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Sound Waves: Characteristics and Applications – Class 9 Science notes, important questions and MCQs

NCERT Class 9 Science, chapter 10. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Sound is a mechanical longitudinal wave produced by vibrating objects. It needs a medium, and its frequency, amplitude and speed decide pitch, loudness and echoes.

Nature of sound

Sound travels as compressions and rarefactions (longitudinal wave).
It cannot travel through vacuum.
Speed is greatest in solids, less in liquids, least in gases (about 343 m/s in air at 20 °C).

Characteristics

Frequency (Hz) decides pitch; amplitude decides loudness; quality (timbre) distinguishes sources.
v = f × λ; time period T = 1/f.
Human hearing range: 20 Hz to 20 kHz; below is infrasound, above is ultrasound.

Reflection and uses

Echo: reflected sound heard separately — needs at least 17.2 m to the reflecting surface (0.1 s gap).
Reverberation: repeated reflections; reduced by soft materials in halls.
Uses of ultrasound: SONAR, cleaning, detecting cracks, ultrasound scans; bats use echolocation.

Key terms

Compression: region of high pressure in a sound wave
Rarefaction: region of low pressure in a sound wave
Echo: sound heard after reflection from a surface
SONAR: Sound Navigation And Ranging, uses ultrasound to find depth and objects

Formulas

v = f λ
Echo distance d = v t / 2

Important questions with answers

Why can sound not travel through a vacuum?2 marks

Sound is a mechanical wave and needs particles of a medium to vibrate and pass on the energy; a vacuum has no particles.

A person hears an echo 2 s after shouting near a cliff. If the speed of sound is 340 m/s, how far is the cliff?3 marks

Sound travels to the cliff and back: total distance = 340 × 2 = 680 m. Distance to the cliff = 680/2 = 340 m.

Explain how pitch and loudness depend on frequency and amplitude. What are ultrasounds? Give three uses.5 marks

Pitch depends on frequency: higher frequency gives a higher (shriller) pitch. Loudness depends on amplitude: larger amplitude gives a louder sound. Ultrasounds are sounds above 20,000 Hz, which humans cannot hear. Uses: ultrasound scans of internal organs and babies in the womb; SONAR to find sea depth and underwater objects; cleaning hard-to-reach parts and detecting cracks in metal blocks.

Quick revision

Sound needs a medium.
Pitch ↔ frequency; loudness ↔ amplitude.
Ultrasound is above 20 kHz.

Exam tips

In echo problems, sound travels to the wall and back — divide total distance by 2.
Loudness is measured in decibels (dB).

Practice MCQs

Sound cannot travel through
1. water
2. iron
3. vacuum
4. air
Show answer
Answer: (c) vacuum. It needs a medium.
Pitch of a sound depends on its
1. amplitude
2. frequency
3. speed
4. wavelength only
Show answer
Answer: (b) frequency. Frequency.
Sound waves in air are
1. longitudinal
2. transverse
3. electromagnetic
4. light waves
Show answer
Answer: (a) longitudinal. Compressions and rarefactions.
A wave has frequency 200 Hz and wavelength 1.7 m. Its speed is
1. 34 m/s
2. 117 m/s
3. 201.7 m/s
4. 340 m/s
Show answer
Answer: (d) 340 m/s. 200 × 1.7.
Bats find obstacles using
1. infrasound
2. radio waves
3. light
4. ultrasound
Show answer
Answer: (d) ultrasound. Echolocation.
The audible range of humans is
1. 2–200 Hz
2. 20 Hz–20 kHz
3. 20–200 kHz
4. above 20 kHz
Show answer
Answer: (b) 20 Hz–20 kHz. 20 Hz to 20 kHz.
A person hears an echo 2 s after shouting. If v = 340 m/s, the wall is
1. 680 m away
2. 85 m away
3. 170 m away
4. 340 m away
Show answer
Answer: (d) 340 m away. d = 340 × 2 / 2.
A ship's SONAR gets an echo after 4 s. Speed of sound in water is 1500 m/s. Depth is
1. 6000 m
2. 3000 m
3. 1500 m
4. 375 m
Show answer
Answer: (b) 3000 m. 1500 × 4 / 2.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.