Sound is a form of energy produced by vibrating objects and carried through a medium as waves. Important Questions Class 9 Science Chapter 10 cover sound waves, longitudinal waves, frequency, wavelength, amplitude, speed of sound, echo, reverberation, ultrasound, SONAR, and the human ear.
A bat finds insects in the dark. A ship measures sea depth without touching the ocean floor. A singer’s voice sounds different from a flute even when both play the same note.
Class 9 Science Chapter 10 explains these everyday examples through sound waves. The chapter becomes easier when students connect each idea to one clear concept: frequency decides pitch, amplitude decides loudness, and reflection explains echo, reverberation, and SONAR.
Key Takeaways
| Detail |
Information |
| Chapter |
Class 9 Science Chapter 10 |
| Topic |
Sound Waves: Characteristics and Applications |
| Syllabus |
CBSE 2026 |
| Question Types |
VSA, Short Answer, Long Answer, Numericals, Assertion-Reason, Case Study |
| High-Value Topics |
Speed of Sound, Echo, SONAR, Ultrasound, Human Ear |
| Main Formulas |
v = fλ, T = 1/f, d = vt/2 |
| Best Revision Method |
Revise concepts first, then solve numericals and case studies |
Class 9 Science Chapter List
Important Topics in Class 9 Science Chapter 10 Sound Waves
Class 9 science chapter 10 sound waves questions usually test definitions, formulas, applications, and reasoning.
Students should revise the wave terms first. Then they should practise speed of sound numericals class 9 and echo numericals class 9 because these are high-scoring.
- Production of sound by vibrating objects
- Sound as a mechanical wave
- Sound as a longitudinal wave
- Compression and rarefaction
- Frequency, wavelength, time period, and amplitude
- Speed of sound in different media
- Pitch, loudness, and timbre
- Reflection of sound
- Echo and reverberation
- Infrasound and ultrasound
- SONAR working and applications
- Human hearing range
- Structure and working of the human ear
- Noise pollution and its control
Important Questions Class 9 Science Chapter 10 with Answers
Important Questions Class 9 Science Chapter 10 should be practised by question type. Start with definitions, then move to short answers, long answers, numericals, assertion-reason, and case studies.
These class 9 science chapter 10 important questions with answers cover theory, formulas, diagrams, and applications from Sound Waves: Characteristics and Applications.
Very Short Answer Important Questions Class 9 Science Chapter 10
Very short answer questions test direct definitions and basic facts. Keep answers short and accurate.
Class 9 Science Chapter 10 Important Questions with Answers
Q1. How is sound produced?
Ans. Sound is produced by vibrating objects.
When an object vibrates, it disturbs the surrounding medium and transfers sound energy as a wave.
Q2. What type of wave is sound?
Ans. Sound is a mechanical and longitudinal wave.
It needs a medium to travel. The particles vibrate in the same direction as the wave travels.
Q3. What is the SI unit of frequency?
Ans. The SI unit of frequency is hertz.
One hertz means one vibration per second.
Q4. Define amplitude of a sound wave.
Ans. Amplitude is the maximum displacement of a particle from its mean position.
Greater amplitude produces louder sound.
Q5. What is the speed of sound in air at 25°C?
Ans. The speed of sound in air at 25°C is about 346 m/s.
Speed increases when temperature rises.
Q6. Define echo.
Ans. Echo is the reflected sound heard distinctly after the original sound.
For a clear echo, the reflecting surface should be about 17.2 m away at room temperature.
Q7. What is the audible range for humans?
Ans. The human audible range is 20 Hz to 20,000 Hz.
Sounds below 20 Hz are called infrasound. Sounds above 20,000 Hz are called ultrasound.
Q8. What does SONAR stand for?
Ans. SONAR stands for Sound Navigation And Ranging.
It uses ultrasound to detect underwater objects and measure distances.
Short Answer Questions Class 9 Science Chapter 10
Short answers test whether students can explain a concept in 2 to 4 clear points.
Use correct terms like mechanical wave, longitudinal wave, compression, rarefaction, frequency, and amplitude.
Sound Waves Class 9 Important Questions
Q1. Distinguish between compression and rarefaction in a longitudinal wave.
Ans.
| Compression |
Rarefaction |
| Particles are closer together |
Particles are farther apart |
| Pressure is higher than normal |
Pressure is lower than normal |
| Density is higher |
Density is lower |
| Forms alternately with rarefaction |
Forms alternately with compression |
Sound travels through a medium as alternate compressions and rarefactions.
Q2. Why can sound not travel in outer space?
Ans. Sound is a mechanical wave.
It needs particles of a medium to transfer vibrations. Outer space is almost a vacuum, so there are no particles to carry sound.
Q3. Explain the relationship between frequency and pitch.
Ans. Pitch depends on frequency.
A higher frequency produces a higher-pitched sound. A lower frequency produces a lower-pitched sound.
A child’s voice usually has a higher pitch than an adult’s voice because it has higher frequency.
Q4. What is reverberation? How is it different from echo?
Ans. Reverberation is the persistence of sound due to repeated reflections inside an enclosed space.
Echo is a distinct repetition of sound heard after a clear time gap.
Reverberation happens when reflected sounds reach the ear very quickly. Echo needs a minimum time gap of 0.1 seconds.
Q5. State two applications of ultrasound.
Ans. Ultrasound is used in medical sonography to examine internal organs.
It is also used in industries to detect cracks and flaws inside metal objects.
Q6. What are the three characteristics of musical sound?
Ans. The three characteristics are pitch, loudness, and timbre.
Pitch depends on frequency. Loudness depends on amplitude. Timbre helps distinguish sounds from different sources.
Long Answer Important Questions Class 9 Science Chapter 10
Long answers need a structured explanation. Use headings, formulas, and applications wherever needed.
Ultrasound and SONAR Class 9 Questions
Q1. Explain how SONAR works. Write its formula and two applications.
Ans. SONAR uses ultrasound waves to detect underwater objects and measure distance.
A transmitter sends ultrasound pulses into water. These waves travel to the seabed or an underwater object. They reflect back and are detected by a receiver.
The time taken by the echo is recorded.
Formula:
Distance = (Speed of sound in water × Time taken) / 2
The division by 2 is used because sound travels to the object and then returns.
Applications:
- Measuring sea depth
- Detecting submarines, shipwrecks, and fish shoals
Q2. Describe the structure and working of the human ear.
Ans. The human ear has three main parts: outer ear, middle ear, and inner ear.
The outer ear includes the pinna and ear canal. The pinna collects sound waves and directs them to the eardrum.
The middle ear contains three small bones: hammer, anvil, and stirrup. These bones amplify the vibrations and pass them to the inner ear.
The inner ear contains the cochlea. The cochlea converts vibrations into electrical signals.
The auditory nerve carries these signals to the brain. The brain interprets them as sound.
Class 9 Science Chapter 10 Numericals and Problem-Based Questions
Three formulas cover most numericals from this chapter.
| Concept |
Formula |
| Speed of sound |
v = f × λ |
| Time period |
T = 1/f |
| Echo or SONAR distance |
d = (v × t)/2 |
Speed of Sound Numericals Class 9
Speed of sound questions usually ask students to calculate speed, frequency, wavelength, or time period.
Always write units before substitution.
Speed of Sound Numericals Class 9 Science Chapter 10
Q1. A sound wave has frequency 500 Hz and wavelength 0.68 m. Calculate the speed of sound.
Ans.
v = f × λ
v = 500 × 0.68
v = 340 m/s
Q2. Speed of sound in a medium is 1500 m/s. Frequency is 3000 Hz. Find the wavelength.
Ans.
λ = v/f
λ = 1500/3000
λ = 0.5 m
Q3. Sound travels 1700 m in 5 seconds. Calculate the speed of sound.
Ans.
Speed = Distance/Time
Speed = 1700/5
Speed = 340 m/s
Frequency and Time Period Questions Class 9 Science Chapter 10
Frequency and time period are reciprocal quantities.
If frequency increases, time period decreases.
Frequency and Time Period Questions
Q1. A tuning fork completes 256 vibrations per second. Find its frequency and time period.
Ans.
Frequency = 256 Hz
T = 1/f
T = 1/256
T = 0.0039 s approximately
Q2. The time period of a sound wave is 0.005 s. Calculate its frequency.
Ans.
f = 1/T
f = 1/0.005
f = 200 Hz
Q3. A wave has frequency 440 Hz. How many vibrations does it complete in 2 seconds?
Ans.
Number of vibrations = Frequency × Time
= 440 × 2
= 880 vibrations
Wavelength Questions Class 9 Science Chapter 10
Wavelength questions often combine v = fλ and T = 1/f.
Write both formulas when the time period is given.
Wavelength Questions with Answers
Q1. Speed of sound in air is 340 m/s. Time period is 0.002 s. Find wavelength.
Ans.
f = 1/T
f = 1/0.002
f = 500 Hz
Now,
λ = v/f
λ = 340/500
λ = 0.68 m
Q2. Wavelength is 1.7 m. Speed of sound is 340 m/s. Find frequency and time period.
Ans.
f = v/λ
f = 340/1.7
f = 200 Hz
T = 1/f
T = 1/200
T = 0.005 s
Echo Numericals Class 9
Echo numericals class 9 use one main formula:
Distance = (Speed × Time) / 2
The factor 2 is used because sound travels to the reflecting surface and returns.
Echo Numericals Class 9 Science Chapter 10
Q1. A person claps near a cliff and hears the echo after 2 seconds. Speed of sound is 340 m/s. Find the distance to the cliff.
Ans.
Distance = (v × t)/2
Distance = (340 × 2)/2
Distance = 340 m
Q2. A ship hears the seabed echo after 4 seconds. Speed of sound in water is 1500 m/s. Find the sea depth.
Ans.
Depth = (v × t)/2
Depth = (1500 × 4)/2
Depth = 3000 m
Q3. What is the minimum wall distance for a distinct echo? Speed of sound is 340 m/s and minimum time is 0.1 s.
Ans.
Distance = (v × t)/2
Distance = (340 × 0.1)/2
Distance = 17 m
So, the reflecting wall should be at least 17 m away.
SONAR Numericals Class 9 Science Chapter 10
SONAR numericals use the same echo formula. The speed of sound in seawater is often taken as 1500 m/s.
SONAR Numerical Questions
Q1. A SONAR pulse returns after 6 seconds. Speed of sound in seawater is 1500 m/s. Find the ocean depth.
Ans.
Depth = (v × t)/2
Depth = (1500 × 6)/2
Depth = 4500 m
Q2. An ultrasound pulse returns after 0.8 seconds. Speed is 1500 m/s. Find the distance to an underwater rock.
Ans.
Distance = (v × t)/2
Distance = (1500 × 0.8)/2
Distance = 600 m
Important Questions on Production and Propagation of Sound
Sound is not a particle moving through air. It is a wave created by vibrations.
These questions test the basic nature of sound.
Sound Needs a Medium to Travel Questions Class 9
Q1. Prove that sound needs a medium to travel.
Ans. Place an electric bell inside a glass jar connected to a vacuum pump.
When air is present, the sound of the bell is heard.
As air is removed by the vacuum pump, the sound becomes faint. After most air is removed, the sound is not heard, even though the bell continues to vibrate.
When air is allowed back in, the sound is heard again.
This proves that sound needs a medium to travel.
Q2. In which medium does sound travel fastest: solid, liquid, or gas?
Ans. Sound travels fastest in solids.
Particles in solids are closely packed, so vibrations transfer quickly.
Sound travels slower in liquids and slowest in gases.
Longitudinal Wave Questions Class 9 Science
Sound in air travels as a longitudinal wave.
The particles vibrate back and forth in the same direction as the wave.
Longitudinal Wave Questions
Q1. Why is sound called a longitudinal wave?
Ans. Sound is called a longitudinal wave because particles of the medium vibrate parallel to the direction of wave propagation.
The wave moves forward through alternate compressions and rarefactions.
Q2. What happens to pressure at compressions and rarefactions in a sound wave?
Ans. At compressions, particles are closer together.
So, pressure and density are higher than normal.
At rarefactions, particles are farther apart.
So, pressure and density are lower than normal.
Characteristics of Sound Wave Class 9 Important Questions
Frequency, amplitude, and waveform decide how sound is heard.
Frequency controls pitch. Amplitude controls loudness. Waveform controls timbre.
Amplitude and Intensity Questions Class 9 Science
Q1. How does amplitude affect the loudness of sound?
Ans. Greater amplitude means greater energy in the sound wave.
So, the sound is louder.
A loud drum beat has greater amplitude than a soft whisper.
Q2. What is the unit used to measure loudness level?
Ans. Loudness level is measured in decibel.
Normal conversation is around 60 dB. Long exposure to sound above 85 dB can harm hearing.
Frequency, Pitch and Loudness Questions Class 9
Q1. Two sounds have the same amplitude but different frequencies. How will they differ?
Ans. They will have the same loudness but different pitch.
The sound with higher frequency will have higher pitch.
Q2. Why does a woman’s voice generally have higher pitch than a man’s voice?
Ans. A woman’s vocal cords usually vibrate at higher frequency.
Higher frequency produces higher pitch.
Sound Wave Graph Questions Class 9 Science
Q1. On a sound wave graph, what do the x-axis and y-axis represent?
Ans. The x-axis usually represents time.
The y-axis represents displacement of particles from their mean position.
Peaks show maximum positive displacement. Troughs show maximum negative displacement.
Q2. Two sound waves A and B are shown on a graph. Wave A has more peaks per unit time. Which has higher pitch?
Ans. Wave A has higher pitch.
More peaks per unit time mean higher frequency. Higher frequency gives higher pitch.
Reflection of Sound Class 9 Important Questions
Reflection of sound explains echo, reverberation, auditorium design, and SONAR.
Hard surfaces reflect sound better. Soft surfaces absorb sound.
Echo Questions Class 9 Science Chapter 10
Q1. State the conditions required for hearing a distinct echo.
Ans. Two conditions are required:
- The reflecting surface should be hard and large.
- The reflecting surface should be about 17.2 m away at room temperature.
This gives a time gap of at least 0.1 seconds between original sound and echo.
Q2. Why are echo numericals important in Class 9 Science Chapter 10?
Ans. Echo numericals test the direct use of d = (v × t)/2.
They connect sound reflection with real-life examples like cliffs, wells, ships, and ocean depth measurement.
Reverberation Questions Class 9 Science Chapter 10
Q1. What is reverberation? Give one example.
Ans. Reverberation is the persistence of sound due to repeated reflections in an enclosed space.
Example: Sound remains for some time in an empty hall after clapping.
Q2. How is reverberation reduced in auditoriums?
Ans. Reverberation is reduced by using sound-absorbing materials.
Curtains, carpets, false ceilings, padded seats, and acoustic panels absorb sound and reduce repeated reflections.
Echo and Reverberation Class 9
Q1. Tabulate the differences between echo and reverberation.
Ans.
| Echo |
Reverberation |
| Distinct repetition of sound |
Persistence of sound |
| Heard after a clear time gap |
Caused by rapid repeated reflections |
| Needs a reflecting surface far away |
Occurs in enclosed spaces |
| Minimum time gap is 0.1 seconds |
Reflections arrive within less than 0.1 seconds |
| Example: Shouting near a cliff |
Example: Sound in an empty hall |
Ultrasound and SONAR Class 9 Important Questions
Ultrasound and SONAR class 9 questions combine definitions, applications, and numericals.
Learn the frequency range, working principle, and formula.
Ultrasound Applications Questions Class 9
Q1. List four applications of ultrasound.
Ans.
- Medical sonography for examining internal organs
- Industrial flaw detection in metal components
- Cleaning delicate parts such as jewellery and electronic components
- SONAR for underwater detection and depth measurement
Q2. Why is ultrasound preferred over normal sound in medical imaging?
Ans. Ultrasound has high frequency and short wavelength.
It can reflect from internal body tissues and produce clearer images.
Normal audible sound cannot give such fine details.
SONAR Working Questions Class 9 Science
Q1. Explain the working principle of SONAR.
Ans. SONAR works on the reflection of sound.
A transmitter sends ultrasound pulses into water.
The sound waves reflect from the seabed or underwater object.
A receiver detects the echo.
The distance is calculated using:
Distance = (Speed of sound in water × Time taken) / 2
Infrasound and Ultrasound Questions Class 9
Q1. Define infrasound and ultrasound. Which animals detect infrasound?
Ans. Infrasound has frequency below 20 Hz.
Ultrasound has frequency above 20,000 Hz.
Elephants, whales, and rhinoceroses can detect infrasound.
Q2. Name two animals that use ultrasound and explain how.
Ans. Bats use ultrasound for echolocation in darkness.
Dolphins use ultrasound for communication and hunting underwater.
Both animals use reflected sound to locate objects.
Human Ear and Hearing Range Class 9 Important Questions
Human ear class 9 questions often appear as diagram-based or long-answer questions.
Know the function of each part clearly.
Human Ear Class 9 Questions
Q1. What is the audible range for humans? What limits it?
Ans. The human audible range is 20 Hz to 20,000 Hz.
The cochlea and hair cells in the inner ear help detect this range. Ageing and exposure to loud sounds can reduce hearing ability.
Q2. Compare the hearing ranges of humans, dogs, and bats.
Ans.
| Animal |
Approximate Hearing Range |
| Humans |
20 Hz to 20,000 Hz |
| Dogs |
About 40 Hz to 65,000 Hz |
| Bats |
About 1,000 Hz to 1,20,000 Hz |
Bats can hear ultrasound that humans cannot detect.
Pitch, Loudness and Timbre Questions Class 9
Q1. What is timbre? Why do a guitar and sitar sound different on the same note?
Ans. Timbre is the quality of sound that helps distinguish two sounds of the same pitch and loudness.
A guitar and sitar have different shapes, strings, and resonance chambers. So, their waveforms differ, and they sound different.
Noise Pollution Questions Class 9 Science
Noise pollution questions test daily-life applications of sound.
Use examples and effects in the answer.
Noise Pollution Questions
Q1. What is noise pollution? Name two sources and two harmful effects.
Ans. Noise pollution is unwanted and excessive sound that harms living beings.
Sources:
- Traffic noise
- Industrial machinery and construction
Harmful effects:
- Hearing damage
- Stress, headache, and sleep disturbance
Q2. Suggest three measures to reduce noise pollution.
Ans.
- Plant trees along roads.
- Use sound-absorbing materials in buildings.
- Control loudspeakers, horns, and industrial noise through rules.
Class 9 Science Chapter 10 Assertion Reason Questions
Assertion-reason questions test whether students understand the reason behind each fact.
Read both statements separately before selecting the answer.
Directions:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Class 9 Science Chapter 10 Assertion Reason Questions
Q1. Assertion (A): Sound cannot travel through vacuum. Reason (R): Sound is a mechanical wave that requires a medium for propagation.
Ans. (a)
Both A and R are true, and R is the correct explanation of A.
Vacuum has no particles to carry sound vibrations.
Q2. Assertion (A): Sound travels faster in solids than in liquids or gases. Reason (R): Particles in solids are closer together, so vibrations transfer more quickly.
Ans. (a)
Both A and R are true, and R correctly explains A.
Q3. Assertion (A): Bats can navigate in complete darkness. Reason (R): Bats emit ultrasound and use reflected echoes to detect obstacles.
Ans. (a)
Both A and R are true, and R correctly explains A.
This process is called echolocation.
Q4. Assertion (A): Loudness of sound depends on frequency. Reason (R): Amplitude of a sound wave determines the energy it carries.
Ans. (d)
A is false, but R is true.
Loudness depends on amplitude, not frequency.
Q5. Assertion (A): An echo cannot usually be heard in a small room. Reason (R): In a small room, reflected sound returns before 0.1 seconds.
Ans. (a)
Both A and R are true, and R correctly explains A.
A clear echo needs a minimum time gap of 0.1 seconds.
Case Study Questions Class 9 Science Chapter 10
Case study questions test application. Read the passage fully, then identify the concept and formula.
Case Study 1: Bat Navigation and Echolocation
Bats are nocturnal animals that hunt in darkness. They emit ultrasound pulses and detect reflected sound through their sensitive ears. By measuring the time between sending sound and receiving the echo, they find the position of prey.
Q1. What type of sound do bats use for navigation?
Ans. Bats use ultrasound.
Ultrasound has frequency above 20,000 Hz and cannot be heard by humans.
Q2. What principle of sound does echolocation depend on?
Ans. Echolocation depends on reflection of sound.
The bat emits sound and detects the echo to locate objects.
Q3. A bat emits a sound and hears the echo after 0.01 seconds. Speed of sound is 340 m/s. How far is the insect?
Ans.
Distance = (v × t)/2
Distance = (340 × 0.01)/2
Distance = 1.7 m
Case Study 2: SONAR in Ocean Exploration
A research vessel maps the ocean floor using SONAR. The ship sends ultrasound pulses and records the time taken by the echo. Scientists use this data to map underwater terrain and locate objects.
Q1. Why does SONAR use ultrasound instead of audible sound?
Ans. Ultrasound has high frequency and short wavelength.
It gives better reflection and helps detect underwater objects more accurately.
Q2. A SONAR pulse returns after 3 seconds. Speed of sound in seawater is 1500 m/s. What is the ocean depth?
Ans.
Depth = (v × t)/2
Depth = (1500 × 3)/2
Depth = 2250 m
Q3. How does SONAR locate a shipwreck on the ocean floor?
Ans. Ultrasound pulses reflect from the shipwreck.
The returning echo is detected by the receiver. The time taken by the echo helps calculate the distance and position of the shipwreck.
Important Definitions and Formulas Class 9 Science Chapter 10
| Term |
Definition / Formula |
| Sound |
Energy produced by vibrating objects |
| Mechanical Wave |
Wave that needs a medium |
| Longitudinal Wave |
Wave in which particles vibrate parallel to wave direction |
| Compression |
High-pressure, high-density region |
| Rarefaction |
Low-pressure, low-density region |
| Frequency |
Number of vibrations per second |
| Time Period |
T = 1/f |
| Wavelength |
Distance between two consecutive compressions or rarefactions |
| Speed of Sound |
v = f × λ |
| Echo / SONAR Distance |
d = (v × t)/2 |
| Amplitude |
Maximum displacement from mean position |
| Audible Range |
20 Hz to 20,000 Hz |
| Infrasound |
Frequency below 20 Hz |
| Ultrasound |
Frequency above 20,000 Hz |
| Reverberation |
Persistence of sound due to repeated reflections |