Sound — Class 8 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 8 Science chapter "Sound" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 8 Science chapter "Sound" — 8 important questions with detailed answers for CBSE board exam preparation.
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
NCERT Solutions for Class 8 Science — Sound. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Sound — All 8 Questions Solved
Q1. Define sound. How is it produced?
Sound is a form of energy that produces a sensation of hearing. It is produced by vibrating objects. Vibrations create disturbances in the medium (air, water, solid), forming sound waves that travel and reach our ears, causing the eardrum to vibrate, transmitting signals to the brain.
Q2. A tuning fork vibrates 256 times per second. Find its frequency and time period.
Step 1: Frequency f = 256 Hz (given: 256 vibrations/second)
Step 2: Time period T = 1/f = 1/256 = 0.0039 seconds
Frequency = 256 Hz, Time period ≈ 0.004 s or 3.9 milliseconds
Q3. What is the relationship between frequency and pitch? Explain with example.
Pitch is directly related to frequency. Higher frequency → higher pitch (shriller sound). Lower frequency → lower pitch (deeper sound). Example: A small whistle produces high-frequency sound (high pitch), while a drum produces low-frequency sound (low pitch). Human ear perceives frequency as pitch.
Q4. Can sound travel in vacuum? Why or why not?
No, sound cannot travel in vacuum. Sound requires a medium (air, water, or solid) to propagate. Sound waves are longitudinal waves that need particles to vibrate and transfer energy. In vacuum, there are no particles, so sound waves cannot form or travel. This is why space is silent.
Q5. Define amplitude and loudness. How are they related?
Amplitude is the maximum displacement of a vibrating object from its mean position. Loudness is the perception of sound intensity by the human ear. Loudness is directly proportional to amplitude: greater amplitude → louder sound. Loudness is measured in decibels (dB).
Q6. A sound wave has a frequency of 50 Hz and travels at 340 m/s. Find its wavelength.
Step 1: Use v = f × λ (where v = speed, f = frequency, λ = wavelength)
Step 2: 340 = 50 × λ
Step 3: λ = 340/50 = 6.8 m
Wavelength = 6.8 meters
Q7. What is reflection of sound? Give two applications.
Reflection of sound is the bouncing back of sound waves from a hard surface. Applications: (1) Sonar systems use sound reflection to detect submarines and map the ocean floor, (2) Ultrasound in medical imaging, (3) Echo used to measure distances, (4) Stethoscope uses reflected sound to hear heartbeat.
Q8. Why is there an echo in a large empty hall but not in a small room?
An echo occurs when reflected sound waves reach the ear after a noticeable time delay. In a large hall, the distance between the listener and reflecting surface is sufficient for the sound to travel to the wall and back with a time gap > 0.1 seconds (minimum for human perception). In a small room, the reflected sound reaches too quickly to be perceived as a separate echo.
Sound — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Sound is produced by:
- A. Stationary objects
- B. Vibrating objects
- C. Reflecting objects
- D. Absorbing objects
Answer: B. Vibrating objects — Sound is always produced by vibrations. When an object vibrates, it sets the surrounding medium into vibration, which travels as sound.
MCQ 2. Which of the following media can sound NOT travel through?
- A. Air
- B. Water
- C. Vacuum
- D. Steel
Answer: C. Vacuum — Sound requires a material medium (solid, liquid, or gas) to propagate. It cannot travel through a vacuum because there are no particles to transmit the vibrations.
MCQ 3. A sound with a high pitch corresponds to a high:
- A. Amplitude
- B. Loudness
- C. Frequency
- D. Wavelength
Answer: C. Frequency — Pitch is the characteristic of sound determined by its frequency. A higher frequency sound is perceived as having a higher pitch.
MCQ 4. The loudness of sound is determined by its:
- A. Frequency
- B. Pitch
- C. Amplitude
- D. Wavelength
Answer: C. Amplitude — Loudness of sound is directly proportional to the square of its amplitude. A larger amplitude means more energy is carried by the sound wave, resulting in a louder sound.
MCQ 5. What is the SI unit of frequency?
- A. Metre
- B. Second
- C. Hertz
- D. Decibel
Answer: C. Hertz — The SI unit for frequency is Hertz (Hz), named after Heinrich Hertz. One Hertz is defined as one cycle per second.
MCQ 6. Which part of the human ear vibrates when sound waves strike it?
- A. Pinna
- B. Auditory nerve
- C. Eardrum
- D. Cochlea
Answer: C. Eardrum — The eardrum (tympanic membrane) is a thin, stretched membrane that vibrates when sound waves reach it. These vibrations are then transmitted to the inner ear.
MCQ 7. In which of the following media would sound travel fastest?
- A. Air at 0°C
- B. Water at 20°C
- C. Steel
- D. Hydrogen gas
Answer: C. Steel — Sound generally travels fastest in solids, then liquids, and slowest in gases. This is because particles in solids are more closely packed and transmit vibrations more efficiently, making steel the fastest medium among the options.
MCQ 8. What is the approximate audible range of frequency for human beings?
- A. 200 Hz to 20,000 Hz
- B. 2 Hz to 20 Hz
- C. 20 Hz to 20,000 Hz
- D. 20,000 Hz to 40,000 Hz
Answer: C. 20 Hz to 20,000 Hz — The typical range of hearing for a young, healthy human ear is from 20 Hertz (Hz) to 20,000 Hertz (Hz). Sounds below this are infrasound and above are ultrasound.
MCQ 9. Which of the following is NOT a measure to reduce noise pollution?
- A. Planting trees along roads
- B. Using loudspeakers at high volume
- C. Limiting horn usage in residential areas
- D. Using sound-absorbing materials in buildings
Answer: B. Using loudspeakers at high volume — Using loudspeakers at high volume directly contributes to noise pollution, rather than reducing it. The other options are effective measures to mitigate noise pollution.
MCQ 10. The sound produced by humans is due to the vibrations of the:
- A. Lungs
- B. Diaphragm
- C. Vocal cords
- D. Trachea
Answer: C. Vocal cords — In humans, sound is produced by the vibration of the vocal cords, which are located in the larynx (voice box). Air passing through them causes them to vibrate, generating sound.
MCQ 11. If a sound wave completes 50 oscillations in 1 second, its frequency is:
- A. 1 Hz
- B. 0.02 Hz
- C. 50 Hz
- D. 5 Hz
Answer: C. 50 Hz — Frequency is defined as the number of oscillations per second. If there are 50 oscillations in 1 second, the frequency is 50 Hertz.
MCQ 12. An echo is formed due to the ______ of sound.
- A. Refraction
- B. Diffraction
- C. Absorption
- D. Reflection
Answer: D. Reflection — An echo is the phenomenon of hearing a distinct repetition of a sound caused by the reflection of sound waves from a surface such as a wall, mountain, or cliff.
MCQ 13. Bats use which type of sound to navigate and find prey?
- A. Infrasound
- B. Audible sound
- C. Ultrasound
- D. Subsonic sound
Answer: C. Ultrasound — Bats emit high-frequency sounds, known as ultrasound (frequencies above 20,000 Hz), and listen for the echoes to create a 'sound map' of their surroundings, a process called echolocation.
MCQ 14. A mosquito's buzz has a higher pitch than a lion's roar. This is because the mosquito's wings vibrate at a:
- A. Lower amplitude
- B. Higher amplitude
- C. Lower frequency
- D. Higher frequency
Answer: D. Higher frequency — Pitch is determined by frequency. A higher pitch means a higher frequency of vibration, which is why a mosquito's buzz is high-pitched.
MCQ 15. The unit used to measure the intensity of sound (loudness) is:
- A. Hertz
- B. Metre
- C. Decibel
- D. Kilogram
Answer: C. Decibel — The intensity or loudness of sound is commonly measured in decibels (dB). Higher decibel values indicate louder sounds.
MCQ 16. If the amplitude of a sound wave is doubled, its loudness will:
- A. Be halved
- B. Remain the same
- C. Be doubled
- D. Be quadrupled
Answer: D. Be quadrupled — The loudness of sound is proportional to the square of its amplitude. If the amplitude is doubled (2x), the loudness will increase by (2x)^2 = 4 times, or be quadrupled.
MCQ 17. Continuous exposure to sounds above _______ decibels can cause hearing damage.
- A. 30 dB
- B. 60 dB
- C. 85 dB
- D. 120 dB
Answer: C. 85 dB — Prolonged exposure to sounds exceeding 85 decibels can lead to permanent hearing loss. For reference, normal conversation is around 60 dB.
MCQ 18. Can sound travel through the Moon's surface, which lacks an atmosphere?
- A. Yes, because it's a solid
- B. No, because it's a vacuum
- C. Yes, but only very slowly
- D. Only if it's a very loud sound
Answer: B. No, because it's a vacuum — The Moon's surface is surrounded by a vacuum. Since sound requires a medium to propagate, it cannot travel in a vacuum, regardless of the solid surface below.
MCQ 19. The human voice box is also known as the:
- A. Pharynx
- B. Larynx
- C. Trachea
- D. Esophagus
Answer: B. Larynx — The larynx, commonly called the voice box, is a cartilaginous structure in the throat containing the vocal cords, which vibrate to produce sound.
MCQ 20. Which of the following differentiates musical sound from noise?
- A. Loudness
- B. Regularity of vibrations
- C. Amplitude
- D. Medium of propagation
Answer: B. Regularity of vibrations — Musical sound is produced by regular, periodic vibrations, resulting in a pleasant sound. Noise, on the other hand, is produced by irregular or non-periodic vibrations, often perceived as unpleasant.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.