Extra Questions on Sound – Class 9 Science

Question 1 – Name two animals which can produce infrasonic waves.

Answer: Elephant and whale.

Question 2 – What is reverberation?

Answer: Reverberation is the persistence of sound in a closed space due to multiple reflections from walls, ceiling, and other surfaces even after the source has stopped producing sound.

Question 3 – What is an echo? Under what conditions can it be heard clearly?

Answer: Echo is the repetition of a sound heard after reflection from a distant surface when the reflected sound reaches the ear after at least 0.1 s.

Question 4 – What is infrasonic sound? Give one example.

Answer: Infrasonic sound is sound having frequency less than 20 Hz.

Example: Sound produced by elephants.

Question 5 – Give examples of organisms which can hear infrasonic sound.

Answer: Elephants and whales can hear infrasonic sound.

Question 6 – What are infrasonic and ultrasonic sounds?

Answer:

• Infrasonic sounds have frequencies less than 20 Hz.
• Ultrasonic sounds have frequencies greater than 20,000 Hz.

Question 7 – What is the audible range of the human ear?

Answer: The audible range of the human ear is from 20 Hz to 20,000 Hz.

Question 8 – Why do we hear the sound of an approaching truck before the truck reaches us?

Answer: Sound travels faster than the truck, so the sound waves reach our ears before the truck itself arrives.

Question 9 – What is the frequency of a wave with time period 0.025 s?

Answer: We know, f = 1/T

f = 1/0.025 = 40 Hz

So, the frequency of the wave is 40 Hz.

Question 10 – A baby recognises her mother by her voice. Name the characteristic of sound involved.

Answer: The characteristic of sound involved is quality (timbre).

Question 15 – How can multiple echoes of a single sound be produced? Explain.

Answer: Multiple echoes are produced when a sound is reflected repeatedly between two or more distant reflecting surfaces. Each reflection takes some time to reach the listener. If the time gap between successive reflections is greater than 0.1 s, each reflected sound is heard separately as multiple echoes.

Question 16 – How does temperature affect the velocity of sound?

Answer: The velocity of sound in air increases with increase in temperature. As temperature rises, air molecules move faster, allowing sound waves to travel more quickly. Thus, sound travels faster in warm air than in cold air.

Question 17 – When a sound is reflected from a distant object, an echo is produced and heard on a winter night. Will you hear the echo of the same sound on a summer day if the distance between reflecting surface and the source of sound remains the same? Justify your answer.

Answer: On a summer day, the velocity of sound is higher due to higher temperature. Because of this, the reflected sound returns in less time. The time gap may become less than 0.1 s, so the echo may not be heard distinctly. Hence, the echo heard in winter may not be heard clearly in summer.

Question 18 – Why are the walls and roof of an auditorium covered with sound-absorbent materials?

Answer: The walls and roof of an auditorium are covered with sound-absorbent materials to reduce excessive reflection of sound. This helps to control reverberation and makes speech and music clearer to the audience.

Question 19 – Define reverberation. What arrangements should be made in an auditorium to control excessive reverberation?

Answer: Reverberation is the persistence of sound in an enclosed space due to repeated reflections after the sound source has stopped.

To control excessive reverberation:

• Walls and ceilings should be lined with sound-absorbing materials like fibre boards or curtains.
• Upholstered seats should be used.
• False ceilings with acoustic panels may be installed.

Question 20 – Name the sound of the following frequencies: (a) 5 Hz, (b) 50 Hz, (c) 50,000 Hz and (d) 50 kHz

Answer:

(a) 5 Hz

Infrasonic sound

(b) 50 Hz

Audible sound

(c) 50,000 Hz

Ultrasonic sound

(d) 50 kHz

Ultrasonic sound

Question 21 – What is the frequency of a wave whose time period is 0.05 s?

Answer: We know, f = 1/T

f = 1/0.05 = 20 Hz

So, the frequency of the wave is 20 Hz.

Question 22 – How do frequency and amplitude affect musical sound?

Answer:

• Frequency determines the pitch of the sound. Higher frequency produces a shriller sound, while lower frequency produces a deeper sound.
• Amplitude determines the loudness of the sound. Greater amplitude means louder sound, and smaller amplitude means softer sound.

Question 23 – What is SONAR? For what is it used?

Answer: SONAR stands for Sound Navigation and Ranging. It uses ultrasonic sound waves to detect and locate underwater objects.

SONAR is used to measure sea depth, locate submarines, and detect underwater obstacles.

Question 24. Distinguish between echo and reverberation.

Answer:

Question 25. When a vertically upward jerk is given to a string, transverse waves are formed. Give three features of these waves.

Answer: In transverse waves, the particles of the medium vibrate perpendicular to the direction of wave propagation. These waves consist of alternate crests and troughs. Transverse waves can be polarised. They generally propagate in solids and on the surface of liquids.

Question 26. Differentiate between infrasonic and ultrasonic sound.

Answer:

Question 27. (A) What is meant by the reflection of sound waves? (B) Describe an activity to study the reflection of sound.

Answer:

(A) Reflection of sound refers to the phenomenon in which sound waves bounce back into the same medium after striking a hard surface such as a wall, cliff, or building. Just like light, sound follows the laws of reflection. This reflected sound can be heard as an echo if the reflecting surface is sufficiently far away.

(B) To study the reflection of sound, take a long cardboard or PVC pipe and place a ticking clock or mobile phone at one end. Stand near a large wall and direct the pipe towards it. When you listen through the pipe, the reflected sound can be clearly heard. This shows that sound waves travel, strike the wall, and return to the listener due to reflection.

Question 28 – How do bats detect their prey?

Answer: Bats detect their prey by a process called echolocation. They produce ultrasonic sound waves which travel through air and strike insects or other objects. These sound waves are reflected back as echoes. By analysing the time delay and nature of the reflected sound, bats determine the distance, size, and position of their prey.

Question 29 – Explain with the help of a diagram the working of SONAR.

Answer: SONAR stands for Sound Navigation and Ranging. It works on the principle of reflection of sound waves.

In SONAR, a transmitter fitted on a ship produces ultrasonic sound waves. These waves travel through water and strike an underwater object such as a submarine or the sea bed. The reflected waves (echoes) are received by a detector. The time taken for the sound to travel to the object and return is noted.

If t is the time taken and v is the velocity of sound in water, then the distance d of the object is given by:

d = (v × t) / 2

Question 30 – Explain the phenomenon of echo and state the conditions necessary for hearing an echo.

Answer: An echo is the repetition of a sound heard after it is reflected from a distant surface such as a wall, cliff, or mountain. When a sound is produced, it travels through a medium and strikes a reflecting surface. The sound waves are reflected back and reach the listener's ear after some time delay, producing an echo.

For an echo to be heard distinctly, the following conditions must be satisfied:

1. The time gap between the original sound and the reflected sound should be at least 0.1 s, so that the human ear can distinguish between the two sounds.
2. The distance between the source of sound and the reflecting surface should be at least 17 m (in air at room temperature).
3. The reflecting surface should be large, smooth, and hard so that it can reflect sufficient sound energy.
4. The sound produced should be sufficiently loud and of short duration.

Thus, echoes are commonly heard in large open spaces like valleys, hills, and empty halls.

Question 31 – Distinguish between transverse waves and longitudinal waves.

Answer:

Question 32 – Explain the characteristics of wave motion. Describe any four important characteristics.

Answer: Wave motion is the propagation of a disturbance through a medium in which energy is transferred from one point to another without any net transfer of matter.

1. Transfer of Energy, Not Matter

In wave motion, energy travels through the medium while the particles only oscillate about their mean positions. There is no permanent displacement of particles.

2. Motion of Particles

Each particle of the medium starts vibrating when the wave reaches it. The vibration is periodic and occurs with the same frequency as the source.

3. Definite Wave Speed

A wave travels with a definite speed that depends on the nature of the medium, not on particle speed.

4. Time Lag Between Particles

Successive particles receive vibrations with a small time delay, producing the wave pattern.

Question 33 – (a) Explain why sound waves are called mechanical waves. (b) Describe, with the help of points, how a sound wave travels through air in the form of compressions and rarefactions.

Answer:

(a) Sound waves as mechanical waves

Sound waves are called mechanical waves because they require a material medium for their propagation. Sound cannot travel in vacuum, which shows that particles of a medium are necessary to transmit the disturbance. The vibration of particles in solids, liquids, or gases helps in the transfer of sound energy from one point to another.

(b) Propagation of sound through compressions and rarefactions

When a vibrating source moves forward, it compresses the air particles, forming a region of high pressure called compression. When it moves backward, a region of low pressure called rarefaction is formed. These compressions and rarefactions travel through the medium as a longitudinal wave, carrying sound energy forward.

Question 34 – Briefly mention three uses of ultrasound in the field of medicine.

Answer: Ultrasound has several important applications in the field of medicine due to its ability to penetrate soft tissues without causing damage.

1. Medical Imaging (Ultrasonography)

Ultrasound is widely used to obtain images of internal organs such as the liver, kidneys, heart, and foetus. It helps doctors diagnose diseases without surgery.

2. Breaking Kidney Stones (Lithotripsy)

High-frequency ultrasonic waves are used to break kidney stones into very small fragments, which can then pass out naturally through urine.

3. Echocardiography

Ultrasound is used to study the structure and functioning of the heart, helping in the detection of heart-related problems.

Thus, ultrasound plays a safe and vital role in medical diagnosis and treatment.