🎵 Sound – Class 9 Science MCQs

Section 4: Numericals, Applications of Sound in Daily Life & HOTS (Q76–Q100)

Q76. A sound wave has frequency 256 Hz and speed 340 m/s. Wavelength = ?

A. 0.5 m
B. 1.0 m
C. 1.25 m
D. 2.0 m

✅ Correct Answer: C. 1.25 m

Explanation: λ = v/f = 340 / 256 ≈ 1.25 m.

Q77. Sound travels 1650 m in water in 1 s. Its speed = ?

A. 330 m/s
B. 1650 m/s
C. 2000 m/s
D. 2500 m/s

✅ Correct Answer: B. 1650 m/s

Q78. If speed of sound in air = 340 m/s, distance of wall for echo (time gap = 0.2 s) = ?

A. 17 m
B. 34 m
C. 68 m
D. 85 m

✅ Correct Answer: B. 34 m

Explanation: Distance = (v × t)/2 = 340 × 0.2 / 2 = 34 m.

Q79. A sonar echo is received after 3 s. Depth of sea = ? (speed in water = 1500 m/s)

A. 1500 m
B. 2000 m
C. 2250 m
D. 4500 m

✅ Correct Answer: C. 2250 m

Explanation: Total distance = 1500 × 3 = 4500 m → depth = 4500/2 = 2250 m.

Q80. If time period of wave = 0.01 s, frequency = ?

A. 10 Hz
B. 50 Hz
C. 100 Hz
D. 200 Hz

✅ Correct Answer: C. 100 Hz

Explanation: f = 1/T = 1/0.01 = 100 Hz.

Q81. A source produces 20 vibrations in 2 seconds. Frequency = ?

A. 5 Hz
B. 10 Hz
C. 20 Hz
D. 40 Hz

✅ Correct Answer: B. 10 Hz

Q82. Ultrasonic waves are useful for:

A. Cleaning objects
B. Medical imaging
C. Detecting cracks in metals
D. All of these

✅ Correct Answer: D. All of these

Q83. Which is an application of ultrasound in medicine?

A. Sonography
B. X-rays
C. MRI
D. CT scan

✅ Correct Answer: A. Sonography

Q84. The use of ultrasound in cleaning works because:

A. It is audible
B. It produces heating
C. It produces tiny bubbles that scrub dirt
D. It changes colour

✅ Correct Answer: C. It produces tiny bubbles that scrub dirt

Q85. Which uses sound reflection to measure sea depth?

A. Radar
B. SONAR
C. Telescope
D. Periscope

✅ Correct Answer: B. SONAR

Q86. Which instrument uses reflection of sound in tubes?

A. Loudspeaker
B. Stethoscope
C. Trumpet
D. Microphone

✅ Correct Answer: B. Stethoscope

Q87. Which property of sound is used in RADAR?

A. Reflection of sound
B. Reflection of radio waves
C. Refraction of sound
D. Diffraction of sound

✅ Correct Answer: B. Reflection of radio waves

Explanation: RADAR works with electromagnetic (radio) waves, not sound.

Q88. Which of these is an infrasonic source?

A. Dog whistle
B. Whales
C. Earthquakes
D. Bats

✅ Correct Answer: C. Earthquakes

Q89. Which of these uses ultrasound?

A. Echocardiography
B. Ultrasonography
C. Cleaning machine parts
D. All of these

✅ Correct Answer: D. All of these

Q90. Which is a hazard of very loud sound?

A. Headache
B. Ear damage
C. Nausea
D. All of these

✅ Correct Answer: D. All of these

Q91. The phenomenon of persistence of hearing for 0.1 s is called:

A. Reverberation
B. Echo
C. Persistence of vision
D. Persistence of hearing

✅ Correct Answer: D. Persistence of hearing

Q92. Loudness of sound is proportional to:

A. Amplitude
B. Amplitude²
C. Frequency
D. Frequency²

✅ Correct Answer: B. Amplitude²

Q93. Which type of waves are sound waves in air?

A. Transverse
B. Longitudinal
C. Electromagnetic
D. Matter waves

✅ Correct Answer: B. Longitudinal

Q94. What determines pitch of sound?

A. Frequency
B. Amplitude
C. Waveform
D. Loudness

✅ Correct Answer: A. Frequency

Q95. Which sound property is affected in ‘noise pollution’?

A. Loudness
B. Pitch
C. Quality
D. Speed

✅ Correct Answer: C. Quality

Q96. Sound travels faster in humid air because:

A. Density increases
B. Water vapour is lighter than nitrogen/oxygen
C. Temperature falls
D. Pressure rises

✅ Correct Answer: B. Water vapour is lighter than nitrogen/oxygen

Q97. Which sound wave property allows us to distinguish two musical instruments playing same note?

A. Loudness
B. Pitch
C. Quality (timbre)
D. Frequency

✅ Correct Answer: C. Quality (timbre)

Q98. If a sound wave has frequency 500 Hz and wavelength 0.68 m, its speed = ?

A. 300 m/s
B. 320 m/s
C. 340 m/s
D. 360 m/s

✅ Correct Answer: C. 340 m/s

Explanation: v = fλ = 500 × 0.68 = 340 m/s.

Q99. The sound of thunder is heard a few seconds after lightning because:

A. Light is faster than sound
B. Sound is faster than light
C. Both travel same speed
D. Light travels in zig-zag path

✅ Correct Answer: A. Light is faster than sound

Q100. Which of these is NOT a property of sound waves?

A. Reflection
B. Refraction
C. Polarisation
D. Diffraction

✅ Correct Answer: C. Polarisation

Explanation:

• Sound waves are longitudinal → cannot be polarised.

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