Selina Concise Solutions for ICSE Class 7 Physics Chapter 6 Sound

Points to Remember

• Sound is a form of energy which produces the sensation of hearing.
• Sound requires a medium to travel. It cannot travel in vacuum.
• Sound travels in all directions. Sound can travel through solids, liquids and gases.
• Sound travels faster in solids than in liquids and gases.
• The loudness of sound is measured on a special scale called the decibel scale.
• Sound is classified into two groups. (i) Musical sound, (ii) Noise.
• Musical sound is produced by periodic vibrations.
• Noise is produced by non-periodic motions.
• What the sound gets repeated after reflection from a distant body, is called an echo.
• Multiple echoes are called reverberations.
• Sonar is method used to measure the depth of a sea and is based on the principle of echo.
• Music is pleasant to hear and is caused by periodic vibrations.
• Noise is irritating and is produced by an irregular pattern of waves.
• The three characteristics of sound are loudness, pitch, quality or timbre.
• Loudness depends upon many factors, e.g. amplitude, distance, waves of vibrating body, humidity, pressure, wind velocity etc.
• Pitch depends on the frequency of sound. Higher is the pitch, shriller is the sound.
• Loudness is merely a sensation.

Test Yourself

A. Objective Questions

1. Write true or false for each statement

(a) Sound can travel in vacuum.
Answer: False.
Correct - Sound requires medium to travel.
Sound is a mechanical wave that works by bumping particles into each other. Since a vacuum has no particles to bump, the energy has no way to move from one place to another.
Teacher's Tip: Remember "No Particles, No Sound."
Exam Tip: Mention that sound is a mechanical wave to justify why it needs a medium.

(b) Sound is a form of energy.
Answer: True.
Sound can do work, such as vibrating your eardrum or even breaking a thin glass if it is loud enough. It is created by converting kinetic energy from a vibrating source into sound waves.
Teacher's Tip: Anything that can make something else move (like your eardrum) is a form of energy.
Exam Tip: State that sound energy is produced by the mechanical vibrations of matter.

(c) Sound can only be produced by vibrating bodies.
Answer: True.
Whether it is a guitar string or your vocal cords, something must move back and forth to create sound. Without this initial vibration, the surrounding air particles would remain still and no sound would exist.
Teacher's Tip: Touch your throat while talking to feel the vibrations yourself!
Exam Tip: Use the term "vibratory motion" when defining the origin of sound.

(d) Larger is the amplitude, feeble is the sound.
Answer: False.
Correct - Larger the amplitude, greater is the sound.
Amplitude measures how much energy a wave carries and how far the particles move from their resting spot. The more energy you put into a vibration, the larger the amplitude and the louder the resulting sound.
Teacher's Tip: Think of amplitude as the "height" of the wave; taller waves mean bigger sound.
Exam Tip: Remember the relationship: Loudnesspropto Amplitude².

(e) The frequency is measured in hertz.
Answer: True.
Frequency tells us how many vibrations occur in exactly one second. One Hertz (Hz) is equal to one full vibration per second.
Teacher's Tip: Hertz is named after Heinrich Hertz, a famous scientist who studied waves.
Exam Tip: Always write the unit symbol as "Hz" with a capital H.

(f) Loudness depends on frequency.
Answer: False.
Correct - Loudness depends on the amplitude.
Frequency determines the pitch (how high or low the sound is), while amplitude determines the volume. You can have a high-pitched sound that is very quiet or a low-pitched sound that is very loud.
Teacher's Tip: Amplitude = Volume dial; Frequency = Bass/Treble dial.
Exam Tip: Do not confuse loudness with pitch; they depend on different wave properties.

(g) Waveforms of two different stringed instruments can be the same.
Answer: False.
Correct - Waveforms of two different stringed instruments cannot be the same.
Every instrument has a unique "quality" or "timbre" because of the different overtones it produces along with the main note. This unique shape of the wave is what allows our ears to tell a violin apart from a guitar.
Teacher's Tip: Think of waveforms as the "fingerprints" of musical instruments.
Exam Tip: Use the term "Quality" or "Timbre" to explain why waveforms differ.

(h) Female voice is shriller than the male voice.
Answer: True.
The vocal cords of women are generally shorter and thinner, which allows them to vibrate faster. This higher frequency of vibration results in a higher pitch, making the voice sound shriller.
Teacher's Tip: Shrillness is just another way to say "High Pitch."
Exam Tip: Explain shrillness in terms of high frequency for full marks.

(i) A ticking clock sound is heard late when heard through a metal.
Answer: False.
Correct - A ticking clock sounds is heard early when heard through a metal.
Sound travels much faster in solids like metal because the atoms are packed tightly together, allowing them to pass vibrations quickly. In contrast, air is much less dense, so the vibrations take longer to reach your ear.
Teacher's Tip: Sound is a "fast traveler" in solids because particles are already touching.
Exam Tip: State the order of speed: Solids > Liquids > Gases.

2. Fill in the blanks

(a) Sound is produced when a body vibrates.
Answer: vibrates.
Vibration is the rapid to-and-fro motion of an object about its central position. This motion pushes against nearby air molecules, starting the sound wave chain reaction.
Teacher's Tip: No shaking, no making (of sound)!
Exam Tip: The word "vibrate" is the fundamental key to any sound-related definition.

(b) The number of times a body vibrates in one second is called its frequency.
Answer: frequency.
Frequency measures the "speed" of the vibration cycle. A high frequency means the object is moving back and forth many times very quickly.
Teacher's Tip: Use the "1-second rule" to identify frequency.
Exam Tip: Define frequency as f = 1/T where T is the time period.

(c) The pitch of a sound depends on its frequency.
Answer: frequency.
Pitch is the psychological perception of frequency. When frequency increases, the sound becomes "higher" or "shriller," like a bird chirping versus a lion roaring.
Teacher's Tip: High frequency = High Pitch; Low frequency = Low Pitch.
Exam Tip: Always link "pitch" to "frequency" and "loudness" to "amplitude."

(d) Sound can travel in a medium solid, liquid or gas.
Answer: solid, liquid or gas.
These three states of matter contain the atoms necessary to carry sound vibrations. Whether through the ground, underwater, or through the air, sound always finds a path if matter is present.
Teacher's Tip: Sound can move through anything except "nothing" (vacuum).
Exam Tip: List all three states of matter to show a complete answer.

(e) We can hear sounds of frequency in the range of 20 Hz to 20,000 Hz.
Answer: 20 Hz to 20,000 Hz.
This is known as the "audible range" for human beings. Sounds below this range are infrasonic, and sounds above it are ultrasonic.
Teacher's Tip: Think of 20 to 20k—it's easy to remember!
Exam Tip: If you use kilohertz, the range is 20 Hz to 20 kHz.

(f) Sound requires a medium for propagation.
Answer: medium.
A medium is any substance (solid, liquid, or gas) through which energy can travel. Since sound waves are mechanical, they cannot exist in the empty space of a vacuum.
Teacher's Tip: Medium means "the stuff in the middle" between the sound source and you.
Exam Tip: Use the "bell jar experiment" as evidence that a medium is required.

(g) Sound travels faster in solids than in liquids.
Answer: solids.
The rigid structure of solids allows vibrations to be passed from particle to particle with very little delay. In liquids, the particles are further apart, so the transmission is slightly slower.
Teacher's Tip: Think of a "solid" wall versus "sloshy" water; the wall passes a knock much faster.
Exam Tip: Clarify that the high elasticity of solids contributes to higher sound speed.

(h) The sound heard after reflection is echo.
Answer: echo.
An echo occurs when sound waves hit a hard surface and bounce back to your ears. To hear a distinct echo, the reflecting surface must be at least 17 meters away.
Teacher's Tip: An echo is just a sound wave playing "ping-pong" with a wall.
Exam Tip: Mention "reflection" as the physical phenomenon that creates an echo.

(i) Sound produces sensation in ears.
Answer: ears.
The vibrating air particles enter the ear canal and hit the eardrum, causing it to vibrate. These vibrations are then converted into electrical signals that the brain recognizes as sound.
Teacher's Tip: Your ears are like tiny "microphones" that catch air vibrations.
Exam Tip: Specify that the sensation happens when the eardrum (tympanum) vibrates.

3. Match the following

Column A (a) Vibrations cause
(b) A shriller sound is
(c) Unit of frequency
(d) Unit of time period
(e) Curtains
Column B
(i) absorb sound
(ii) second
(iii) sound
(iv) of high pitch
(v) hertz
Answer:
Column A
(a) Vibrations cause
(b) A shriller sound is
(c) Unit of frequency
(d) Unit of time period
(e) Curtains
Column B
(iii) sound
(iv) of high pitch
(v) hertz
(ii) second
(i) absorb sound

4. Select the correct alternative

(a) We can distinguish a shrill sound from a flat sound by its
1. amplitude
2. loudness
3. pitch
4. none of the above.
Answer: 3. pitch.
Pitch is the property that defines whether a sound is high (shrill) or low (flat). It is determined entirely by the frequency of the vibrating source.
Teacher's Tip: "Flat" sounds are like low bass notes; "Shrill" sounds are like high treble notes.
Exam Tip: Select "pitch" whenever the question mentions shrillness or flatness.

(b) We can hear sound of frequency
1. 10 Hz
2. 500 Hz
3. 100,000 Hz
4. 50,000 Hz
Answer: 2. 500 Hz.
The human ear can only hear sounds between 20 Hz and 20,000 Hz. 500 Hz falls comfortably within this range, while the other options are either too low or too high.
Teacher's Tip: Use the range 20-20k to filter out the wrong answers.
Exam Tip: Remember that anything above 20,000 Hz is called "ultrasound."

(c) Sound cannot travel in
1. gases
2. liquids
3. solids
4. vacuum
Answer: 4. vacuum.
A vacuum is a space entirely devoid of matter. Because sound needs a material medium to propagate, it is completely blocked by a vacuum.
Teacher's Tip: "In space, no one can hear you scream" because space is a vacuum.
Exam Tip: Identify "vacuum" as the absence of a medium.

(d) The minimum distance required between the source and the reflector so as to hear the echo in air is
1. 10m
2. 17m
3. 34 m
4. 50 m
Answer: 2. 17m.
The human brain needs a 0.1-second gap between sounds to distinguish them. Given the speed of sound is roughly 340 m/s, the sound must travel 34 meters total (17m there and 17m back) for that gap to exist.
Teacher's Tip: Think of 17 as the "magic number" for echos.
Exam Tip: Calculate the distance using Distance = Speed × Time/2.

(e) Wavelength is measured in
1. kg
2. second
3. litre
4. metre
Answer: 4. metre.
Wavelength is the physical distance between two consecutive peaks (crests) or troughs of a wave. Since it is a measurement of length, the standard S.I. unit is the meter.
Teacher's Tip: "Length" is in the word "Wavelength"—length is always in meters!
Exam Tip: Don't confuse wavelength (distance) with frequency (cycles per second).

(f) The speed of sound in water is
1. 332 m
2. 1500 m
3. 5000 m s
4. 1000 m s
Answer: 2. 1500 m (Note: should be 1500 m/s)
Sound travels about four times faster in water than in air because liquid particles are closer together. This fast travel is why marine animals like whales can communicate over huge distances.
Teacher's Tip: Air approx 330, Water approx 1500, Steel approx 5000.
Exam Tip: Be careful with units; speed must always include "per second" (m/s).

(g) Sound travels the fastest in
1. liquids
2. solids
3. gases
4. vacuum
Answer: 2. solids.
In a solid, the atoms are locked in a tight grid, making them very efficient at passing along kinetic energy. The "springiness" or elasticity of solids allows sound waves to zoom through them.
Teacher's Tip: "Solid is speedy" – remember this for speed comparisons.
Exam Tip: Order the mediums by density and elasticity to explain why solids are fastest.

B. Short/Long Answer Questions

Question 1: What do you mean by a vibratory motion ?
Answer: The oscillatory motion in which the body assumes a new shape during its motion, is called the vibratory motion.
This means the object not only moves back and forth but may also stretch or bend as it moves. Common examples include the skin of a drum being hit or a rubber band being plucked.
Teacher's Tip: Vibration is just "fast, shaky oscillation."
Exam Tip: Distinguish vibratory motion from simple oscillatory motion by mentioning the "change in shape."

Question 2: What is sound ?
Answer: Sound is a form of energy which produces the sensation of hearing.
It travels as a wave through a medium and is interpreted by our brain when our ears detect those vibrations. Sound energy is mechanical energy because it depends on the movement of matter.
Teacher's Tip: Sound is energy you can "feel" with your ears.
Exam Tip: Always define sound as both "energy" and a "sensation" for a complete answer.

Question 3: How is sound produced ?
Answer: Sound is produced by vibrating bodies.
When an object vibrates, it creates a disturbance in the surrounding medium (like air). This disturbance travels outward as a sound wave until it reaches a receiver.
Teacher's Tip: If something is making noise, something is shaking!
Exam Tip: Use the word "disturbance" to describe how the vibrating body affects the medium.

Question 4: Describe an experiment to show that each source of sound is a vibrating body.
Answer: Sound is produced when a body vibrates. In other words, each source of sound is a vibrating body. This can be demonstrated by the following experiment.
Take a ruler. Press its one end on the table with the left hand as shown in figure. Pull down the other end of the ruler with the right hand and then leave it.
You will notice that the ruler vibrates i.e., the ruler moves to and fro and a humming sound is heard.
After some time, the ruler stops vibrating. No sound is then heard.
This shows that the humming sound is produced only because of the vibrations of the ruler.
The experiment clearly links the presence of sound to the physical act of vibrating. When the movement stops, the sound dies instantly, proving that the two are inseparable.
Teacher's Tip: You can do this at home with a plastic or metal ruler on your desk.
Exam Tip: Mention the "start" and "stop" of both vibration and sound to prove the link.

Question 5: Name two sources of sound.
Answer: Each vibrating body is a source of sound. We, the human beings, produce sound when our vocal cords vibrate on blowing air through them by our lungs. Some animals like birds, frogs etc., also produce sound due to vibration of their vocal cords. But bees do not have the voice-boxes. They produce sound by moving their wings up and down very fast.
Biological sources like our throat and mechanical sources like a ringing bell are both valid examples. Even insects like bees create sound by using their wings as vibrating "plates."
Teacher's Tip: Think of one "natural" source (vocal cords) and one "artificial" source (musical instrument).
Exam Tip: Clearly identify the vibrating part for each source you name.

Question 6: How do we produce sound ?
Answer: Our throat has a larynx. The voice is produced in the larynx. Larynx is also called the voice box. It is designed to produce voice. It is a box like structure with walls of tough tissues. Inside two folds of the tissue, there is a gap. They are the vocal cords. When we breathe, the vocal cords become loose and the gap between them increases. When we talk, shout or sing, the cords become tight and hence they vibrate, thus produce sound. Given figure shows the part of the body which vibrates to produce sound.
Our lungs provide the "wind" power that pushes air through the narrow gap in the tightened vocal cords. This airflow forces the cords to shake rapidly, turning your breath into the sound of your voice.
Teacher's Tip: Your larynx is like a biological "musical instrument."
Exam Tip: Name the "Larynx" and "Vocal Cords" as the specific parts involved.

Question 7: The bees do not have voice-boxes. How do they produce sound ?
Answer: The bees do not have the voice-boxes. Still they produce sound. This happens by the vibrations produced by the quick movement of their wings. Bees buzz while flying and depositing pollen among flowers.
Because their wings move hundreds of times per second, they disturb the air at a frequency we can hear as a "buzz." The speed of the wing-beat determines the pitch of the buzzing sound.
Teacher's Tip: Buzzing is just "wing-flapping" sound energy!
Exam Tip: Point out that sound doesn't always need a larynx; any fast vibration works.

Question 8: Can sound travel through a vacuum ? Describe an experiment to explain your answer.
Answer: Experiment - Arrange an electric bell, a glass bell jar, a vacuum pump, a battery and a switch as shown in the figure. When the circuit is closed by pressing the switch, the bell starts ringing and sound can be heard. Now remove the air from the jar with the help of vacuum pump. The loudness of the sound gradually decreases and a stage comes when no sound is heard. Sound requires a medium to travel but cannot travel in vacuum.
This "Bell Jar Experiment" proves that without air molecules to carry the vibration, sound cannot bridge the gap between the bell and our ears. Even though we can see the bell moving, the silence confirms that sound needs a medium.
Teacher's Tip: "See but not hear" is the result when the air is pumped out.
Exam Tip: Label your diagram with "Bell Jar," "Vacuum Pump," and "Electric Bell."

Question 9: Describe an experiment to show that sound can travel in water.
Answer: Take a tub filled with water. Hold a bell in one hand and dip it in water. Keep one of your ears gently on the surface of water without letting water into the ear. Now ring the bell inside water. You will be able to hear the sound clearly. This shows that sound can travel through liquids.
Water is a better conductor of sound than air because its molecules are closer together. This allows the energy to pass through the liquid efficiently until it reaches your ear.
Teacher's Tip: This is why divers can hear boat engines from far away under the sea.
Exam Tip: Emphasize that the sound must be "clear" to prove the liquid is a good medium.

Question 10: Describe an experiment to show that sound can travel in a solid.
Answer: Take two empty ice-cream cups. Make a small hole at the bottom of each cup and pass a long thread (about 20 m long) through them. Tie a knot or match-stick at each end of the thread so that the thread does not slip out through the holes. This makes a toy - telephone.
Now use the toy-telephone as shown in figure and talk to your friend. You will be able to hear the sound of your friend. This shows that sound travels through the thread and reaches your ear. Thus, sound can travel through a solid.
The vibrations from your voice shake the cup, which pulls on the thread and sends the vibration all the way to the other end. The string acts as a solid bridge for the sound energy to cross.
Teacher's Tip: Keep the string "tight" or the vibrations won't travel properly!
Exam Tip: Identify the "thread" as the specific solid medium in this experiment.

Question 11: Can two person hear each other on moon’s surface ? Give reason to support your answer.
Answer: No, we cannot hear each other since sound requires medium for transmission. It cannot travel through vacuum.
The Moon has no atmosphere, meaning it is surrounded by a total vacuum. Without air to carry vibrations between astronauts, they must use radios (which use light-based waves) to talk.
Teacher's Tip: Think of the Moon as a "silent giant" because it has no air.
Exam Tip: Mention "lack of atmosphere" as the reason for the vacuum on the Moon.

Question 12: What is a longitudinal wave ?
Answer: In a longitudinal wave, the particles of air vibrate to and fro about their mean positions in the direction of travel of sound.
This is like a Slinky toy being pushed and pulled; the wave moves forward and the coils move forward and backward. It creates regions of high pressure (compressions) and low pressure (rarefactions).
Teacher's Tip: Longitudinal = Along (moves along the same line as the particles).
Exam Tip: Contrast this with "Transverse" waves where particles move up and down.

Question 13: Define the following terms : Amplitude, Time period, Frequency.
Answer: (a) Amplitude (A) : The maximum displacement of a wave on either side of its mean position is called Amplitude. A = XY is amplitude.
(b) Time Period (T) : Time taken to complete one vibration is called Time Period, i.e. from A to B.
(c) Frequency (f) or u : Number of oscillations made by a wave in one second is known as its frequency.
These three terms define everything about how a sound wave behaves and looks. Amplitude tells us the "size," Time Period tells us the "duration," and Frequency tells us the "speed" of the wave.
Teacher's Tip: Amplitude = Height; Time Period = Time for 1; Frequency = Number in 1s.
Exam Tip: Use units (m, s, Hz) when defining these terms to get full marks.

Question 14: Write the audible range of frequency for the normal human ear.
Answer: The range of frequency from 20 Hz to 20,000 Hz is called the audible range for the normal human ear.
This is the specific window of sound that our biological equipment can process. As people get older, the upper limit of this range usually drops lower.
Teacher's Tip: Think of it as your ear's "reception frequency."
Exam Tip: State the range clearly and label the limits with the unit "Hz".

Question 15: What are ultrasonics ? Can you hear the ultrasonic sound ?
Answer: Sounds of frequency higher than 20,000 Hz are called the ultrasonics. We cannot hear the ultrasonic sounds.
Even though these sounds exist and vibrate air particles, they move too fast for our eardrums to respond to them. Many animals, like dogs and bats, can hear these high-frequency sounds easily.
Teacher's Tip: "Ultra" means beyond—beyond our hearing range!
Exam Tip: Define ultrasonics as frequencies > 20 kHz.

Question 16: What are infrasonics ? Can you hear them ?
Answer: Sounds of frequency lower than 20 Hz are called the infrasonics. We cannot hear the infrasonic sounds.
These are very deep, low-frequency rumbles, often produced by earthquakes or large animals like elephants. They are too slow for the human ear to register as sound sensation.
Teacher's Tip: "Infra" means below—below our hearing range!
Exam Tip: Define infrasonics as frequencies < 20 Hz.

Question 17: How does a bat make use of ultrasonics waves to find its way?
Answer: Use of ultrasonics by bats : Bats have no eyes. But they easily move about without colliding with any object (or obstacle). The reason is that they produce ultrasonic sound as they fly. When this ultrasonic sound comes back after reflection from any object (or obstacle) in their way, they hear it and thus they detect the presence of the object (or obstacle).
This biological sonar system is called echolocation. By measuring the time it takes for the "ping" to return, the bat's brain builds a 3D map of the world in total darkness.
Teacher's Tip: Bats "see" with their ears using sound bounce-backs.
Exam Tip: Use the term "Echolocation" to describe this process.

Question 18: Name the two characteristics of sound which differentiate two sounds from each other.
Answer: A sound wave is characterized by its amplitude and frequency. Depending upon the amplitude and frequency of the sound wave, the following two characteristics of sound : (1) Loudness, and (2) Pitch.
Loudness allows us to tell the difference between a whisper and a shout. Pitch allows us to distinguish between a high-noted bird call and a low-noted lion's growl.
Teacher's Tip: Loudness = How strong; Pitch = How high.
Exam Tip: Connect each characteristic to its physical cause (Loudness Amplitude, PitchFrequency).

Question 19: On what factor does the loudness of a sound depend ?
Answer: The loudness of a sound depends on the amplitude of vibration of the vibrating body producing the sound.
When you hit a drum harder, you increase the amplitude of the drumhead's vibration. This larger physical movement pushes the air with more force, creating a louder sound.
Teacher's Tip: More energy = More Amplitude = More Volume.

Question 20: How does the loudness of sound produced depend on the vibrating area of the body ?
Answer: The loudness of sound also depends on the area of the vibrating body. Greater the area of the vibrating body, louder is the sound produced.
If you take two drums, one small and the other big, and beat both of them to produce vibrations in them, We will notice that the sound produced from the big drum is louder than that produced from the small drum. In temples, you must have noticed that the bell with a big case produces a louder sound than that with a small case.
A larger surface area can push a much greater volume of air at once. This creates a more powerful pressure wave that reaches our ears with higher intensity.
Teacher's Tip: Big bells make big booms because they touch more air!
Exam Tip: Use the "two drums" example to illustrate the effect of surface area on loudness.

Question 21: The outer case of the bell in a temple is made big. Give a reason.
Answer: The outer case of the bell in a temple is made big. So that there is multiple reflection of sound and the sound can be amplified.
The large, hollow shape acts like a resonator that traps and bounces the sound waves inside. This adds more energy to the sound before it escapes, making the bell ring loudly throughout the temple grounds.
Teacher's Tip: The bell's shape is like a "megaphone" that points the sound outward.
Exam Tip: Mention "amplification" and "reflection" as the two key reasons for the size.

Question 22: State the factors on which the pitch of a sound depends.
Answer: The pitch of a sound depends on its frequency (i.e., on the frequency of the vibrating body).
Pitch is our ear's way of measuring how fast vibrations are happening. A rubber band vibrating 100 times per second will have a much higher pitch than one vibrating only 20 times per second.
Teacher's Tip: Frequency is the "ruler" we use to measure pitch.
Exam Tip: Identify frequency as the primary physical factor for pitch.

Question 23: Differentiate between a high pitch sound and a low pitch sound.
Answer: Higher the pitch, the shriller is the sound. Lower the pitch, the flat (or grave) is the sound.
High pitch sounds are associated with high-frequency sources like a sparrow or a child's voice. Low pitch sounds come from low-frequency sources like a thunderclap or a bass guitar.
Teacher's Tip: Think of a whistle (High) versus a foghorn (Low).
Exam Tip: Use words like "shrill" for high pitch and "flat/grave" for low pitch in your answers.

Question 24: How does a man’s voice differ from a woman’s voice ?
Answer: A female voice is shriller than a male voice because of higher frequency. Higher is the frequency, shriller is the sound. Female has higher frequency.
Biological differences in the length and tension of vocal cords cause this change. Men generally have thicker, longer cords that vibrate slowly, creating a deeper, lower-pitched sound.
Teacher's Tip: Men have "heavy" vocal strings; women have "light" vocal strings.
Exam Tip: Contrast the two voices using the terms "frequency" and "pitch."

Question 25: Name the characteristic which differentiates two sounds of the same pitch and same loudness.
Answer: The quality is the characteristic of sound which distinguishes the two sounds of the same pitch and same loudness.
This is why you can tell a piano and a flute apart even if they are playing the exact same note at the same volume. Each instrument creates a specific mix of extra vibrations called overtones.
Teacher's Tip: Quality is the "personality" of the sound.
Exam Tip: Use the alternative name "Timbre" for quality if you want to sound like a pro!

Question 26: You recognize your friend by hearing his voice on a telephone. Explain.
Answer: We can recognize our friend by hearing his voice on a telephone due to quality of sound and pitch of sound.
Every person's vocal tract has a unique shape, which gives their voice a specific timbre or quality. Even through the limited speaker of a phone, these unique characteristics remain recognizable to our brain.
Teacher's Tip: Your voice is as unique as your thumbprint!
Exam Tip: Always mention "Quality" (Timbre) as the main reason for voice recognition.

Question 27: A musician recognizes the musical instrument by hearing the sound produced by it, even without seeing the instrument. Which characteristic of sound makes this possible ?
Answer: It is the pitch and quality that helps a musician recognize the musical instrument by hearing the sound produced by it, even without seeing the instrument.
Quality describes the specific "texture" of the sound wave produced by materials like wood, brass, or strings. A musician's trained ear can identify these textures instantly.
Teacher's Tip: Sound quality is like the "flavor" of music.
Exam Tip: Specify that quality is the primary characteristic for identifying different instruments.

Question 28: Describe an experiment to show the production of sound having low and high pitch.
Answer: Take few rubber bands some thicker and longer, few thinner and of shorter length. Cut and stretch these rubber bands by holding one end of the string in your mouth under the teeth and the other end in your hand. Now pluck these rubber bands one by one. The rubber bands thicker and longer will produce sound with a lower pitch. The rubber bands thinner and shorter will produce sound with a higher pitch.
The mass and length of the rubber band change how fast it can vibrate back and forth. Lighter, tighter bands snap back much faster, creating the high-frequency waves we hear as high pitch.
Teacher's Tip: Thickness acts like a "weight" that slows down the vibration.
Exam Tip: Link the physical properties (thickness/length) to the resulting frequency and pitch.

Question 29: How does a musician playing on a flute change the pitch of sound produced by it ?
Answer: In musical instruments like flute and clarinet, the pitch of sound is changed by changing the length of vibrating air column when different holes in it are closed.
Closing more holes makes the air column longer, which makes it vibrate more slowly (low pitch). Opening the holes shortens the column, allowing it to vibrate faster (high pitch).
Teacher's Tip: Short pipe = Short waves = High Pitch; Long pipe = Long waves = Low Pitch.
Exam Tip: Use the term "vibrating air column" to explain pitch changes in wind instruments.

Question 30: Why are musical instruments provided with more than one string ?
Answer: The stringed instruments are provided with a number of strings of different thickness and under different tensions so that each string produces sound of a different pitch.
Having multiple strings allows a musician to play a wide variety of notes very quickly without constantly retuning. Each string is a "specialist" for a specific part of the musical scale.
Teacher's Tip: One string can only do so much; more strings mean more musical possibilities!
Exam Tip: Mention that thickness and tension are adjusted to set the base pitch for each string.

Question 31: How can the pitch of sound produced in a piano be changed ?
Answer: In a piano, the string is struck to make the string vibrate and produce sound. The pitch of sound produced can be changed by stretching or loosening the strings of piano.
Increasing the tension makes a string tighter and faster to vibrate, which raises the pitch. This process, called tuning, ensures that every key on the piano plays the correct musical note.
Teacher's Tip: Tighter strings always sing a higher note!
Exam Tip: Explain that tension is the key factor used for permanent pitch adjustment in a piano.

Question 32: Explain why you can predict the arrival of a train by placing your ear on the rails without seeing it.
Answer: The sound produced by the moving wheels of train travels much faster through the track than through the air. Therefore they hear through the track much before it is heard through the - air.
Because steel is a solid, its molecules are very close together and very elastic, allowing them to pass vibrations at about 5000 m/s. This is more than 15 times faster than the sound traveling through the air.
Teacher's Tip: The metal rails are like a "superhighway" for sound waves.
Exam Tip: Mention the specific speed of sound in steel (approx 5000 m/s) to show deep knowledge.

Question 33: Write the approximate speed of sound in (i) air, (ii) water and (iii) steel.
Answer:
Medium - Speed of sound
(i) Gas (Air) - 330 m s^-1
(ii) Liquid (Water) - 1500 m ss^-1
(iii) Solid (Iron or Steel) - 5000 m ss^-1
The speed increases as the medium becomes denser and more rigid. This data proves that solids are the most efficient conductors of sound energy among the three states of matter.
Teacher's Tip: Memorize the sequence: 330, 1500, 5000.
Exam Tip: Use the unit ms^-1 or m/s for every value in the table.

Question 34: During a thunderstorm, the sound of a thunder is heard after the lightning is seen. Why ?
Answer: The velocity of light is 3 × 10⁸ m/s whereas velocity of sound is 332 m/s. First we see the flash of light and then we hear the thunder.
Light is almost a million times faster than sound, so it reaches your eyes nearly instantly. The sound wave is much "lazier" and takes about 3 seconds to travel just one kilometer.
Teacher's Tip: You can count the seconds between light and sound to estimate how far away a storm is!
Exam Tip: Compare the two specific speeds (3 × 10⁸ vs 332) to justify the delay.

Question 35: Describe an experiment to estimate the speed of sound in air.
Answer: To estimate the speed of sound in air suppose we choose two hills A and B about a kilometer apart. A person at the hill A fires a gun. Another person at the hill B starts a stop watch as he sees the flash of the fire and stops it on hearing the sound. Thus, he measures the time interval between the seeing of flash and hearing of the sound. Let it be t second. Then measure the distance between the hills A and B. Let it be S metre.
The speed of sound V = Distance (S)/Time (t) = S/tms^-1
Experimentally, it is found that the speed of sound in air is nearly 330 m ss^-1.
This method works because the flash of light is so fast that we can assume it reaches the observer at exactly the moment the gun is fired. The delay on the stopwatch is entirely caused by the slower sound wave traveling across the gap.
Teacher's Tip: This is basically a "race" where light gets a massive head start!
Exam Tip: Write the final formula V = S/t and specify the result as approximately 330 m/s.

Question 36: Can sound travel through solids and liquids ? In which of these two does it travel faster ?
Answer: Sound travels with highest speed in - solids. and Sound travels with lowest speed in - gases.
Sound travels through all states of matter, but its speed depends on how closely the particles are packed. Between solids and liquids, solids are faster because their structure is more rigid and better at transmitting mechanical force.
Teacher's Tip: Solid > Liquid > Gas. It's an alphabetical trend for speed!
Exam Tip: Mention that sound travels faster in solids because of their high elasticity.

Question 37: What do you mean by reflection of sound ?
Answer: Reflection of Sound - When a sound wave strikes a rigid surface, it retraces from its path is called reflection of sound.
Just like light hitting a mirror, sound waves bounce off hard objects like walls or cliffs. This redirected energy is what we recognize as an echo.
Teacher's Tip: Reflection is just sound "bouncing" off a hard surface.
Exam Tip: Use the term "rigid surface" to describe what causes sound to bounce back.

Question 38: State one use of reflection of sound.
Answer: The reflection of sound is used in making the speaking tube (or megaphone), sound board and trumpet.
By shaping the instrument to bounce sound in a specific direction, we can concentrate the energy and make it travel much further. This prevents the sound from scattering in all directions, making it louder for the listener.
Teacher's Tip: A megaphone is a tool that "points" reflections where you want them.
Exam Tip: Name the "Megaphone" or "Trumpet" as a practical application of reflection.

Question 39: What is echo ?
Answer: Echo is the sound heard after reflection from a rigid surface such as a cliff, a hillside, the wall of a building etc.
For an echo to be clear, it must arrive back at our ears at least 0.1 seconds after the original sound. This time delay allows our brain to process the two sounds as separate events.
Teacher's Tip: An echo is the sound's "mirror image."
Exam Tip: Mention that an echo is a "repeated sound" heard after a delay.

Question 40: What minimum distance is required between the source of sound and the reflecting surface to hear an echo ? Give reason.
Answer: Since sound has to travel an equal distance in going up to the reflecting surface and in coming back from the reflecting surface, therefore it must travel nearly 33/2 = 16.5 m either way. Thus, to hear the echo clearly in air, the reflecting surface should be at a minimum distance of 16.5 m from the source of sound.
Our brain has "persistence of hearing" which lasts for about 0.1 seconds. To cross 33 meters at the speed of sound takes exactly 0.1 seconds, which is why 16.5m (half of 33) is the required distance to the wall.
Teacher's Tip: Round 16.5 up to 17m to be safe and remember it easier.
Exam Tip: Show the math (Speed × 0.1 / 2) to prove the 16.5m result.

Question 41: List four substances which are good absorbers of sound.
Answer: When sound falls on sofa, fluffs and light substances such as clothes, papers, thermocol, coating of plaster of paris, carpets, curtains, furniture, wood etc., they absorb the sound to a good extent. These are called good absorbers of sound.
These materials have tiny holes or loose fibers that trap the sound wave and turn its energy into a tiny amount of heat. Because the energy is trapped, it doesn't bounce back, which eliminates noise and echoes.
Teacher's Tip: Soft and "holey" materials are the best sound sponges.
Exam Tip: Give common household examples like "Carpets" or "Curtains."

Question 42: List the measures that you will take when designing a sound-proof room.
Answer: In order to design such a sound proof room we take the following measures
(1) The roof of the enclosure must be covered by plaster of paris after putting the sheets of thermocol.
(2) The walls of the enclosure should be covered by the wooden strips.
(3) The floor must be laid down by thick carpets.
(4) The machine parts of all the electrical equipments such as fan, air conditioner etc. must be placed outside the enclosure.
(5) Thick curtains should be used to cover the doors and keep them closed.
(6) Thick stripping must be used to cover the openings of doors and windows.
A sound-proof room works by blocking sound from coming in and absorbing any sound produced inside. By using carpets, thermocol, and curtains, we ensure that every surface is a "sound sponge" that prevents reflection.
Teacher's Tip: Think of it as wrapping the room in a giant, soft blanket!
Exam Tip: Group your answers by surface (Ceiling, Walls, Floor) for a better structure.

C. Numericals

Question 1: A boy fires a gun and another boy at a distance of 1020 m hears the sound of firing the gun 3 s after seeing its smoke. Find the speed of sound.
Answer:
Speed = Distancespace travelled/timespace taken
Speed = 1020/3 = 340 m ss^-1
By dividing the total distance by the time delay, we find how many meters the sound covered each second. This result is close to the standard speed of sound in air, confirming the calculation is correct.
Teacher's Tip: Distance div Time is the formula for any speed calculation.
Exam Tip: Don't forget to include the unit ms^-1 in your final answer.

Question 2: A boy on a hill A fires a gun. The other boy on hill B hears the sound after 4 s. If the speed of sound is 330 m s^-1, find the distance between the two hills.
Answer:
Speed, v = 330 m s^-1
t = 4 s
Distances = v × t
= 330 × 4 s = 1320 m Ans.
To find the distance, we multiply the speed of the wave by the number of seconds it traveled. This shows that the two hills are over a kilometer apart.
Teacher's Tip: Rearrange the triangle: Distance = Speed × Time.
Exam Tip: Double-check your multiplication to avoid simple arithmetic errors.