Sound — Class 9 MCQs with Answers

Sound — important questions & MCQs with answers (Class 9 Science)

14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 76 are timed and scored when you sign in.

1. Q1Easy

   Sound waves cannot travel through:

   A.Air

   B.Water

   C.Vacuum✓ Correct

   D.Steel

   Answer: C. Vacuum

   Explanation: Sound is a mechanical wave — it needs particles of a medium to vibrate and pass on the disturbance. A vacuum has no particles, so unlike light, sound cannot cross it.

2. Q2Easy

   SI unit of frequency is:

   A.Hertz✓ Correct

   B.Newton

   C.Pascal

   D.Joule

   Answer: A. Hertz

   Explanation: Frequency counts oscillations per second; its SI unit is the hertz (1 Hz = 1 oscillation/s), unlike newton (force), pascal (pressure), or joule (energy).

3. Q3Easy

   An echo is heard due to:

   A.Refraction of sound

   B.Reflection of sound✓ Correct

   C.Diffraction

   D.Resonance

   Answer: B. Reflection of sound

   Explanation: An echo is the original sound wave bouncing off a hard, distant surface and returning to the listener — the same reflection principle as light off a mirror.

4. Q4Easy

   Sound waves are:

   A.Transverse

   B.Longitudinal✓ Correct

   C.Electromagnetic

   D.Stationary only

   Answer: B. Longitudinal

   Explanation: Particles of the medium vibrate parallel to the direction sound travels, forming compressions and rarefactions — this makes sound a longitudinal wave, not transverse.

5. Q5Easy

   The pitch of a sound depends on:

   A.Frequency✓ Correct

   B.Amplitude

   C.Wavelength only

   D.Speed only

   Answer: A. Frequency

   Explanation: Pitch tracks a sound's frequency: more vibrations per second gives a shriller, higher-pitched sound. Amplitude instead controls loudness, not pitch.

6. Q6Easy

   SONAR stands for:

   A.Sound Operation Navy And Range

   B.Signal Navigation And Radar

   C.Sound Navigation And Ranging✓ Correct

   D.System Of Navigation And Radar

   Answer: C. Sound Navigation And Ranging

   Explanation: SONAR expands to Sound Navigation And Ranging — sending out sound pulses and timing their echoes to locate objects, most often used underwater.

7. Q7Medium

   Speed of sound in a medium depends mainly on the medium's:

   A.Colour

   B.Frequency

   C.Volume

   D.Elasticity and density✓ Correct

   Answer: D. Elasticity and density

   Explanation: Wave speed in a medium depends on how strongly its particles are bonded (elasticity) and how much mass they carry (density) — not on colour, frequency, or sample volume.

8. Q8Medium

   Speed of a wave v relates to frequency f and wavelength λ as:

   A.v = f / λ

   B.v = f + λ

   C.v = λ / f

   D.v = f λ✓ Correct

   Answer: D. v = f λ

   Explanation: For any wave, speed equals frequency times wavelength: v = f λ, linking how fast a wave moves to how often it oscillates and how far apart its repeats are.

9. Q9Medium

   SONAR uses:

   A.Radio waves

   B.Visible light

   C.Ultrasonic waves✓ Correct

   D.X-rays

   Answer: C. Ultrasonic waves

   Explanation: SONAR (Sound Navigation And Ranging) sends ultrasonic pulses underwater and times the echo, since sound propagates far better than radio waves or light through water.

10. Q10Medium

    In a longitudinal wave, particles of the medium vibrate:

    A.Perpendicular to wave direction

    B.In circles

    C.Parallel to wave direction✓ Correct

    D.Not at all

    Answer: C. Parallel to wave direction

    Explanation: A longitudinal wave moves particles back and forth along the same line the wave travels, unlike a transverse wave where particles vibrate perpendicular to it.

11. Q11Medium

    Audible range of frequency for humans is approximately:

    A.1 – 100 Hz

    B.20 Hz – 20 kHz✓ Correct

    C.20 kHz – 200 kHz

    D.100 Hz – 1000 Hz

    Answer: B. 20 Hz – 20 kHz

    Explanation: The human ear typically detects sound from about 20 Hz to 20 kHz. Frequencies below this are infrasonic and those above are ultrasonic — both inaudible to people.

12. Q12Hard

    Which is INCORRECT about sound waves?

    A.They are longitudinal

    B.They need a medium

    C.They transfer energy

    D.They are transverse in air✓ Correct

    Answer: D. They are transverse in air

    Explanation: Sound in air is longitudinal — particles vibrate parallel to the direction of travel, forming compressions and rarefactions, never perpendicular to it as in a transverse wave.

13. Q13Hard

    Wavelength of a sound wave with frequency 340 Hz in air (v = 340 m/s) is:

    A.0.1 m

    B.1 m✓ Correct

    C.3.4 m

    D.34 m

    Answer: B. 1 m

    Explanation: λ = v/f = 340 ÷ 340 = 1 m. Since the frequency here numerically equals the speed, the wavelength conveniently works out to exactly 1 m.

14. Q14Hard

    Minimum distance from a wall to hear a clear echo (sound speed 340 m/s, persistence 0.1 s) is approximately:

    A.170 m

    B.34 m

    C.68 m

    D.17 m✓ Correct

    Answer: D. 17 m

    Explanation: The echo's round trip 2d must fit within the ear's 0.1 s persistence of hearing: 2d = 340 × 0.1 = 34 m, so d = 34 ÷ 2 = 17 m.

Sound — FAQs

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