Class 9 Science – Sound Waves | Original Visual Revision Notes

📝 Section 1 — Classroom Notes

💡 Think about this first! Drop a stone into still water and watch the ripples spread outward in circles. Sound works in a very similar way — except it travels through the air as invisible push-and-pull waves instead of water ripples. Every time something vibrates (a drum skin, a guitar string, your own vocal cords) it pushes the air particles around it. Those particles push their neighbors, and so on — carrying the sound all the way to your ears. In this lesson, we explore how these invisible waves work, travel, and bring us all the sounds we hear.

🥁 Topic 1: How Sound is Made

Sound comes from objects that vibrate quickly.
Vibration means quick to-and-fro movement.
A guitar string vibrates when you pluck it.
Stop the vibration and the sound stops.
Vibrating objects push nearby air molecules too.

🌊 Topic 2: Sound Needs a Medium

Sound needs a medium to travel through.
A medium can be solid, liquid, or gas.
Sound cannot travel through empty space or vacuum.
Air carries most sounds we hear daily.
Sound also travels well through water and metal.

🔘 Topic 3: Sound Waves — Compressions and Rarefactions

Sound is a longitudinal mechanical wave in air.
Particles vibrate parallel to the wave direction.
Compression is a region with tightly packed particles.
Rarefaction is a region with spread out particles.
Compressions and rarefactions travel one after another.

📏 Topic 4: Wavelength, Frequency, and Time Period

Wavelength is distance between two nearby compressions.
Wavelength symbol is lambda, measured in metres.
Frequency counts oscillations happening every single second.
Frequency unit is hertz, written as Hz.
Time period is time for one full oscillation.

🚀 Topic 5: Speed of Sound

Speed equals wavelength multiplied by frequency value.
Sound moves fastest through solid materials.
Sound moves slower through liquid materials than solids.
Sound moves slowest through gases like air.
Air carries sound near 340 metres per second.

📣 Topic 6: Amplitude and Loudness

Amplitude is the maximum change from average position.
Bigger amplitude means the sound is louder.
Loudness is measured using a unit called decibel.
Soft whispers have small sound wave amplitude.
Loud shouts have much larger wave amplitude.

🎵 Topic 7: Pitch and Frequency

Pitch tells us how high or low sound is.
High frequency waves give a high, shrill pitch.
Low frequency waves give a low, deep pitch.
A small bird chirps with a high pitch.
A big drum beats with a low pitch.

👂 Topic 8: Audible, Infrasonic, and Ultrasonic

Humans hear sound between 20 Hz and 20000 Hz.
This range is called the audible range.
Sound below 20 Hz is called infrasonic sound.
Sound above 20000 Hz is called ultrasonic sound.
Some animals can hear beyond human hearing range.

🏔 Topic 9: Echo and Reflection of Sound

Sound reflects off hard surfaces like walls.
A reflected sound we hear again is echo.
The reflecting surface must be far enough for the echo to arrive at least 0.1 s after the original sound.
Our ear holds sound for 0.1 seconds.
Reverberation is many echoes mixing inside a hall.

🦇 Topic 10: SONAR and Echolocation

SONAR means Sound Navigation And Ranging system.
Ships use SONAR to find objects underwater.
SONAR sends ultrasonic waves toward the seabed.
Bats use echolocation to fly in darkness.
Bats send ultrasound and listen for its echo.

🔬 Section 3 — Worked Examples

💡 Example 1: Speed of Sound

A sound wave has frequency 250 Hz and wavelength 1.36 m. Find its speed.

Speed = frequency × wavelength.

Speed = 250 × 1.36.

Speed = 340 m/s.

v = ν × λ

Answer: The speed of sound is 340 m/s.

💡 Example 2: Echo Distance

You clap and hear the echo after 0.4 s. Speed of sound is 340 m/s. Find the distance to the wall.

Total distance travelled = speed × time.

Total distance = 340 × 0.4 = 136 m.

Sound travels to wall and back, so divide by 2.

Distance = (Speed × Time) ÷ 2

Answer: The wall is 68 m away.

💡 Example 3: SONAR Depth

A ship sends an ultrasonic signal. It returns after 1.2 s. Speed of sound in seawater is 1500 m/s. Find the depth.

Total distance travelled = speed × time.

Total distance = 1500 × 1.2 = 1800 m.

Signal goes down and comes back, so divide by 2.

Depth = (Speed × Time) ÷ 2

Answer: The depth of the seabed is 900 m.

📋 Section 4 — Cheat Sheet

Term	Meaning
Vibration	Quick to-and-fro motion of an object.
Medium	Material through which sound travels.
Longitudinal Wave	Particles vibrate along the wave direction.
Compression	Region where particles are packed close together.
Rarefaction	Region where particles are spread far apart.
Wavelength (λ)	Distance between two nearby compressions or rarefactions.
Frequency (ν)	Number of oscillations completed every second.
Time Period (T)	Time taken for one complete oscillation.
Amplitude	Maximum change in density from the average.
Speed of Sound	Distance sound covers in one second.
Pitch	How high or low a sound feels.
Loudness	How soft or loud a sound feels.
Audible Range	Sound humans can hear, 20 Hz to 20000 Hz.
SONAR	System using ultrasound to find underwater objects.

🏠 Section 5 — Take-Home Handout

Part A: Fill in the Blanks

1. Sound is produced by a _______ object.
2. Sound cannot travel through a _______.
3. Compressions are regions of _______ density.
4. The unit of frequency is _______.
5. High frequency sound has a high _______.

Part B: True or False

1. Sound can travel through empty vacuum space.
2. Amplitude decides how loud a sound is.
3. Ultrasonic sound has frequency below 20 Hz.
4. SONAR uses ultrasonic waves under the water.
5. Echo is heard when the reflected sound reaches your ears at least 0.1 s after the original sound.

Part C: Solve These

1. A wave has frequency 200 Hz and wavelength 1.7 m. Find its speed.

2. A ship hears its echo after 2 s in water where speed is 1500 m/s. Find the depth.

3. A sound of frequency 50 Hz repeats how many times in 4 seconds?

Part D: Match the Columns

Column A	Column B	Your Answer
1. Wavelength	A. Number of oscillations per second
2. Frequency	B. Distance between two compressions
3. Amplitude	C. Sound reflected back to our ears
4. Echo	D. Decides how loud a sound is
5. SONAR	E. Finds objects underwater using sound

Part E: Draw and Label

Draw a sound wave in a tube. Label the compression and rarefaction regions.

✅ Detailed Solutions — Section 5 Take-Home Handout

Part A: Fill in the Blanks — Answers

Sound is produced by a vibrating object.
Sound cannot travel through a vacuum.
Compressions are regions of high density.
The unit of frequency is hertz (Hz).
High frequency sound has a high pitch.

Part B: True or False — Answers

False. Sound cannot travel through vacuum because there are no particles to vibrate and pass the sound energy forward.
True. Larger amplitude means the particles compress and expand more, creating a louder sound.
False. Ultrasonic sound has frequency above 20,000 Hz. Below 20 Hz is called infrasonic.
True. SONAR sends ultrasonic pulses from a ship and measures how long it takes for the echo to return.
True. Our ears retain sound for 0.1 s, so the echo must arrive at least 0.1 s after the original sound to be heard separately.

Part C: Solve These — Step-by-Step Solutions

Q1. A wave has frequency 200 Hz and wavelength 1.7 m. Find its speed.

Speed = frequency × wavelength.

Speed = 200 × 1.7 = 340 m/s.

Answer: Speed of sound = 340 m/s.

Q2. A ship hears its echo after 2 s in water where speed is 1500 m/s. Find the depth.

Total distance = speed × time = 1500 × 2 = 3000 m.

Sound travels to the seabed and comes back, so divide by 2.

Depth = 3000 ÷ 2 = 1500 m.

Answer: The depth of the seabed is 1500 m.

Q3. A sound of frequency 50 Hz repeats how many times in 4 seconds?

Frequency = 50 Hz means 50 oscillations every second.

In 4 seconds: 50 × 4 = 200 oscillations.

Answer: The sound completes 200 oscillations (cycles) in 4 seconds.

Part D: Match the Columns — Answers

Column A	Match	Column B
1. Wavelength	→ B	Distance between two compressions
2. Frequency	→ A	Number of oscillations per second
3. Amplitude	→ D	Decides how loud a sound is
4. Echo	→ C	Sound reflected back to our ears
5. SONAR	→ E	Finds objects underwater using sound

🔊 Class 9 Science – Sound Waves
Original Visual Revision Notes Aligned with the Class 9 Science syllabus. Independently created study material. Not affiliated with or endorsed by NCERT.

📑 What is inside this lesson

📝 Section 1 — Classroom Notes

🥁 Topic 1: How Sound is Made

🌊 Topic 2: Sound Needs a Medium

🔘 Topic 3: Sound Waves — Compressions and Rarefactions

📏 Topic 4: Wavelength, Frequency, and Time Period

🚀 Topic 5: Speed of Sound

📣 Topic 6: Amplitude and Loudness

🎵 Topic 7: Pitch and Frequency

👂 Topic 8: Audible, Infrasonic, and Ultrasonic

🏔 Topic 9: Echo and Reflection of Sound

🦇 Topic 10: SONAR and Echolocation

🎨 Section 2 — Easy Illustrations

🔬 Section 3 — Worked Examples

💡 Example 1: Speed of Sound

💡 Example 2: Echo Distance

💡 Example 3: SONAR Depth

📋 Section 4 — Cheat Sheet

🏠 Section 5 — Take-Home Handout

Part A: Fill in the Blanks

Part B: True or False

Part C: Solve These

Part D: Match the Columns

Part E: Draw and Label

✅ Detailed Solutions — Section 5 Take-Home Handout

Part A: Fill in the Blanks — Answers

Part B: True or False — Answers

Part C: Solve These — Step-by-Step Solutions

Part D: Match the Columns — Answers

🔄 Section 6 — Revision Flashcards

🧠 Section 7 — Mind Map

🔊 Class 9 Science – Sound WavesOriginal Visual Revision Notes Aligned with the Class 9 Science syllabus. Independently created study material. Not affiliated with or endorsed by NCERT.

📑 What is inside this lesson

📝 Section 1 — Classroom Notes

🥁 Topic 1: How Sound is Made

🌊 Topic 2: Sound Needs a Medium

🔘 Topic 3: Sound Waves — Compressions and Rarefactions

📏 Topic 4: Wavelength, Frequency, and Time Period

🚀 Topic 5: Speed of Sound

📣 Topic 6: Amplitude and Loudness

🎵 Topic 7: Pitch and Frequency

👂 Topic 8: Audible, Infrasonic, and Ultrasonic

🏔 Topic 9: Echo and Reflection of Sound

🦇 Topic 10: SONAR and Echolocation

🎨 Section 2 — Easy Illustrations

🔬 Section 3 — Worked Examples

💡 Example 1: Speed of Sound

💡 Example 2: Echo Distance

💡 Example 3: SONAR Depth

📋 Section 4 — Cheat Sheet

🏠 Section 5 — Take-Home Handout

Part A: Fill in the Blanks

Part B: True or False

Part C: Solve These

Part D: Match the Columns

Part E: Draw and Label

✅ Detailed Solutions — Section 5 Take-Home Handout

Part A: Fill in the Blanks — Answers

Part B: True or False — Answers

Part C: Solve These — Step-by-Step Solutions

Part D: Match the Columns — Answers

🔄 Section 6 — Revision Flashcards

🧠 Section 7 — Mind Map

🔊 Class 9 Science – Sound Waves
Original Visual Revision Notes Aligned with the Class 9 Science syllabus. Independently created study material. Not affiliated with or endorsed by NCERT.