Sound Waves Characteristics and Applications Class 9 Extra Questions and Answers Science Chapter 10

Students can keep Class 9 Science Extra Questions and Class 9 Exploration Chapter 10 Extra Question Answer Sound Waves Characteristics and Applications handy for quick reference during exams.

Class 9 Science Chapter 10 Sound Waves Characteristics and Applications Extra Questions

Class 9 Science Chapter 10 Extra Questions on Sound Waves Characteristics and Applications

Sound Waves Characteristics and Applications Class 9 Very Short Question Answer

Question 1.
What is a sound wave?
Answer:
A sound wave is a disturbance consisting of alternating compressions and rarefactions that propagates through a material medium due to the vibrations of its particles.

Question 2.
Name the two types of mechanical waves.
Answer:
Longitudinal waves and transverse waves.

Question 3.
What is a compression in a sound wave?
Answer:
A compression is a region in the medium where the particle density is higher than the average density, formed when the source of sound moves forward.

Question 4.
What is the audible range of human hearing?
Answer:
The audible range for humans is from 20 Hz to 20,000 Hz (20 kHz).

Question 5.
Define wavelength of a sound wave.
Answer:
The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength (λ) of a sound wave. Its SI unit is metre (m).

Question 6.
What is a rarefaction?
Answer:
A rarefaction is a region in the medium where the particle density is lower than the average density, formed when the source moves backward.

Question 7.
In which medium does sound travel the fastest?
Answer:
Sound travels fastest in solids.

Question 8.
What is an echo?
Answer:
An echo is the repetition of a sound due to the reflection of sound waves off a hard, distant surface.

Question 9.
What do you mean by infrasonic waves?
Answer:
Sound waves with frequencies below 20 Hz, which cannot be heard by humans, are called infrasonic waves. Elephants can detect them.

Question 10.
State the relation between speed, frequency and wavelength of a sound wave.
Answer:
Speed = Frequency × Wavelength, i.e., υ = v ×λ.

Sound Waves Characteristics and Applications Class 9 Short Question Answer

Question 1.
Explain, with an example, that sound requires a material medium to propagate.
Answer:
The vacuum bell jar experiment demonstrates this clearly. When an electric bell is placed inside a bell jar and switched on, the ringing can be heard. As air is slowly removed using a vacuum pump, the sound becomes fainter. Once a near-vacuum is created, almost no sound is heard even though the bell is still ringing visibly. This shows that sound cannot travel through vacuum and requires a material medium (solid, liquid, or gas) to propagate.

Question 2.
Distinguish between longitudinal and transverse waves.
Answer:
In a longitudinal wave, particles of the medium vibrate back and forth parallel to the direction of wave propagation. Sound waves are longitudinal waves. In a transverse wave, particles vibrate perpendicular to the direction of wave propagation. Light waves and waves on a string are examples of transverse waves.

Question 3.
Why does sound travel faster in solids than in liquids or gases?
Answer:
The speed of sound depends on the elasticity and density of the medium. Solids have much higher elasticity (they resist compression more strongly) compared to liquids and gases. This means disturbances (compressions and rarefactions) are transmitted more quickly from one particle to the next. As a result, sound travels about 15-20 times faster in solids like steel (~5000 ms^-1) than in air (~340 ms^-1).

Question 4.
What is reverberation? How is it controlled in auditoriums?
Answer:
Reverberation is the persistence of sound in a large enclosed space due to multiple reflections from walls, ceiling, and floor, even after the original sound source has stopped. It occurs when successive reflections arrive within 0.05 s of each other.

In modern auditoriums, reverberation is controlled by using soft, porous materials such as curtains, upholstered chairs, carpets, and sound-absorbing panels on walls and ceilings. These materials absorb sound rather than reflecting it, reducing unwanted echoes and ensuring clear sound quality.

Question 5.
Explain how bats navigate in the dark using echolocation.
Answer:
Bats are nocturnal creatures that emit short bursts of ultrasonic waves (above 20 kHz) from their mouths. These waves travel outward and reflect off nearby objects, obstacles, or prey. The bat detects the returning echoes with its large, sensitive ears and uses the time delay and direction of the echo to determine the exact position, distance, and size of objects. This ability to navigate and hunt using reflected sound waves is called echolocation. Dolphins and whales also use echolocation.

Question 6.
The sound wave is given below. Label A-D and explain the nature of the wave.

Answer:
A: Amplitude (maximum displacement from the equilibrium line to the trough)
B: Half wavelength (distance between two consecutive equilibrium crossings in the same direction)
C: Wavelength (distance between two successive crests or compressions)
D: Amplitude (maximum displacement from the equilibrium line to the crest). Nature of the wave is a longitudinal wave (particles vibrate parallel to the direction of propagation).

Question 7.
The four waves are shown below: Compare and describe each wave.

Answer:
A = This wave has high amplitude and is loud
B = Amplitude is very small and is not loud; it has higher frequency and hence has high pitch.
C = The wave has low amplitude and very small frequency. Hence, it is soft sound and has low pitch.
D = This wave has high amplitude and hence loud; it also has high frequency and has high pitch.

Question 8.

Label the two types of waves given above and explain how you can classify it.
Answer:
The wave in which the propagation direction is perpendicular to the disturbance direction is a transverse wave. It does not need any medium to travel. The second wave in which the propagation direction is same in the direction of the disturbance is called longitudinal wave. It needs medium to travel.

Question 9.
Which wave is of a soothing musical note and why?

Answer:
The second wave is having low amplitude and low frequency hence, it is soothing music sound as it is not loud sound and has low pitch. The wave above is having high amplitude and is very loud.

Question 10.
Draw the sketches of two waves A and B such that wave A has twice the wavelength and half the amplitude of wave B.
Answer:

Sound Waves Characteristics and Applications Class 9 Long Question Answer

Question 1.
Describe in detail how sound propagates through air using the piston-tube model.
Answer:
Consider a long tube filled with air with a piston at one end and open at the other. When the piston is stationary, the air inside has a uniform average density.

Forward stroke: When the piston moves forward, it compresses the nearby air particles, creating a region of higher density called a compression (C). This compression propagates forward as compressed particles collide with neighbouring particles.

Backward stroke: When the piston moves backward, the air near it becomes less dense, creating a region of lower density called a rarefaction (R). This rarefaction also moves forward as neighbouring particles spread out to fill the gap.

As the piston oscillates back and forth continuously, it produces alternating compressions and rarefactions that travel through the air away from the source. This travelling disturbance of alternating compressions and rarefactions is called a sound wave.

Question 2.
Explain the characteristics of sound waves:
wavelength, frequency, time period, amplitude, and speed. Give the relationship between speed, frequency, and wavelength.
Answer:
Wavelength (λ): The distance between two consecutive compressions or rarefactions is called the wavelength. SI unit: metre (m).

Frequency (ν): The number of complete oscillations (cycles) per unit time at a fixed point in the medium. SI unit: hertz (Hz or s^-1). High frequency = high-pitched sound.

Time Period (T): The time taken to complete one full oscillation. T = 1/v. SI unit: second (s).
Amplitude: The maximum displacement of particles from their mean position. Larger amplitude = louder sound.
Speed (ν): The distance travelled by a sound wave per unit time. It depends on the medium: fastest in solids, slowest in gases. In air at 22°C: approximately 344 ms^-1’.
Relationship: υ = ν × λ (speed frequency × wavelength). This means for a given medium (fixed speed), as frequency increases, wavelength decreases proportionally.

Question 3.
Write a note on the applications of ultrasonic and infrasonic waves in science and technology.
Answer:
Ultrasonic waves (frequency > 20 kHz) have many important applications:
Medical Imaging (Ultrasonography):
Used to image internal organs such as the liver, kidney, and uterus without any surgery or harmful radiation.

• Industrial Cleaning: Ultrasonic waves can clean delicate machine parts, jewellery, and electronic components by dislodging dirt particles.
• Detecting Defects: Ultrasonic waves detect cracks or flaws inside metal blocks used in construction and engineering (non-destructive testing).
• Sonar: Ships and submarines use sonar (Sound Navigation and Ranging) to detect underwater objects, measure ocean depths, and locate shipwrecks using reflected ultrasonic waves.

Question 4.
(a) A laboratory uses an ultrasonic scanner to trace tumour in tissue. The operating frequency of the scanner is 4.2 × 10⁶ Hz. Calculate the wavelength of sound in the tissue if the speed of the sound is 1700 ms^-1.
(b) A person clapped his hands near a cliff and heard the echo after 2 s. What is the distance of the cliff from the person if the speed of the sound, v, is taken as 346 ms^-1?
Answer:
(a) To calculate the wavelength of sound in the tissue, we can use the wave equation:
Wavelength (λ) = Speed of Sound/ Frequency
Where : Speed = 1700 m/s
Frequency = 4.2 × 10⁶ Hz
Now, substitute the values into the formula:
Wavelength (λ) = 1700 / 4.2 × 10⁶
= 4.05 × 10^-4 m
So, the wavelength of sound in the tissue is 4.05 × 10^-4 m.

(b) Given,
Speed of sound, ν = 346 ms^-1
Time taken for hearing the echo, t = 2 s
Distance travelled by the sound
= v × t = 346 ms^-1 × 2 s = 692 m
In 2 seconds, sound has to travel twice the distance between the cliff and the person. Hence, the distance between the cliff and the person = \(\frac{692 \mathrm{~m}}{2}\) = 346 m.

Question 5.
Draw a curve showing density or pressure variations with respect to distance for a disturbance produced by sound. Mark the position of compression and rarefaction on this curve. Also define wavelengths and time period using this curve.
Answer:

Wavelength: The distance between two consecutive compressions (or rarefactions) is called the wavelength.
Time Period: The time taken to travel between two consecutive compressions (or rarefactions) is called time period.

Sound Waves Characteristics and Applications Class 9 Quiz Questions

Question 1.
What type of wave is a sound wave?
Answer:
Longitudinal (mechanical) wave

Question 2.
What is the full form of SONAR?
Answer:
Sound Navigation and Ranging

Question 3.
Name one animal that can hear infrasonic waves.
Answer:
Elephant

Question 4.
What is the speed of sound in dry air at 0°C?
Answer:
331 ms^-1

Question 5.
Name the technique used to image internal organs using sound.
Answer:
Ultrasonography.

Sound Waves Characteristics and Applications Class 9 Skill Based Questions

Question 1.
(a) What is loudness of sound?
(b) How is loudness of sound related to amplitude?
Answer:
(a) The loudness of sound is determined by its amplitude. The amplitude of the sound wave depends upon the force with which an object is made to vibrate.
(b) Loud sound can travel a larger distance as it is associated with higher energy. A sound waves spreads out from its source. As it moves away from the source, its amplitude as well as its loudness decreases.

Sound Waves Characteristics and Applications Class 9 Case Based Questions

Question 1.
Priya and her friends are on a school trip near a large mountain range. Priya shouts loudly and, after about 2 seconds, hears her own voice again. Her science teacher explains that the sound reflected off the mountain. On another day, the class visits a hospital where the doctor uses an instrument to view the image of a patient’s kidney without any operation.

The doctor uses high-frequency sound waves that are beyond the range of human hearing to create images of the internal organs.
I. What phenomenon did Priya experience when she heard her own voice after shouting? Explain it.
II. What type of sound waves was the doctor using to view the patient’s kidney? Name the medical technique.
OR
Why is it not possible to hear an echo in a small room?
Answer:
I. Priya experienced an echo. An echo is the reflection of sound off a hard, distant surface (the mountain). For an
echo to be heard distinctly, the reflected sound must reach the listener at least 0.1 s after the original sound. Since
Priya heard her voice after 2 seconds, the mountain was quite far away. The sound travelled to the mountain and
back in 2 s, meaning the mountain was at a distance of (340 × 2)/2 = 340m from
Priya (using speed of sound ≈ 340 ms^-1).

II. The doctor was using ultrasonic waves – sound waves with frequencies above 20,000 Hz (20 kHz), which are beyond human hearing. The medical technique used to view internal organs using such waves is called ultrasonography (or ultrasound imaging). These waves are safe, non-invasive, and can pass through soft tissues, reflecting back at boundaries to create images.
OR
In a small room, the walls are too close to the listener. Sound reflected from the walls reaches the listener in less than
0.1 s, which is the minimum time gap needed for the human brain to distinguish between the original sound and its echo. Since the two sounds merge, no distinct echo is heard. Instead, the combined effect creates reverberation.

Question 2.
For hearing the loudest ticking sound by the ear, following experimental setup is made:

(a) Find the angles x and y in the figure above.
(b) Name the phenomenon observed here.
(c) Sound waves and light waves are similar in that both ……………………. .
(i) are produced by mechanical vibrations
(ii) are longitudinal waves
(iii) can travel through vacuum
(iv) obey the laws of reflection
(d) Which of the following is an elastic wave?
(i) Sound waves
(ii) light waves
(iii) X-rays
(iv) Radio waves
Answer:
∠i + 50° = 90° ⇒ ∠i = x = 40°
∠r = y = 40°

(b) Reflection of sound is the phenomenon observed.
(c) (iv) obey the laws of reflection
(d) (i) Sound waves: Elastic waves require a medium for their propagation.

Suggested Projects and Activities

Question 1.
Investigate Noise Levels in Your Surroundings Study how noise levels vary in different environments and their possible impact on health. Use a mobile app (sound level meter) to measure noise (in decibels) at places like home, school, roadside, and a park. Record readings at different times of the day. Classify the noise levels into safe, moderate, and harmful ranges.

Research recommended noise limits by organizations like World Health Organization. Suggest ways to reduce noise pollution in your area. Present your findings with charts, graphs, and a short report.

Question 2.
Create a musical instrument using easily available materials like rubber bands and a box. Experiment by changing the thickness and tension of the bands. Observe the changes in sound produced and relate them to pitch and frequency. Explain your findings with examples.

Sound Waves Characteristics and Applications Extra Questions for Practice

Multiple Choice Questions

Question 1.
A sound wave travels from steel into air. Which of the following increases?
(a) Frequency
(b) Wavelength
(c) Speed
(d) Time period

Question 2.
A tuning fork completes 440 oscillations per second. What is its time period?
(a) 440 s
(b) 0.00227 s
(c) 44 s
(d) 0.44 s

Question 3.
The graph of a sound wave plots density of the medium on the y-axis and distance on the x-axis. The highest points of this graph represent
(a) rarefactions
(b) troughs
(c) compressions
(d) mean density.

Question 4.
Read the statements below and choose the correct option.
Assertion (A): The particles of the medium travel along with a sound wave.
Reason (R): Sound waves are longitudinal mechanical waves.
(a) Both A and R are true, but R is not the correct explanation of A.
(b) Both A and R are true, and R is the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.

Question 5.
Match the Column A with Column B.

Column A	Column B
(i) Frequency	(p) Loudness
(ii) Amplitude	(q) Number of vibrations per second
(iii) Echo	(r) Reflection of sound
(iv) Time period	(s) Time taken for one vibration

(a) (i) – (q), (ii) – (p), (iii) – (r), (iv) – (s)
(b) (i) – (p), (ii) – (q), (iii) – (s), (iv) – (r)
(c) (i) – (r), (ii) – (s), (iii) – (p), (iv) – (q)
(d) (i) – (s), (ii) – (r), (iii) – (q), (iv) – (p)

Very Short Answer Type Questions

Question 1.
Define time period of a sound wave.

Question 2.
State one use of infrasonic waves.

Question 3.
What is the speed of sound in water at 15°c (approximately)?

Question 4.
Name the quantity that remains unchanged when sound passes from one medium to another.

Question 5.
What does SONAR stand for?

Short Answer Type-I Questions

Question 1.
Distinguish between echo and reverberation.

Question 2.
A sound wave has a frequency of 250 Hz and a wavelength of 1.32 m. Calculate its speed and time period.

Question 3.
What is pitch? How is it related to frequency?

Question 4.
Why do we hear two separate sounds when Gunjan places her ear on a steel fence, and her friend strikes the fence 340 m away?

Question 5.
Explain, with the help of a slinky analogy, how compressions and rarefactions are produced in a sound wave.

Short Answer Type-II Questions

Question 1.
A lightning flash is seen and thunder is heard 6 seconds later. Taking the speed of sound as 340 ms^-1, calculate the distance to the lightning strike. Why is light seen first?

Question 2.
State and explain three factors that affect the speed of sound.

Question 3.
A naval vessel uses sonar to detect a submarine. The sonar signal returns after 1.2 seconds. If the speed of sound in seawater is 1530 ms^-1’, how far is the submarine?

Question 4.
A person hears an echo after 2 seconds. If the speed of sound is 330 m/s, calculate the distance of the reflecting surface.

Question 5.
A sound wave travels 1700 m in 5 seconds. Calculate its speed and wavelength if frequency is 100 Hz.

Long Answer Type Questions

Question 1.
With the help of the graphical representation of a sound wave, explain the terms: wavelength, frequency, amplitude, crest, and trough. Also write the wave equation relating speed, frequency, and wavelength.

Question 2.
Describe in detail the applications of reflection of sound. Include echo, reverberation, use in medicine, industry, and navigation.

Question 3.
Explain how sound travels in different mediums such as solids, liquids, and gases. Why does sound travel fastest in solids?

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