10. Short and Long Answer Questions: Sound Waves: Characteristics and Applications

Short Answer Type Questions

Ques 1: What is vibration? How does it help in the production of sound?

Ans: Vibration refers to the periodic to and fro motion (oscillations) of an object about its mean position. When an object vibrates, it disturbs the surrounding medium by creating alternating compressions and rarefactions, which travel as a sound wave. Sound is produced only as long as the object continues to vibrate; once the vibration stops, the sound ceases.

Ques 2: What is a tuning fork? How is it used to demonstrate that sound is produced by vibrating objects?

Ans: A tuning fork is a U-shaped metal bar with a stem, usually made of steel or aluminium. Its two sides are called prongs or tines. When a prong is struck against a rubber pad, it begins to vibrate and produces sound. If the vibrating prong is gently touched to the surface of water, ripples form immediately, confirming that the prongs are indeed vibrating and that sound is produced by vibrating objects.

Ques 3: Why can sound not travel in vacuum? Give an example to support your answer.

Ans: Sound needs a material medium (solid, liquid, or gas) to propagate, because it travels through compressions and rarefactions of the particles of the medium. Vacuum is a space with no medium (no matter), so there are no particles to pass on the disturbance. In the vacuum bell jar experiment, when air is pumped out of a jar containing a ringing electric bell, the sound becomes fainter and nearly disappears even though the bell can still be seen ringing, proving that sound cannot propagate in vacuum.

Ques 4: Define compression and rarefaction in the context of a sound wave.

Ans: When a vibrating source moves forward, it pushes nearby air particles together, creating a region of higher air density than average. This high-density region is called a compression (C). When the source moves backward, air particles spread apart, forming a region of lower air density than average, called a rarefaction (R). A sound wave consists of a series of alternating compressions and rarefactions travelling through the medium.

Ques 5: What is wavelength of a sound wave? What is its SI unit? How is it related to the speed and frequency of the wave?

Ans: The distance between two consecutive crests (or two consecutive troughs) in the graphical representation of a sound wave is called its wavelength, represented by $λ$ . Its SI unit is the metre (m). The relationship between speed $v$ , frequency $ν$ , and wavelength is given by: $v = λ \times ν$ This means wavelength and frequency are inversely related for a fixed speed of sound in a given medium.

Ques 6: Define frequency and time period of a sound wave. Write the mathematical relation between them.

Ans: The number of density oscillations (compressions and rarefactions) occurring at a fixed point per unit time is called the frequency $(ν)$ of the sound wave. Its SI unit is hertz (Hz) or s $^{- 1}$ . The time taken for one complete density oscillation at a fixed point is called the time period $(T)$ of the wave. Its SI unit is second (s). They are inversely related: $ν = \frac{1}{T}$

Ques 7: What is amplitude of a sound wave? How is it related to the energy and loudness of sound?

Ans: The amplitude of a sound wave is the maximum change in air density in a compression (or rarefaction) compared to the average density. A wave with a larger amplitude carries more energy and is perceived as louder sound, while a wave with a smaller amplitude carries less energy and is heard as softer sound. This is why striking an object harder produces a louder sound – more energy is transferred to the surrounding medium.

Ques 8: What is an echo? State the minimum distance required between the source of sound and a reflecting surface for an echo to be heard clearly.

Ans: When sound bounces off a hard and distant surface and reaches back to the listener after some time, the reflected sound heard is called an echo. For the brain to distinguish the echo from the original sound, the time gap between the two must be at least 0.1 s. Taking the speed of sound as $340 {m s}^{- 1}$ , the total distance the sound must travel is $340 \times 0.1 = 34$ m, so the minimum distance of the reflecting surface from the source must be at least $\frac{34}{2} = 17$ m.

Ques 9: What is reverberation? How is it controlled in large auditoriums?

Ans: When sound undergoes multiple reflections from the walls of a large hall or auditorium, it persists after the source stops emitting, producing a prolonged sound effect called reverberation. This happens when successive sound reflections arrive with a time difference of less than 0.05 s. In large auditoriums, reverberation is controlled by using sound-absorbing materials such as soft panels, upholstered chairs, curtains, and porous surfaces on the walls and ceiling, which prevent unwanted reflections and ensure clear sound.

Ques 10: What are infrasonic and ultrasonic waves? Name one animal that can detect each type.

Ans: Sound waves with a frequency below 20 Hz are called infrasonic waves, and those with a frequency above 20 kHz (20,000 Hz) are called ultrasonic waves. Both lie outside the human audible range of 20 Hz to 20 kHz. Elephants can detect infrasonic waves, while dogs, bats, and dolphins can detect ultrasonic waves. Humans cannot hear either infrasound or ultrasound.

Ques 11: What is echolocation? Name any two animals that use it and describe how they benefit from it.

Ans: Echolocation is the ability to locate objects by emitting sound waves and sensing the reflected echoes. Bats emit short bursts of ultrasonic waves; by detecting the echoes that bounce back from nearby objects or prey, they can determine the position, distance, and size of obstacles even in complete darkness. Dolphins similarly use echolocation to navigate underwater and locate prey. This remarkable ability allows these animals to hunt and move accurately without relying on vision.

Ques 12: State how the speed of sound depends on the medium and temperature. Give a numerical example to support the effect of temperature.

Ans: The speed of sound depends on the nature of the medium – it travels fastest in solids, slower in liquids, and slowest in gases. For example, sound travels about 4-5 times faster in water and 15-20 times faster in solids than in air. Within the same medium (air), the speed also increases with temperature and humidity. For instance, the speed of sound in dry air is about $331 {m s}^{- 1}$ at $0 ° C$ and rises to nearly $344 {m s}^{- 1}$ at $22 ° C$ .

Long Answer Type Questions

Ques 1: Explain how a sound wave is formed in air. Describe the nature of a sound wave and why it is called a longitudinal mechanical wave.

Ans: Consider a long tube filled with air and fitted with an oscillating piston at one end.

Compression: When the piston moves forward, it pushes nearby air particles together, increasing the air density in that region. This high-density region is called a compression (C). Due to collisions, the compression passes forward through the air, even though the air particles themselves do not travel with it.
Rarefaction: When the piston moves backward, the air near the piston becomes less dense than average. This low-density region is called a rarefaction (R). Like the compression, the rarefaction also moves forward through the medium.
Sound wave: As the piston oscillates continuously, alternating compressions and rarefactions are produced, which travel away from the source. This disturbance – the series of alternating C and R – is called a sound wave. The particles of the medium only vibrate about their mean positions and do not travel with the wave.
Longitudinal wave: In a sound wave, the particles of the medium vibrate parallel to the direction of wave propagation. Such waves are called longitudinal waves. Sound is a longitudinal wave.
Mechanical wave: Sound requires a material medium (solid, liquid, or gas) to propagate. Waves that need a medium for propagation are called mechanical waves. Since sound cannot travel through vacuum, it is a mechanical wave.

Thus, sound is a longitudinal mechanical wave.

Ques 2: Describe the characteristics of a sound wave – wavelength, frequency, time period, amplitude, and speed – and write the mathematical relationship connecting speed, wavelength, and frequency.

Ans:

Wavelength (λ): The distance between two consecutive compressions (crests) or two consecutive rarefactions (troughs) in a sound wave. SI unit: metre (m).
Frequency (ν): The number of complete density oscillations at a fixed point per unit time. SI unit: hertz (Hz) or s $^{- 1}$ .
Time period (T): The time taken for one complete density oscillation at a fixed point. SI unit: second (s). The relation between frequency and time period is: $ν = \frac{1}{T}$
Amplitude: The maximum change in density of the medium at a compression (or rarefaction) compared to the average density. A larger amplitude corresponds to louder sound and more energy carried by the wave.
Speed (v): The speed of sound is defined as the distance a point on the wave (crest or trough) travels in unit time. The mathematical relationship connecting speed, wavelength, and frequency is: $v = λ \times ν$ For example, if the speed of sound in air is $344 {m s}^{- 1}$ and the frequency is $344 Hz$ , the wavelength is $λ = \frac{344}{344} = 1$ m.

These characteristics together describe a sound wave completely. While speed, wavelength, frequency, and amplitude are physical and measurable, the corresponding human perceptions are pitch (related to frequency) and loudness (related to amplitude).

Ques 3: What is the reflection of sound? State the laws of reflection of sound. Explain echo and reverberation with examples, and give the condition necessary to hear an echo.

Ans: Sound waves can bounce off obstacles such as solids or liquids; this bouncing back of sound is called the reflection of sound.

Laws of reflection of sound:

The angle of incidence (the angle between the incident sound and the normal to the reflecting surface) is equal to the angle of reflection (the angle between the reflected sound and the normal).
The incident sound, the reflected sound, and the normal at the point of incidence all lie in the same plane.

Echo: When sound reflects off a hard, distant surface and returns to the listener after some time, the reflected sound is perceived as an echo. For an echo to be heard clearly, the brain must be able to separate the reflected sound from the original. This requires a minimum time gap of 0.1 s. Taking the speed of sound as $340 {m s}^{- 1}$ : $Minimum distance of reflecting surface = \frac{340 \times 0.1}{2} = 17 m$

Reverberation: In a large hall or auditorium, sound undergoes multiple reflections from the walls, ceiling, and floor. When the time gap between successive reflections is less than 0.05 s, the reflected sounds overlap and sound persists even after the source stops – this is reverberation. Modern concert halls use sound-absorbing materials like soft panels, curtains, and upholstered chairs to control unwanted reverberation and maintain clarity of sound.

Ques 4: Write a detailed note on ultrasonic waves: their frequency range, and at least four important applications in science, medicine, and technology.

Ans: Sound waves with frequency above 20 kHz (i.e., above the upper limit of human hearing) are called ultrasonic waves. Humans cannot hear them, but animals like dogs, bats, dolphins, and whales can. Ultrasonic waves have several important applications:

Medical imaging (ultrasonography): Ultrasonic waves are used to image internal organs of the body without surgery. The waves are directed into the body and the reflected waves are used to form images, helping doctors diagnose conditions in the liver, kidney, uterus, heart, etc.
Breaking kidney stones: High-intensity ultrasonic waves can be focused on kidney stones to break them into smaller pieces, which then pass out of the body naturally, avoiding the need for surgery.
Industrial cleaning: Delicate machine parts and odd-shaped objects, such as electronic components, that cannot be cleaned by ordinary methods are placed in a cleaning solution and subjected to ultrasonic waves. The vibrations dislodge all dirt and grease from even hard-to-reach surfaces.
Detecting defects in metals: Ultrasonic waves are used in industrial testing to detect internal cracks or flaws in metal blocks. The waves pass through the metal and any defect causes reflection, revealing its location.
Sonar (Sound Navigation and Ranging): In sonar, ultrasonic waves are sent into water from a ship and the reflected waves (echoes) are detected. By measuring the time taken for the echo to return, the distance, direction, and speed of underwater objects like submarines or shipwrecks can be determined. If the time taken is $t$ and the speed of sound in water is $v$ , then the distance $d$ of the object is: $d = \frac{v \times t}{2}$

Ques 5: Explain human perception of sound with reference to pitch and loudness. Also define intensity and state the difference between intensity and loudness. What is the audible range of humans and what are infrasonic and ultrasonic waves?

Ans: The physical properties of a sound wave – frequency, amplitude, wavelength, and speed – are measurable quantities. However, how humans experience sound is subjective and described in terms of pitch and loudness.

Pitch: Pitch is how the frequency of a sound is perceived by the human ear. Sounds with higher frequency are perceived as having a higher pitch (shrill sounds like a whistle or a siren), while sounds with lower frequency have a lower pitch (deep sounds like thunder or an aircraft engine). In general, high-frequency sounds have high pitch and low-frequency sounds have low pitch.
Loudness: Loudness is the human perception of the amplitude of a sound wave. Sounds with larger amplitude are heard as louder, while those with smaller amplitude sound softer. Loudness also decreases as one moves away from the source, since sound energy spreads over a larger area with distance.
Intensity: Intensity is the amount of sound energy passing through a unit area perpendicular to the direction of propagation of the sound wave in unit time. It is a measurable physical quantity. Loudness, on the other hand, depends on the listener’s hearing ability and is subjective. In everyday language both terms are often used interchangeably, but they are not the same. Sound loudness is commonly measured in decibels (dB); for example, normal conversation is about 60 dB, while very loud sounds like firecrackers can exceed 100 dB.
Audible range: The human ear can hear sounds in the frequency range of 20 Hz to 20,000 Hz (20 kHz). This is called the audible range. This range decreases with age.
Infrasonic waves: Sound waves with frequency below 20 Hz, inaudible to humans but detectable by animals like elephants. Used to detect natural events such as earthquakes and volcanic eruptions.
Ultrasonic waves: Sound waves with frequency above 20 kHz, inaudible to humans but detectable by bats, dogs, and dolphins. Their wide applications is in medicine, industry, and navigation .