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Class 9 Science Important Questions and Answers: Sound Waves — Characteristics and Applications

Premium NCERT, Board Exam, Exemplar and Olympiad-level question answers for Class 9 Science. Learn propagation of sound, longitudinal waves, compression, rarefaction, wavelength, frequency, time period, amplitude, intensity, speed of sound, reflection, echo, reverberation, echolocation, infrasonic waves, ultrasonic waves and SONAR.

Class 9 Science Sound Waves NCERT + Board + Olympiad

Short Introduction

Sound is a form of energy that produces the sensation of hearing in our ears. It is produced by vibrating objects and travels through a material medium in the form of waves.

Sound waves are mechanical waves because they need a medium to propagate. In air, sound travels as longitudinal waves through alternate compressions and rarefactions.

Exam focus: Propagation of sound, longitudinal waves, compression and rarefaction, characteristics of sound waves, speed of sound, echo, reverberation, ultrasonic waves and SONAR are very important for CBSE exams.

Chapter Overview

1. Sound as Energy

Sound is a form of energy produced by vibrating objects.

2. Propagation of Sound

Sound travels through a medium by transferring energy from one particle to another.

3. Longitudinal Waves

In air, sound travels as longitudinal waves with compressions and rarefactions.

4. Wave Characteristics

Wavelength, frequency, time period, amplitude and intensity describe a sound wave.

5. Human Perception

Pitch, loudness, tone, timbre and shrillness explain how we hear sound.

6. Applications

Reflection, echo, reverberation, echolocation, ultrasonic waves and SONAR are useful applications of sound.

Important Keywords

Sound Vibration Medium Mechanical wave Longitudinal wave Transverse wave Compression Rarefaction Density Pressure variation Wavelength Frequency Time period Amplitude Intensity Speed of sound Pitch Loudness Tone Timbre Shrillness Reflection of sound Echo Reverberation Echolocation Ultrasonic Infrasonic SONAR

Important Formulae and Concepts

Speed of Sound

v = λf

Frequency

f = 1/T

Time Period

T = 1/f

Wavelength

λ = v/f

Distance in Echo

Total distance = v × t

Distance of Reflector

d = vt/2

Audible Range

20 Hz to 20,000 Hz

Ultrasonic Sound

Frequency greater than 20,000 Hz

Infrasonic Sound

Frequency less than 20 Hz

Symbols: v = speed of sound, λ = wavelength, f = frequency, T = time period, t = time taken for echo, d = distance of reflecting surface.

Graphical Representation of a Sound Wave

Compression Rarefaction Compression

A sound wave can be represented graphically by showing pressure or density variations with distance or time. Regions of high pressure and high density are called compressions, while regions of low pressure and low density are called rarefactions.

Important Very Short Answer Questions

Q1. What is sound?

Answer: Sound is a form of energy that produces the sensation of hearing in our ears.

Q2. How is sound produced?

Answer: Sound is produced by vibrating objects.

Q3. Does sound need a medium to propagate?

Answer: Yes, sound needs a material medium such as solid, liquid or gas to propagate.

Q4. Can sound travel through vacuum?

Answer: No, sound cannot travel through vacuum because there are no particles to transfer sound energy.

Q5. What type of wave is sound in air?

Answer: Sound travels as a longitudinal wave in air.

Q6. What is compression?

Answer: Compression is the region of a sound wave where particles are close together and pressure is high.

Q7. What is rarefaction?

Answer: Rarefaction is the region of a sound wave where particles are far apart and pressure is low.

Q8. Define wavelength.

Answer: Wavelength is the distance between two consecutive compressions or two consecutive rarefactions.

Q9. Define frequency.

Answer: Frequency is the number of complete oscillations or waves produced per second.

Q10. What is the SI unit of frequency?

Answer: The SI unit of frequency is hertz, written as Hz.

Q11. Define time period.

Answer: Time period is the time taken to complete one oscillation.

Q12. Define amplitude.

Answer: Amplitude is the maximum displacement of a vibrating particle from its mean position.

Q13. What is echo?

Answer: Echo is the repetition of sound heard after reflection from a distant surface.

Q14. What is reverberation?

Answer: Reverberation is the persistence of sound due to repeated reflections from surfaces.

Q15. What is SONAR?

Answer: SONAR is a device that uses ultrasonic waves to find distance, direction and speed of underwater objects.

Short Answer Questions

Q1. Why does sound need a medium to propagate?

Answer: Sound travels by the vibration of particles of a medium. These particles transfer energy from one particle to the next. In vacuum, there are no particles, so sound cannot travel.

Q2. Differentiate between longitudinal and transverse waves.

Longitudinal Waves	Transverse Waves
Particles vibrate parallel to the direction of wave propagation.	Particles vibrate perpendicular to the direction of wave propagation.
They have compressions and rarefactions.	They have crests and troughs.
Sound wave in air is an example.	Wave on a stretched string is an example.

Q3. Differentiate between compression and rarefaction.

Compression	Rarefaction
Particles of medium are close together.	Particles of medium are far apart.
Pressure and density are high.	Pressure and density are low.
It represents a crowded region of the wave.	It represents a stretched region of the wave.

Q4. What is the relation between frequency and time period?

Answer: Frequency and time period are reciprocal of each other. If frequency is f and time period is T, then f = 1/T and T = 1/f.

Q5. Explain amplitude and intensity of sound.

Answer: Amplitude is the maximum displacement of a vibrating particle from its mean position. Intensity is the sound energy passing per second through a unit area. Greater amplitude usually produces louder sound.

Q6. Differentiate between pitch and loudness.

Pitch	Loudness
Pitch depends mainly on frequency.	Loudness depends mainly on amplitude.
Higher frequency means higher pitch.	Greater amplitude means louder sound.
It helps distinguish shrill and flat sounds.	It helps distinguish loud and soft sounds.

Q7. What is timbre?

Answer: Timbre is the characteristic quality of sound that helps us distinguish between two sounds of the same pitch and loudness produced by different sources.

Q8. What is the condition for hearing a distinct echo?

Answer: To hear a distinct echo, the time interval between the original sound and reflected sound should be at least 0.1 second. The reflecting surface should be sufficiently far and large.

Q9. What is echolocation?

Answer: Echolocation is the method used by some animals, such as bats and dolphins, to locate objects by producing sound waves and detecting their echoes.

Q10. What are ultrasonic and infrasonic sounds?

Answer: Sounds with frequency greater than 20,000 Hz are called ultrasonic sounds. Sounds with frequency less than 20 Hz are called infrasonic sounds.

Long Answer Questions

Q1. Explain propagation of sound in air.

Answer:

Sound is produced by a vibrating object.
When an object vibrates, it pushes nearby air particles forward and creates compression.
When the object moves backward, it creates rarefaction.
These compressions and rarefactions move forward through air.
The particles of air only vibrate about their mean positions; they do not travel along with the wave.
Thus, sound travels through air as a longitudinal mechanical wave.

Q2. Derive the relation between speed, wavelength and frequency of a sound wave.

Answer:

Let the wavelength of a sound wave be λ.

Let the time period of the wave be T.

In one time period, the wave travels a distance equal to one wavelength.

Speed = Distance ÷ Time

v = λ ÷ T

But frequency f = 1/T

Therefore, v = λf

Hence, speed of sound = wavelength × frequency.

Q3. Explain the characteristics of sound waves.

Answer: Important characteristics of sound waves are:

Wavelength: Distance between two consecutive compressions or rarefactions.
Frequency: Number of complete vibrations per second.
Time period: Time taken to complete one vibration.
Amplitude: Maximum displacement of a particle from its mean position.
Intensity: Sound energy passing per second through unit area.
Speed: Distance travelled by sound per unit time.

Q4. Explain human perception of sound: pitch, loudness, tone, timbre and shrillness.

Answer:

Pitch: It depends on frequency. Higher frequency gives higher pitch.
Loudness: It depends mainly on amplitude. Greater amplitude gives louder sound.
Tone: A sound of single frequency is called a tone.
Timbre: It is the quality of sound that helps distinguish different sound sources.
Shrillness: A sound with high frequency is more shrill.

Q5. Explain reflection of sound, echo and reverberation.

Answer:

Sound reflects from hard surfaces in the same way as light reflects from surfaces.
Echo: The repetition of sound heard after reflection from a distant surface is called echo.
For a distinct echo, the reflected sound must reach the ear at least 0.1 second after the original sound.
Reverberation: Persistence of sound due to repeated reflections in a closed space is called reverberation.
Excessive reverberation makes sound unclear in halls and auditoriums.
It can be reduced by using curtains, carpets, acoustic panels and sound-absorbing materials.

Q6. Explain SONAR and its uses.

Answer: SONAR stands for Sound Navigation and Ranging.

SONAR uses ultrasonic waves to detect underwater objects.
A transmitter sends ultrasonic waves into water.
These waves reflect from an object and return as echoes.
The receiver detects the reflected waves.
Using the time taken by the echo, the distance of the object is calculated.

Uses:

Measuring depth of the sea.
Detecting submarines and underwater rocks.
Locating schools of fish.
Navigation under water.

Important Numericals with Solutions

Numerical 1: A sound wave has frequency 500 Hz and wavelength 0.68 m. Find its speed.

Solution:

Frequency, f = 500 Hz

Wavelength, λ = 0.68 m

Speed, v = λf

v = 0.68 × 500 = 340 m/s

Numerical 2: A sound wave travels with speed 330 m/s and frequency 110 Hz. Find its wavelength.

Solution:

Speed, v = 330 m/s

Frequency, f = 110 Hz

v = λf

λ = v ÷ f

λ = 330 ÷ 110 = 3 m

Numerical 3: The frequency of a wave is 50 Hz. Find its time period.

Solution:

Frequency, f = 50 Hz

Time period, T = 1/f

T = 1/50 = 0.02 s

Numerical 4: A wave has time period 0.01 s. Find its frequency.

Solution:

Time period, T = 0.01 s

Frequency, f = 1/T

f = 1/0.01 = 100 Hz

Numerical 5: An echo is heard after 2 seconds. If speed of sound is 340 m/s, find the distance of the reflecting surface.

Solution:

Time, t = 2 s

Speed of sound, v = 340 m/s

Total distance travelled by sound = v × t

Total distance = 340 × 2 = 680 m

Distance of reflecting surface, d = 680 ÷ 2 = 340 m

Numerical 6: A ship sends an ultrasonic signal to the sea bed. The echo is received after 4 seconds. If speed of sound in water is 1500 m/s, find the depth of the sea.

Solution:

Time, t = 4 s

Speed of sound in water, v = 1500 m/s

Total distance travelled = v × t = 1500 × 4 = 6000 m

Depth of sea = 6000 ÷ 2 = 3000 m

Numerical 7: Find the minimum distance of a reflecting surface required to hear a distinct echo. Take speed of sound as 340 m/s.

Solution:

Minimum time for distinct echo = 0.1 s

Speed of sound, v = 340 m/s

Total distance travelled = v × t = 340 × 0.1 = 34 m

Minimum distance of reflecting surface = 34 ÷ 2 = 17 m

Numerical 8: A sound wave has wavelength 2 m and speed 340 m/s. Find frequency and time period.

Solution:

Wavelength, λ = 2 m

Speed, v = 340 m/s

v = λf

f = v ÷ λ = 340 ÷ 2 = 170 Hz

T = 1/f = 1/170 = 0.0059 s approximately

Numerical 9: A bat detects an insect by hearing an echo after 0.04 s. If speed of sound is 340 m/s, find the distance of the insect.

Solution:

Time, t = 0.04 s

Speed of sound, v = 340 m/s

Total distance travelled = v × t = 340 × 0.04 = 13.6 m

Distance of insect = 13.6 ÷ 2 = 6.8 m

Numerical 10: A wave produces 1200 vibrations in 4 seconds. Find its frequency.

Solution:

Number of vibrations = 1200

Time = 4 s

Frequency = Number of vibrations ÷ Time

Frequency = 1200 ÷ 4 = 300 Hz

Case-Study Based Questions

Case Study 1: Sound Needs a Medium

A ringing electric bell is placed inside a glass jar connected to a vacuum pump. Initially, the sound of the bell is clearly heard. As air is removed from the jar, the sound becomes weaker and finally almost inaudible.

Q1. Why does the sound become weaker?

Answer: The sound becomes weaker because air particles are removed from the jar.

Q2. What does this experiment prove?

Answer: It proves that sound needs a material medium to travel.

Q3. Can sound travel through vacuum?

Answer: No, sound cannot travel through vacuum.

Q4. Why does light from the bell still remain visible?

Answer: Light can travel through vacuum, but sound cannot.

Case Study 2: Echo and Reflection of Sound

A student shouts near a high cliff and hears the same sound again after a short time. The teacher explains that the sound reflected from the cliff and returned to the student.

Q1. What is the repeated sound called?

Answer: The repeated sound is called an echo.

Q2. Which property of sound is responsible for echo?

Answer: Reflection of sound is responsible for echo.

Q3. What should be the minimum time gap for hearing a distinct echo?

Answer: The minimum time gap should be 0.1 second.

Q4. Name one application of echo.

Answer: Echo is used in SONAR to detect underwater objects.

Case Study 3: Ultrasonic Waves and SONAR

A ship uses SONAR to measure the depth of the sea. Ultrasonic waves are sent from the ship and their echoes are received after reflection from the sea bed.

Q1. What type of waves are used in SONAR?

Answer: Ultrasonic waves are used in SONAR.

Q2. What is the full form of SONAR?

Answer: SONAR stands for Sound Navigation and Ranging.

Q3. Why are ultrasonic waves useful in SONAR?

Answer: Ultrasonic waves have high frequency and can travel in a narrow beam, making them useful for detection.

Q4. Name two uses of SONAR.

Answer: SONAR is used to measure sea depth and detect underwater objects like submarines.

Assertion-Reason Questions

Choose the correct option:

A. Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.