: Traffic noise (85 dB) from the adjacent highway. Noise levels >80 dB cause stress, hearing risk, and cognitive impairment in children. The 45 dB increase above baseline (85 – 40) significantly disrupts learning. **Step 2: Understand noise propagation.** Sound from highway enters via: (a) Windows (glass is poor sound insulator); (b) Doors and ventilation gaps; (c) Building walls (if thin/unmaintained). Noise reflects inside the classroom, amplifying the problem. **Step 3: Propose engineering controls.** (a) Double-glazed windows with air gap (reduces noise by ~25–30 dB). (b) Sound-absorbing wall panels (acoustic foam, mineral wool) inside classrooms, targeting high-traffic-side walls (reduces ~10–15 dB). (c) Weatherproof seals on doors and air vents (~5 dB reduction). (d) Carpeting or rubber flooring (absorbs vibrations and reverberant noise). Expected result: 40 + (sound reduction of 30 dB) = 70 dB or less inside classroom. **Step 4: Propose administrative/urban planning controls.** (a) Coordinate with traffic authorities to divert heavy vehicles away from school roads during 9 AM–2 PM (school hours). (b) Install noise barriers (concrete walls, green hedges 3–4 m tall) on the highway side, 100 m from school. (c) Enforce speed limits near school zone; promote electric buses (quieter than diesel). Expected reduction: 5–10 dB from traffic measures alone. **Step 5: Verify impact with decibel addition formula.** Decibels are logarithmic. If classroom noise is 85 dB and we reduce by 30 dB (windows) + 10 dB (panels) = ~37–40 dB cumulative reduction: New level ≈ 85 – 35 ≈ 50 dB (well below 70 dB target). If reduction is conservative (~25 dB total): New level ≈ 85 – 25 = 60 dB (still achieves target). **Step 6: Cost-benefit analysis.** Investment: Double-glazed windows (~₹2,00,000), acoustic panels (~₹1,50,000), sealing (~₹50,000) = ~₹4,00,000 one-time cost. Benefits: Improved student concentration, better exam scores (studies show 7–10% improvement in noise-reduced schools), reduced teacher fatigue, enhanced health outcomes. ROI justified within 3–4 years. **Conclusion:** A combined approach of engineering controls (primary) and traffic management (supporting) will reduce classroom noise from 85 dB to <70 dB, creating a healthier learning environment. Priority: double-glazed windows and acoustic panels on the highway-facing wall.

CBSETUTOR.ai is designed specifically for Class 9 CBSE Science mastery. Our AI tutor personalizes practice using these exact question patterns and scientifically proven learning methods: **Daily Drill Structure:** (1) Adaptive Question Selection: The AI analyzes your performance on 2-mark and 3-mark questions. If you miss a definition-based question (e.g., "What is frequency?"), the AI auto-escalates to related 5-mark application questions the next day, building depth progressively. (2) Formula-to-Application Path: For Sound, our tutor chains v = f × λ across 6+ problem variations—different speeds (air/water/steel), unknown variables (solve for λ or T), and real-world contexts (ultrasound imaging, frequency of musical notes). This prevents rote memorization and builds problem-solving confidence. (3) Misconception Correction: Common errors students make ("sound travels faster in air than water" or "amplitude affects pitch") are embedded as MCQ traps. When you choose wrong, the AI explains the physics concept underlying the error and pairs it with a 2-mark definition question to reinforce clarity. (4) Timed Board Exam Simulation: Every 10 days, you attempt a full 30-mark mock test mirroring actual board distribution: 5 × 1-mark MCQs, 5 × 2-mark short-answer, 3 × 3-mark questions, and 1 × 5-mark essay. Real-time feedback highlights gaps. (5) Voice-Powered Explanations: CBSETUTOR.ai's tutor explains every answer aloud in clear, conversational Indian English—no jargon. You can replay explanations and adjust speed, ideal for auditory learners. (6) Parent Dashboard Insights: Parents see: "Your child has mastered frequency/wavelength calculations (95% accuracy) but needs practice on noise pollution essay structure." Actionable feedback drives targeted revision. **Example 3-Day Drill on Sound:** Day 1 (Monday): Define frequency, period, pitch (2-mark). MCQ on sound speed ranking. Short calculation: given f and λ, find v. Day 2 (Tuesday): 3-mark question on why sound needs medium. Apply v = f × λ with ultrasound scenario. Review Day 1 weak areas. Day 3 (Wednesday): 5-mark essay on noise pollution. Timed (15 min). AI grades and compares with model answer. Feedback: "Good structure; add one more health effect." Next lesson auto-scheduled. **Start a 3-day free trial at cbsetutor.ai** to experience AI-driven daily drills on Sound and all Class 9 Science chapters. No credit card required. Your first mock Sound test is free.