CBSE Class 9 Physics Chapter 11 Sound — 20 MCQs with Answers

Sound is produced when an object vibrates, creating compressions and rarefactions in the surrounding medium. These disturbances travel as longitudinal waves — meaning particle motion is parallel to the direction of wave travel. Sound cannot propagate through a vacuum because it needs particles to push against each other. The speed of sound varies widely: approximately 343 m/s in air at 20°C, 1480 m/s in water, and 5000 m/s in steel. Solids transmit sound faster than liquids, and liquids faster than gases, because closely packed particles transfer vibrations more efficiently. Temperature also affects speed — sound travels faster in warmer air. These MCQs test your understanding of sound production, medium requirements, and speed variations across different states of matter.

Pitch is the sensation of how 'high' or 'low' a sound is, determined by frequency (measured in Hertz). A piccolo has high pitch (high frequency ~4000 Hz); a tuba has low pitch (low frequency ~100 Hz). Loudness depends on amplitude — the maximum displacement of particles from rest. Larger amplitude means louder sound, measured in decibels (dB). Timbre (or quality) is what makes a piano and a violin sound different even when playing the same note at the same loudness — it is determined by the number and intensity of harmonics (overtones). Understanding these characteristics is crucial for explaining everyday phenomena like musical instruments, voice differences, and noise pollution. The following MCQs cover definitions, units, and real-life applications of pitch, loudness, and timbre.

When sound waves strike a hard, rigid surface like a wall or cliff, they reflect back — following the law of reflection (angle of incidence equals angle of reflection). An echo is a distinct repetition of sound caused by this reflection. For an echo to be heard separately from the original sound, the reflected wave must reach your ear at least 0.1 seconds later. Using the speed of sound in air (~340 m/s), the minimum distance for an echo is calculated as distance = speed × time = 340 m/s × 0.1 s = 34 meters (round trip), meaning the reflecting surface must be at least 17 meters away. In smaller rooms, reflections merge into reverberation — a prolonged, muddled sound rather than a distinct echo. Hard surfaces like marble and concrete reflect sound well; soft materials like carpets and curtains absorb it. These MCQs test calculations, definitions, and practical applications of sound reflection.

SONAR (Sound Navigation and Ranging) uses ultrasound waves (frequency >20,000 Hz, above human hearing) to detect underwater objects and measure ocean depth. A SONAR device sends a sound pulse into water, waits for the reflection from the seabed or an object, and measures the time delay. Using the formula distance = (speed of sound in water × time) ÷ 2, navigators calculate depth or distance. The division by 2 accounts for the round trip — sound travels to the target and back. Typical speed of sound in seawater is ~1500 m/s. SONAR is used by submarines for navigation, fishing boats to locate fish schools, and oceanographers to map the ocean floor. Medical ultrasound imaging uses the same principle — sending sound into the body and analysing reflections to create images of organs or a fetus. These MCQs cover SONAR calculations, principles, and real-world uses.

The human ear is a sophisticated biological sensor that converts sound waves into electrical signals the brain interprets. The outer ear (pinna and ear canal) collects and channels sound to the eardrum, a thin membrane that vibrates. These vibrations pass through three tiny bones in the middle ear — the ossicles (hammer, anvil, stirrup) — which amplify the vibration about 30 times. The stirrup transmits vibrations to the cochlea in the inner ear, a spiral fluid-filled structure lined with thousands of hair cells. Different frequencies stimulate different hair cells, which generate electrical impulses sent via the auditory nerve to the brain. The normal human hearing range is 20 Hz to 20,000 Hz. Sounds below 20 Hz are infrasound (felt as vibrations, like earthquakes); sounds above 20 kHz are ultrasound (inaudible to humans but heard by dogs, bats, dolphins). Prolonged exposure to sounds above 85 dB can damage hearing; sounds above 120 dB cause pain. These MCQs test ear structure, function, and hearing limits.

Multiple-choice questions in CBSE Class 9 Physics papers typically carry one mark each and test both recall and application. Read each question carefully — often a single word (like 'not', 'maximum', 'minimum') changes the correct answer. Eliminate obviously wrong options first to narrow your choices. For numerical MCQs (like echo distance or SONAR depth), write the formula in the margin, substitute values, and calculate before choosing. For assertion-reason questions (common in CBSE), evaluate the assertion and reason independently first, then check if the reason correctly explains the assertion. If you are unsure, mark your best guess and flag the question to revisit if time permits — there is no negative marking in CBSE board exams, so never leave an MCQ blank. Practice time management: aim to spend no more than 30–45 seconds per MCQ during the exam. Regular practice with NCERT-based MCQs builds speed and confidence. Use the last five minutes of the exam to review flagged questions and ensure you have not misread any option or left any question unattempted. Smart MCQ technique can easily add 4–5 extra marks to your Physics score.

Many students confuse pitch with loudness — remember pitch is frequency-based (how high or low), loudness is amplitude-based (how strong or weak). Another frequent error is forgetting to divide by 2 in echo and SONAR distance calculations; always account for the round trip (sound travels to the obstacle and back). Students often mix up ultrasound and infrasound definitions — ultrasound is above 20 kHz, infrasound is below 20 Hz. In questions about the human ear, candidates sometimes confuse the cochlea (inner ear, converts vibrations to signals) with the eardrum (middle ear, vibrates in response to sound). When asked about the speed of sound in different media, students may incorrectly assume vacuum can carry sound — it cannot, because sound needs particles. Lastly, in assertion-reason questions, many mark an option without checking whether the reason actually explains the assertion logically. Avoiding these pitfalls and practicing with NCERT-aligned MCQs will significantly boost your exam confidence and accuracy.

Preparing for CBSE Class 9 Physics requires clarity on concepts, regular practice with MCQs, and instant doubt resolution — especially when you are stuck on a tricky numerical or a confusing assertion-reason question at 10 PM before your exam. CBSETUTOR.ai gives your child a 24×7 AI tutor that answers questions in seconds, solves problems step-by-step when you upload a photo of the question, and provides NCERT-aligned explanations for every chapter including Sound. Whether your child needs help understanding why sound travels faster in solids, how to calculate SONAR depth, or what timbre means, the AI tutor responds in simple language with worked examples. The platform covers all CBSE classes (6 to 12) and subjects at one flat price of ₹999 per month — no hidden charges, no per-subject fees. Parents in metro cities and Tier-2 towns alike appreciate the convenience: no commute to coaching centres, no fixed class timings, just on-demand learning whenever your child needs it. Start with a free 3-day trial to see how AI-powered tutoring complements school teaching, sharpens exam skills, and builds genuine confidence in Physics and every other subject.