Sound Waves: Characteristics and Applications — Class 9 MCQs with Answers

Sound Waves: Characteristics and Applications — important questions & MCQs with answers (Class 9 Science)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Sound waves cannot travel through:

   AAirBWaterCVacuumDSteel
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   Answer: 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. Q2 of 6Easy

   SI unit of frequency is:

   AHertzBNewtonCPascalDJoule
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   Answer: 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. Q3 of 6Medium

   SONAR uses:

   ARadio wavesBVisible lightCUltrasonic wavesDX-rays
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   Answer: 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.

4. Q4 of 6Medium

   Assertion (A): Sound waves are longitudinal waves. Reason (R): In a sound wave, the particles of the medium vibrate parallel to the direction in which the wave travels.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: Both are true. A wave in which the particles vibrate parallel to the direction of propagation is a longitudinal wave, so R explains A.

5. Q5 of 6Hard

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

   A0.1 mB1 mC3.4 mD34 m
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   Answer: 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.

6. Q6 of 6Hard

   Assertion (A): A person who claps while standing 10 m from a tall wall will hear a distinct echo (speed of sound = 340 m s⁻¹). Reason (R): Two sounds are heard separately only if they reach us at least 0.1 s apart.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is false but R is true

   Explanation: A is false: the sound travels 20 m to the wall and back in 20 ÷ 340 ≈ 0.06 s, which is less than 0.1 s. The wall must be at least 17 m away. R is true.

Sound Waves: Characteristics and Applications notes: key concepts (Class 9 Science)

Sound is a mechanical wave produced by a vibrating body and carried to the ear by the particles of a medium. This chapter covers how sound propagates, the characteristics that distinguish one sound from another, reflection and echoes, and applications such as SONAR.

Production of sound

Sound is produced by a vibrating object; when the vibration stops, so does the sound.

Sound needs a medium

Sound is a mechanical wave and cannot travel through vacuum, which is why a bell ringing in an evacuated jar cannot be heard.

Longitudinal wave

Sound travels as a longitudinal wave in which particles vibrate along the direction of propagation, creating compressions and rarefactions.

Compression and rarefaction

Compressions are regions of high pressure and density; rarefactions are regions of low pressure and density.

Wavelength

The distance between two consecutive compressions or rarefactions, measured in metres.

Frequency

The number of complete oscillations per second, measured in hertz; it is fixed by the source and does not change with the medium.

Time period

The time taken for one complete oscillation, which is the reciprocal of the frequency.

Amplitude

The maximum displacement of a particle from its rest position, which determines how loud the sound is.

Pitch and frequency

Pitch is how shrill or flat a sound seems, and it rises with frequency — a higher frequency gives a higher pitch.

Loudness and amplitude

Loudness depends on amplitude, and it is a measure of the energy carried by the wave rather than a property of the source alone.

Quality (timbre)

The characteristic that distinguishes two sounds of the same pitch and loudness, letting you tell a violin from a flute.

Speed of sound in different media

Sound travels fastest in solids, slower in liquids and slowest in gases, because the particles are more closely packed in solids.

Reflection of sound and echo

Sound reflects off hard surfaces; a distinct echo is heard only when the reflecting surface is far enough for a 0.1 s gap.

Reverberation

The persistence of sound in a hall by repeated reflection, reduced by using curtains, carpets and sound-absorbing boards.

Range of hearing

Humans hear from 20 Hz to 20 000 Hz; below this is infrasound and above it is ultrasound.

SONAR and uses of ultrasound

Ultrasonic pulses are used to measure sea depth by echo ranging, and in medicine to image internal organs and break kidney stones.