Frank Brothers Solutions for ICSE Class 9 Physics Chapter 7. 3 Sound

Page No: 286

Question 1. Can we hear sound in a dark room?
Answer: Yes, we can hear in the dark room.
In simple words: Hearing does not depend on light. Just like you can hear a bell ring at night, sound travels perfectly well in the dark.

📝 Teacher's Note: Use this to explain that sound energy and light energy are independent. Students often confuse the two when first learning about senses.

🎯 Exam Tip: Simply state "Yes" and mention that sound waves do not require light for propagation.

Question 2. How is sound produced?
Answer: Sound is produced by the vibrating objects.
In simple words: Whenever something shakes back and forth very fast (vibrates), it creates sound waves in the air.

📝 Teacher's Note: Ask students to touch their throats while humming. They will feel the vibrations of their vocal cords, which produces the sound.

🎯 Exam Tip: "Vibration" is the key technical word here. Always include it when explaining the origin of sound.

Question 3. Is sound a form of energy?
Answer: Yes, sound is a form of energy.
In simple words: Sound is energy because it can move things (like your eardrum) and perform work.

📝 Teacher's Note: You can demonstrate this by placing a speaker near a candle; the sound vibrations can make the flame dance or even blow it out.

🎯 Exam Tip: Categorize sound as "mechanical energy" to show a deeper understanding.

Question 4. Can sound travel through a vacuum?
Answer: No, sound cannot travel through vacuum. It needs a material medium to travel.
In simple words: Sound needs "stuff" like air, water, or metal to ride on. In empty space (vacuum), there is nothing for the sound to travel through.

📝 Teacher's Note: This is a classic "Bell Jar Experiment" topic. Remind students that movies often get this wrong by showing loud explosions in space!

🎯 Exam Tip: Always specify that sound is a "mechanical wave," which by definition requires a medium.

Question 5. Does sound require a material medium for transmission?
Answer: Yes, sound requires a material medium for transmission.
In simple words: To get from one place to another, sound must pass through a solid, liquid, or gas.

📝 Teacher's Note: Use the analogy of a boat needing water to sail; without the water (medium), the boat (sound) cannot move.

🎯 Exam Tip: List the three types of media (solids, liquids, and gases) to provide a complete answer.

Question 6. On what factors does the speed of sound depend?
Answer: The speed of sound depends on the following factors: Density of air, Temperature, Humidity in air as well the wind.
In simple words: Sound travels faster or slower depending on how hot the air is, how much water is in it, and which way the wind is blowing.

📝 Teacher's Note: Explain that on a hot day, particles move faster, helping sound travel more quickly compared to a cold day.

🎯 Exam Tip: Memorize these four factors: Density, Temperature, Humidity, and Wind.

Question 7. What is the effect of wavelength and amplitude on the speed of sound?
Answer: There is no effect on the wavelength and amplitude on the speed of sound.
In simple words: Making a sound louder (amplitude) or changing its pitch (wavelength) doesn't make it travel faster or slower.

📝 Teacher's Note: Students often think loud sounds travel faster. Use the analogy of a car: a red car and a blue car can both go the same speed regardless of their color.

🎯 Exam Tip: Remember that speed is a property of the *medium*, not the sound wave's loudness or pitch.

Question 8. What is the speed of sound at room temperature?
Answer: Speed of sound at room temperature is 330 m/s.
In simple words: In a normal room, sound travels about 330 meters in one second.

📝 Teacher's Note: Note that this value varies slightly in textbooks (sometimes \( 340\text{ m/s} \)), but \( 330\text{ m/s} \) is the standard used here.

🎯 Exam Tip: Always include the unit "m/s" (meters per second) to get full marks.

Question 9. Is the speed of sound the same in different media?
Answer: Yes, the speed of sound is different in different medium.
In simple words: Sound moves much faster through solids (like a metal rail) than it does through air.

📝 Teacher's Note: Explain that since particles are closer together in solids, they can pass vibrations to their neighbors more quickly.

🎯 Exam Tip: Order of speed: \( \text{Solids} > \text{Liquids} > \text{Gases} \).

Question 10. Does the speed of sound depend on the pressure of a gas?
Answer: No, the speed of sound in the gas is independent on the change in pressure of the gas.
In simple words: Even if you squeeze the air (increase pressure), sound still travels at the same speed as long as the temperature stays the same.

📝 Teacher's Note: This is because as pressure increases, density also increases, and these two effects cancel each other out in the speed formula.

🎯 Exam Tip: This is a very common "trick" question. Always answer "independent" or "no effect."

Question 11. State the formula for the speed of sound in gas using Laplace correction.
Answer: Using Laplace correction, the speed of sound in the gas is given by:
\( V = \sqrt{\frac{\gamma P}{d}} \) where \( \gamma \) is ratio of specific heat of gas at constant pressure to that at constant volume.
In simple words: This math formula helps scientists calculate the exact speed of sound by looking at the gas pressure, its thickness (density), and its heat properties.

📝 Teacher's Note: Newton originally thought the process was isothermal, but Laplace corrected it to "adiabatic" because sound moves too fast for heat to escape.

🎯 Exam Tip: Identify the symbols: \( P \) is pressure, \( d \) is density, and \( \gamma \) (gamma) is the adiabatic constant.

Question 12. Does wind direction affect the speed of sound?
Answer: Yes, the speed of sound depends on the direction of wind. When wind is blowing in the direction of propagation of sound, the speed of sound increases. But, when wind blows in the direction opposite to that of sound, the speed of sound decreases.
In simple words: If the wind is blowing the same way you are shouting, your voice travels faster. If it's blowing against you, your voice slows down.

📝 Teacher's Note: Use the analogy of walking on a moving walkway at the airport. Walking with the belt makes you go faster; walking against it slows you down.

🎯 Exam Tip: Use the term "resultant velocity" to explain how wind speed adds to or subtracts from sound speed.

Question 13. What are ultrasonic sounds?
Answer: Ultrasonic are sounds having frequency higher that 20 KHz and not audible to the human being.
In simple words: These are "super high" sounds that are vibrating so fast our ears can't even hear them.

📝 Teacher's Note: Mention that while humans can't hear these, animals like dogs and bats can. That's why a dog whistle seems silent to us.

🎯 Exam Tip: The limit for human hearing is 20 Hz to 20,000 Hz (or 20 KHz). Anything above this is "ultrasonic."

Question 14. What does the acronym SONAR stand for?
Answer: SONAR stands for the “SOund Navigation And Ranging”.
In simple words: It's a system that uses sound to find things underwater, like how we use radar to find planes in the sky.

📝 Teacher's Note: Explain how ships use this to find the depth of the ocean or locate schools of fish by bouncing sound off the bottom.

🎯 Exam Tip: Memorize the full form carefully; it is a very common one-mark question.

Question 15. Compare the frequencies of ultrasonic and infrasonic sounds.
Answer: Ultrasonic sound has higher frequency than infrasonic sound.
In simple words: Ultrasonic is super fast (high pitch), and infrasonic is super slow (low pitch).

📝 Teacher's Note: Infrasonic sounds are below 20 Hz (like earthquake waves), while ultrasonic are above 20,000 Hz.

🎯 Exam Tip: Use the prefixes: "Ultra" means beyond/above, "Infra" means below/under.

Question 16. Can we hear the sound produced by a second’s pendulum?
Answer: No, we cannot hear the sound produced due to the vibration of second’s pendulum.
In simple words: A pendulum swings too slowly (low frequency) for our ears to hear it as a sound.

📝 Teacher's Note: A second's pendulum has a frequency of 0.5 Hz, which is way below the human hearing limit of 20 Hz.

🎯 Exam Tip: Categorize this as an "infrasonic" vibration.

Question 17. Does SONAR use infrasonic waves?
Answer: No, SONAR does not make use of infrasonic waves. Instead, it make use of ultrasonic waves.
In simple words: SONAR needs high-energy sounds that bounce back clearly, so it uses ultrasonics, not the low infrasonic waves.

📝 Teacher's Note: Ultrasonic waves are used because they can travel long distances in water without spreading out too much.

🎯 Exam Tip: SONAR = Ultrasound. Always remember this pairing.

Question 18. Does ultrasound travel faster than audible sound in the same medium?
Answer: Yes, ultrasound in the medium travels faster than the audible sound.
In simple words: In some cases, these high-frequency waves can move quicker through a material.

📝 Teacher's Note: (Note: In a non-dispersive medium like air, all frequencies travel at the same speed. This answer suggests a specific medium property or "ultrasound" speed characteristics often cited in introductory texts).

🎯 Exam Tip: Follow your textbook's specific claim on this; usually, speed depends only on the medium, not frequency.

Question 19. Is it possible to hear a friend's voice in a vacuum?
Answer: No, it is not possible to hear the friend’s voice in vacuum because sound needs a material medium to travel.
In simple words: If you were in a glass room with no air, you could see your friend's mouth move, but you wouldn't hear a single word.

📝 Teacher's Note: This reinforces that sound is a mechanical wave. Contrast this with light, which *can* travel in a vacuum.

🎯 Exam Tip: Use the phrase "absence of a medium" to explain why sound doesn't travel in a vacuum.

Question 20. What are the requirements for a medium to transmit sound?
Answer: The following are the requisites of medium to travel:

• The medium should possess elasticity so that particles of medium have tendency to come back to their original position after the displacement.
• The medium should be frictionless so that there should be loss of energy.
• The medium must possess inertia so that particles of medium have ability to store energy.

In simple words: For sound to move, the medium needs to be "springy" (elastic) so it can bounce back, it shouldn't rub too much (frictionless), and it needs to be able to carry movement (inertia).

📝 Teacher's Note: Explain "elasticity" using a spring. If the air wasn't elastic, once a particle moved, it would stay there, and the sound couldn't continue.

🎯 Exam Tip: Memorize the three terms: Elasticity, Low Friction, and Inertia.

Question 21. Why is a material medium required for the propagation of sound?
Answer: Sound is produced by the vibrating objects. We require a material medium for the propagation of sound because vibrations of object need to get transferred or transmitted from one place to another.
In simple words: Sound is like a relay race. One air particle has to bump into the next one to pass the message along. If there are no particles, the message stops.

📝 Teacher's Note: This explains the "mechanism" of sound. It's a chain reaction of collisions between molecules.

🎯 Exam Tip: Mention the "transfer of vibrations" as the reason for needing a medium.

Question 22. How does temperature affect the speed of sound in a gas?
Answer: With increase in temperature of gas, there is increase in speed of gas because increase in temperature decreases the density of gas. And speed of sound is inversely proportional to density of gas. The speed of gas is independent of the pressure of gas.
In simple words: Hot air is "thinner" (less dense), which makes it easier for sound to move through it quickly.

📝 Teacher's Note: Point out that on a hot summer evening, you can often hear sounds from further away because they travel faster.

🎯 Exam Tip: Explain that speed is directly proportional to the square root of the absolute temperature.

Question 23. State the effect of wavelength and density on the speed of sound.
Answer:

• No, there is no effect of wavelength and amplitude on the speed of sound.
• With increase in density, there is decrease in the speed of sound in gas.

In simple words: Changing the wave's shape doesn't change speed, but making the gas "heavier" (denser) slows the sound down.

📝 Teacher's Note: Be careful—solids are denser than air but sound is faster in them because their *elasticity* increases much more than their density.

🎯 Exam Tip: Remember: In gases, higher density = lower speed.

Question 24. Define sound and explain how it travels through wave motion.
Answer: Sound is defined as form of energy that produces the sensation of hearing in our ears. The sound travels in medium through energy transfers by wave motion from one place to another. For example, when the prong of the tuning fork vibrate, the waves start moving from one fork to another, the air start getting compressed. The compression is pushed forward. Now, when the fork vibrate in another direction, the air in the immediate neighbor which was compressed, now get rarefied. As a result, a series of compression and rarefactions are produced and sound energy travels in the direction where compression and rarefactions travel.
In simple words: Sound is energy you hear. It travels by squeezing and stretching the air (like a slinky toy) in a pattern called a wave.

📝 Teacher's Note: Use a Slinky to show compressions (squeezed parts) and rarefactions (stretched parts). This is the best visual for longitudinal waves.

🎯 Exam Tip: Define sound using the phrase "sensation of hearing" and mention "compression and rarefaction."

Question 25. What is the ratio of the speed of sound in air to that in steel?
Answer: The ratio of speed of sound in air to speed of sound in steel is 1/15.
In simple words: Sound travels 15 times faster in steel than it does in air!

📝 Teacher's Note: This is why you can hear an approaching train by putting your ear to the track long before you hear it through the air.

🎯 Exam Tip: If the speed in air is 330 m/s, the speed in steel would be roughly \( 330 \times 15 \approx 5000\text{ m/s} \).

Question 26. An observer hears the sound of a gun 2 seconds after the flash. If the speed of sound is 350 m/s, find the distance.
Answer: The distance of observer from the gun = speed of sound \( \times \) time taken for the explosion = \( 350 \times 2 = 700\text{m} \).
In simple words: Since sound travels 350 meters every second, in two seconds it covers double that distance.

📝 Teacher's Note: This assumes we see the flash instantly (which is true because light is so fast). Use the formula \( \text{Distance} = \text{Speed} \times \text{Time} \).

🎯 Exam Tip: Always show the multiplication step clearly to ensure full marks.

Question 27. Does light travel faster in air than in water? Why?
Answer: Yes, light travels faster in air than water. Because, light does not require any medium for their propagation and they are not mechanical waves. The quantities which oscillate in light are mechanical and electric field. And electric and magnetic field decrease with the medium in between them. So, light travels faster in air than water.
In simple words: Light loves empty space. The more "stuff" (like water) it has to push through, the more it slows down.

📝 Teacher's Note: This is the opposite of sound! Sound needs medium (faster in water), but light is slowed down by medium.

🎯 Exam Tip: Contrast "mechanical waves" (sound) with "electromagnetic waves" (light) in your explanation.

Question 28. Why do we see the flash of lightning before hearing the thunder?
Answer: The flash of light reaches earlier than the sound of thunder because the speed of light (its \( 3 \times 10^8 \)) is much larger than the speed of sound in air (its \( 330\text{ m/s} \)).
In simple words: Light is a million times faster than sound. It finishes the race to your eyes almost instantly, while the sound is still "running" to your ears.

📝 Teacher's Note: You can calculate how far away a storm is by counting the seconds between the flash and the bang. Every 3 seconds is about 1 kilometer.

🎯 Exam Tip: Cite the specific speeds (\( 3 \times 10^8\text{ m/s} \) vs \( 330\text{ m/s} \)) to support your answer.

Question 29. Out of 2Hz, 1000Hz, 200KHz, and 8MHz, which sound is audible to humans?
Answer: The sound of 1000 Hz frequency will be audible to us. Others 2Hz is infrasonic sound frequency and 200 KHz, 8 MHz is ultrasonic sound frequency.
In simple words: We can only hear sounds between 20 and 20,000. 1000 is right in the middle, but the others are too slow or too fast for us.

📝 Teacher's Note: Remind students of the "Audible Range": 20 Hz to 20,000 Hz. Anything outside this range is "silent" to humans.

🎯 Exam Tip: Be careful with units: "KHz" means thousand and "MHz" means million.

Question 30. Describe the Bell Jar experiment using a diagram.
Answer: Take an air tight bell jar having an arrangement of electric bell as shown above in the diagram. The electric bell is suspended inside the glass bell-jar, which is further connected to the vacuum pump. When the circuit of the electric bell is completed, the hammer strikes the gong repeatedly and sound is heard. Now, start withdrawing the air from the glass bell-jar using the vacuum pump and loudness of the sound will keep decreasing. When the whole of the air is withdrawn, no sound will be heard. So, this experiment shows that a material medium is needed for the propagation of sound.
In simple words: If you put a ringing bell in a jar and suck all the air out, the bell keeps ringing, but you won't hear anything! This proves sound needs air to travel.

📝 Teacher's Note: Emphasize that the bell is *suspended* by wires so the sound doesn't travel through the base of the jar (conduction through solids).

🎯 Exam Tip: The main conclusion is: "Sound cannot travel in a vacuum."

Question 31. Why can't two people hear each other on the moon?
Answer: There is no atmosphere on moon. So, we cannot hear each other on moon because sound needs a material medium to travel from one place to another. No kind of sound can be heard.
In simple words: The moon is like a giant vacuum. Since there's no air to carry the sound of your voice, you could shout as loud as you want and no one would hear you.

📝 Teacher's Note: Astronauts use radio waves (a form of light energy) to talk because radio *can* travel through a vacuum, unlike sound.

🎯 Exam Tip: Use the word "atmosphere" or "medium" to explain the lack of sound on the moon.

Question 32. List the characteristics required for a medium to transmit sound.
Answer: The following are the three characteristics of the medium required for the propagation of sound:

• The medium must possess elasticity so that the particles of medium have the tendency to return back to their original positions.
• The medium must be frictionless so that there is no loss of energy during transmission.
• The medium must possess inertia such that the particles of medium have the ability to store energy.

In simple words: A good medium for sound must be springy, smooth, and able to carry energy forward without losing it.

📝 Teacher's Note: (Repetition of Question 20). Reinforce these three terms as they are essential for describing how sound waves interact with matter.

🎯 Exam Tip: "Elasticity" and "Inertia" are the two most important properties to remember.

Question 33. How can a person calculate the speed of sound using an echo?
Answer: The simple experiment that a person can do to calculate the speed of sound in air is that a person stands at a known distance (d meter) from the cliff and fires a pistol and simultaneously start the stop watch. He stops the stop-watch as soon as he hears a echo. The distance travelled by the sound during the time (t) seconds is 2d.
So, \( \text{speed of sound} = \text{distance travelled} / \text{time taken} = 2d/t \)
In simple words: Since the sound has to go to the wall and come back, it travels twice the distance. Dividing that total distance by the time gives you the speed.

📝 Teacher's Note: This is a great outdoor activity. Use a measuring tape for 'd' and a digital stopwatch for 't'. The "2d" factor is the most common student error in echo problems.

🎯 Exam Tip: Always remember to use \( 2d \) for echo-based speed or distance calculations.

Question 34. Define ultrasonic sound and give two applications.
Answer: Ultrasonics sound is the sound having frequency more than 20KHz. The application of ultrasonic sound is as follows:

• It is used to detect the flaws in metal casting of automobile tyres.
• Used in hospitals to detect defects in certain parts of body.

In simple words: These are high-pitched sounds we can't hear. Doctors use them for "scans" (ultrasounds) to see inside the body without surgery.

📝 Teacher's Note: Mention "Echocardiography" or pregnancy scans as familiar examples of hospital applications.

🎯 Exam Tip: Application (i) is specifically for industrial quality control, while (ii) is for medical diagnostics.

Question 35. How are sound waves produced and transmitted in a medium?
Answer: Sound waves are produced by the object motion back and forth in the medium. Due to which it compresses the medium and compression waves are moved forward. The sound travels in medium through energy transfers by wave motion from one place to another. For example, when the prong of the tuning fork vibrate, the waves start moving from one fork to another, the air start getting compressed. The compression is pushed forward. Now, when the fork vibrate in another direction, the air in the immediate neighbor which was compressed, now get rarefied. As a result, a series of compression and rarefactions are produced and sound energy travels in the direction where compression and rarefactions travel.
In simple words: A vibrating object pushes the air molecules together (compression) and then pulls them apart (rarefaction). This "push-pull" pattern moves through the air like a wave.

📝 Teacher's Note: Emphasize that the *energy* moves forward, but the air particles only move back and forth in their own spots.

🎯 Exam Tip: Use a diagram of a tuning fork showing "C" (compression) and "R" (rarefaction) labels.

Question 36. Compare light and sound waves. Why does lightning reach us before thunder?
Answer: Light waves are electromagnetic waves while the sound waves are mechanical waves. Light waves do not require any material medium for their propagation while sound waves need the material medium for its travel from one place to another. Due to large magnitude of difference between speed of light (i.e \( 3 \times 10^8\text{ m/s} \)) to the speed of sound in air (330 m/s), the light reaches first and then the sound from the object. Like, in thunderstorm, the light reaches first to earth and then person hears the sound of light.
In simple words: Light is like a super-fast jet, and sound is like a person walking. Even though they start at the same time in the clouds, the light "jet" finishes the race way earlier.

📝 Teacher's Note: This is a standard comparison question. Ensure students understand that light is much, much faster (\( 300,000\text{ km/s} \)) compared to sound (\( 0.33\text{ km/s} \)).

🎯 Exam Tip: Mention both "wave type" (EM vs Mechanical) and "speed difference" for a full-mark answer.

Question 37. Detail the factors affecting the speed of sound in air (Pressure, Temperature, Humidity).
Answer:

• Pressure of air: The speed of sound in air is independent of pressure of air.
• Temperature of Air: Speed of sound in air is directly proportional to the temperature of air. As the temperature of air increases, the density of air decreases and hence speed of sound increases.
• Humidity: The speed of sound increases with increase in humidity. The increase of moisture decreases the density of the atmosphere and therefore sound travels faster in moist air.

In simple words: Pressure doesn't matter. But hot air and wet (moist) air both make sound travel faster because they are "lighter" for the sound to move through.

📝 Teacher's Note: This is the most detailed part of the chapter. Explain that moist air is actually less dense than dry air because water molecules are lighter than nitrogen or oxygen molecules.

🎯 Exam Tip: Note the "inverse" relationship between density and speed in these gas-based factors.

Question 38. How do bats and dolphins use sound to survive?
Answer:

• The bat during its flight at night: Bats emits its own distinctive sounds. The echoes of the sound helps bat to navigate and to locate their prey during night.
• The dolphin to locate small fish as its prey: Dolphins sends the sound having frequencies between 120 KHz to 150KHz. The returning echoes used to help their preys.

In simple words: These animals use "Echolocation." They shout high sounds and listen to the bounce (echo) to "see" in the dark or underwater.

📝 Teacher's Note: This is biological SONAR. It's so precise that a bat can detect a tiny mosquito in total darkness.

🎯 Exam Tip: Use the term "echolocation" if it is in your syllabus, or describe it as "echo-based navigation."

Question 39. Why does a person hear two sounds when a long metal bar is struck at the other end?
Answer: A person hearing at one end of the long metal bar hears two distinct sounds when the other end is struck with a stone because sound comes from the metal bar as well as from air. The sound travels faster in solids than air. So, first sound comes from metal bar hitting and then from air.
In simple words: Sound takes two different paths to reach you. It "wins the race" through the metal first, and then arrives a bit later through the air.

📝 Teacher's Note: This is a perfect proof that sound speed depends on the medium. The metal "shortcut" is much faster than the air "path."

🎯 Exam Tip: Mention that "speed in solids > speed in gases" to explain the time gap between the two sounds.