Selina Concise Solutions for ICSE Class 8 Physics Chapter 7 Sound

1. SOUND: “Is energy which produces in us the sensation of hearing.” It is produced by vibration of body.
Sound is a form of mechanical energy that travels through a medium as a wave. It is created whenever an object oscillates or moves back and forth rapidly.
Teacher's Tip: Remember that without vibration, there is no sound.
Exam Tip: For full marks, always define sound as a form of energy that requires a vibrating source.

2. Sound Propagation:
Sound needs a medium for its propagation. Sound cannot travel in vacuum.
Unlike light, sound needs molecules to carry its energy from one place to another. This is why in outer space, where there is no air, it is completely silent.
Teacher's Tip: Think of sound like a "delivery service" that needs a road (the medium) to travel.
Exam Tip: Use the term "mechanical wave" to explain why sound requires a material medium.

3. Speed of Sound:
Speed of sound is maximum in solids. 5000  ms^-1 in steel, in water 1500 ms^-1 and in air it is least 330 ms^-1 nearly.
Sound moves fastest in solids because the particles are packed very tightly together, allowing vibrations to pass quickly. In air, the particles are far apart, so the transfer of energy takes more time.
Teacher's Tip: Solids are "fast lanes" for sound waves.
Exam Tip: Memorize the approximate speed of sound in air 330  to 340 ms^-1 as it is frequently asked.

4. Energy in Sound:
When a body vibrates, the particles of medium also start vibrating and K.E. of particles changes into potential energy and P.E. into K.E. This is why sound in energy.
The constant switching between kinetic and potential energy keeps the wave moving through the air. This energy transfer is what eventually reaches our ears to produce the sensation of hearing.
Teacher's Tip: Kinetic energy is "moving energy" and Potential energy is "stored energy".
Exam Tip: Explain that sound is energy because it does work by moving the particles of the medium.

5. Sound Waves:
Sound travels in a medium in the form of wave.
A wave is a disturbance that carries energy through a medium without moving the matter itself permanently. Sound specifically travels as a series of pushes and pulls in the air.
Teacher's Tip: Imagine a wave in a stadium; the people stay in their seats, but the "wave" moves around the circle.
Exam Tip: Define a wave as a periodic disturbance in a medium.

6. Longitudinal wave: When the particles of medium move in the direction of motion of wave by forming compression and rarefaction.
In these waves, the air molecules move back and forth parallel to the way the sound is traveling. This creates areas of high pressure called compressions and low pressure called rarefactions.
Teacher's Tip: Use a "Slinky" toy to visualize longitudinal waves by pushing it forward and back.
Exam Tip: Draw a diagram showing clusters of lines (compressions) and gaps (rarefactions) to illustrate this wave.

7. AMPLITUDE: “The maximum displacement of the particle of medium on either side of mean position.”
Amplitude tells us how far a particle moves from its original resting spot when the sound wave passes. A larger amplitude means a more powerful vibration and a louder sound.
Teacher's Tip: Amplitude is the "height" of the wave's vibration.
Exam Tip: Label the amplitude on a wave diagram from the center line to the peak.

8. TIME PERIOD: “The time taken by a particle of medium to complete its one vibration” “t”
This is a measure of duration for a single full cycle of the wave to occur. It is usually measured in very small fractions of a second for most sounds.
Teacher's Tip: Time period is the "seconds per vibration".
Exam Tip: The S.I. unit for time period is the second (s).

9. FREQUENCY: “The number of vibrations made by a particle of the medium in one second. f measured in Hertz ({Hz})”
Frequency describes how fast the source is vibrating. High-frequency sounds have many vibrations per second and sound very sharp or shrill.
Teacher's Tip: Think of frequency as the "speedometer" of the vibration.
Exam Tip: Remember that 1 { Hz} = 1 { vibration per second}.

10. Frequency Relationship:
FREQUENCY f = 1/t or t = 1/f
This formula shows that frequency and time period are inversely related to each other. If the time for one vibration is small, the number of vibrations per second will be large.
Teacher's Tip: If you double the frequency, you halve the time period.
Exam Tip: Be prepared to calculate one if the other is given in a numerical problem.

11. WAVE LENGTH: “The distance travelled by the wave in one time period of vibration of particle of medium.” Or “The distance between two consecutive compressions or between two consecutive rarefactions.” It is denoted by ‘ lambda ’ and S.I. unit of wave length is metre (m).
Wavelength is the physical length of one complete wave cycle. In a sound wave, you measure it from the center of one compression to the center of the very next one.
Teacher's Tip: lambda is the Greek letter "Lambda".
Exam Tip: Always mention that wavelength is measured in metres (m).

12. CHARACTERISTICS OF SOUND:
(i) Loudness.
(ii) Pitch (or shrillness).
(iii) Quality (or timbre or wave form).
These three properties allow us to identify and distinguish between different sounds. For example, they help us tell the difference between a whisper and a shout, or a flute and a piano.
Teacher's Tip: These are the "Big Three" of sound identity.
Exam Tip: List all three when asked to name the characteristics of sound.

13. LOUDNESS: is the characteristic of sound by virtue of which a loud sound can be distinguished from a faint sound, both having same frequency and same wave form.
Loudness depends on how much energy the sound wave carries. It is primarily determined by the amplitude of the vibrating source.
It depend on: (i) Amplitude of wave (ii) Surface area of vibrating body (iii) Distance from the source of sound (iv) Sensitivity of listener: Unit of loudness is ({dB}) decibel.
Teacher's Tip: Remember: Big Amplitude = Big Loudness.
Exam Tip: Mention "decibel" ({dB}) as the unit of loudness for full marks.

14. PITCH: It depends on number of vibrations per second or frequency: more frequency is high pitch shrilled sound and low frequency is flat sound.
Pitch is how "high" or "low" a note sounds to our ears. A bird's chirp has a high pitch (high frequency), while a lion's roar has a low pitch (low frequency).
Teacher's Tip: Pitch is all about "speed" of vibration.
Exam Tip: Clearly state that Pitch is directly proportional to Frequency.

15. QUALITY: is the characteristic which distinguishes two sounds of the same pitch and same loudness. It depends on wave form.
Quality, or timbre, is why a middle C on a piano sounds different from a middle C on a guitar. Each instrument creates a unique wave shape even if they play the same note at the same volume.
Teacher's Tip: Quality is the "personality" or "voice" of the sound.
Exam Tip: Quality is determined by the "wave form" of the sound.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) When sound propagates in air, it does not carry energy with it.
Answer: False.
Sound is a form of energy that moves through the air by vibrating particles. This energy is transferred from one molecule to the next until it reaches our ears.
Teacher's Tip: If sound didn't carry energy, it couldn't move your eardrum.
Exam Tip: Remember that all waves, including sound, are transporters of energy.

(b) In a longitudinal wave, compression and rarefaction are formed.
Answer: True.
Longitudinal waves consist of areas where particles are pushed together and areas where they are spread out. These high and low-pressure regions allow the sound to move forward.
Teacher's Tip: Compression = "Squeeze", Rarefaction = "Stretch".
Exam Tip: Mention these two terms specifically when describing sound waves in air.

(c) The distance from one compression to nearest rarefaction is called wavelength.
Answer: False.
A wavelength is the distance between two *consecutive* compressions. The distance from a compression to the nearest rarefaction is actually only half a wavelength.
Teacher's Tip: One full wave must include one "squeeze" and one "stretch".
Exam Tip: Be careful with the word "consecutive" in definitions.

(d) The frequency is measured in second.
Answer: False.
Frequency is measured in Hertz ({Hz}), which represents cycles per second. The second is the unit for the Time Period, not frequency.
Teacher's Tip: Hertz ({Hz}) = 1/{second}.
Exam Tip: Always double-check units; Frequency is {Hz}, Time is s.

(e) The quality of a sound depends on the amplitude of wave.
Answer: False.
Quality depends on the shape of the wave, also known as the waveform. Amplitude determines how loud the sound is, not its quality.
Teacher's Tip: Think of quality as the "flavor" of sound and amplitude as the "portion size".
Exam Tip: Associate "Quality" with "Waveform" in your memory.

(f) The pitch of sound depends on frequency.
Answer: True.
Higher frequency vibrations produce a higher-pitched sound. As frequency increases, the sound becomes shriller.
Teacher's Tip: Faster vibrations mean higher notes.
Exam Tip: State that pitch and frequency are directly related.

(g) Decibel is the unit of pitch of a sound.
Answer: False.
The decibel ({dB}) is the unit for loudness. Pitch is related to frequency, which is measured in Hertz ({Hz}).
Teacher's Tip: Decibels measure "volume", Hertz measure "note".
Exam Tip: Don't confuse the characteristics; Loudness ({dB}) vs Pitch ({Hz}).

2. Fill in the blanks

(a) The time period of a wave is 2 s. Its frequency is 0.5 S^-1.
Answer: 0.5 S^-1.
Using the formula f = 1/T, we calculate 1/2 which equals 0.5. This means the wave completes half a vibration every second.
Teacher's Tip: 1 div 2 = 0.5.
Exam Tip: Show the calculation f = 1/T to get partial credit even if you make a math error.

(b) The pitch of a stringed instrument is increased by increasing tension in string.
Answer: increasing.
When a string is tighter, it vibrates much faster when plucked. Faster vibrations result in a higher frequency and therefore a higher pitch.
Teacher's Tip: Tightening a guitar string makes the note higher.
Exam Tip: Remember that tension, thickness, and length all affect string pitch.

(c) The pitch of a flute is decreased by increasing length of air column.
Answer: increasing.
A longer air column vibrates more slowly than a short one. Slower vibrations mean a lower frequency, which results in a lower pitch.
Teacher's Tip: Longer instrument = Lower sound.
Exam Tip: Use the relationship: Length of air column is inversely proportional to frequency.

(d) Smaller the membrane, higher is the pitch.
Answer: higher.
Small membranes can vibrate back and forth much more rapidly than large, heavy ones. This higher frequency creates a sharper, higher-pitched sound.
Teacher's Tip: Small drums sound high, big drums sound low.
Exam Tip: Associate "smaller size" with "faster vibration".

(e) If a drum is beaten hard, its loudness increases.
Answer: increases.
Hitting a drum harder provides more energy, creating a vibration with a larger amplitude. This larger amplitude is perceived by our ears as a louder sound.
Teacher's Tip: More force = More sound.
Exam Tip: Connect "beating hard" to "greater amplitude".

(f) A tuning fork produces sound of single frequency.
Answer: single.
Tuning forks are designed to vibrate at one specific frequency to help musicians tune their instruments. They produce a "pure" tone without extra harmonics.
Teacher's Tip: Tuning forks are like "standard rulers" for sound frequency.
Exam Tip: Use the term "pure tone" if asked to describe a tuning fork's sound.

3. Match the following
Column A                   Column B
(a) Amplitude         (i) frequency
(b) Frequency       (ii) amplitude
(c) Loudness        (iii) maximum displacement on either side
(d) Pitch                (iv) presence of other frequencies
(e) Wave form       (v) 1/time period
Answer:
(a) Amplitude -  (iii) maximum displacement on either side
(b) Frequency - (v) 1/{time period}
(c) Loudness - (ii) amplitude
(d) Pitch - (i) frequency
(e) Wave form - (iv) presence of other frequencies
These pairs link the scientific definitions and physical properties of sound to their corresponding characteristics. Understanding these relationships is fundamental to the study of acoustics.
Teacher's Tip: Remember "L-A-P-F": Loudness-Amplitude, Pitch-Frequency.
Exam Tip: Cross-check each pair before finalizing your answer in a matching question.

4. Select the correct alternative

(a) Sound can not travel in
1. solid
2. liquid
3. gas
4. vacuum
Answer: 4. vacuum.
Sound requires a material medium to propagate because it travels by vibrating molecules. Since a vacuum has no molecules, sound waves have no way to move.
Teacher's Tip: Space is a vacuum, which is why "In space, no one can hear you scream."
Exam Tip: Always choose vacuum as the place where sound cannot travel.

(b) When sound travels in form of a wave
1. the particles of medium move from the source to the listener
2. the particles of medium remains stationary
3. the particles of medium start vibrating up and down
4. the particles of medium transfer energy without leaving their mean positions.
Answer: 4. the particles of medium transfer energy without leaving their mean positions.
The air molecules only move back and forth locally, bumping into their neighbors to pass on the energy. The actual molecule doesn't fly from the speaker to your ear.
Teacher's Tip: Particles "wiggle" but they don't "walk".
Exam Tip: Emphasize that matter is NOT transported by waves, only energy is.

(c) The safe limit of loudness of audible sound is
1. 0 to 80 dB
2. above 80 dB
3. 120 dB
4. above 120 dB
Answer: 1. 0 to 80 dB.
Sounds below 80 { dB} are generally safe for the human ear over long periods. Sounds louder than this can start to cause discomfort or even permanent hearing damage.
Teacher's Tip: 80 { dB} is about as loud as a busy street or a vacuum cleaner.
Exam Tip: 120 { dB} is often called the "threshold of pain".

(d) The unit of loudness is
1. cm
2. second
3. hertz
4. decibel
Answer: 4. decibel.
The decibel is a logarithmic unit used to express the intensity or volume of a sound. Named after Alexander Graham Bell, it is the standard unit for loudness.
Teacher's Tip: Decibel starts with "D" for "Diameter of volume" (a silly way to remember it!).
Exam Tip: The symbol is written as {dB} (small d, capital B).

(e) In a piano, pitch is decreased by
1. using thicker string
2. increasing tension
3. reducing length of string
4. striking it hard
Answer: 1. using thicker string.
Thicker, heavier strings are more massive and vibrate more slowly than thin ones. This slower vibration (lower frequency) creates a deeper, lower-pitched sound.
Teacher's Tip: Thick strings = Slow vibration = Low pitch.
Exam Tip: Strike force (striking hard) only changes loudness, not pitch.

B. Short/Long Answer Questions

Question 1: How does sound travel in air ?
Answer: A periodic disturbance in the medium (Air) is created by the vibration of sound and the particles of the medium vibrate about their mean position and transfer of energy in the form of sound waves takes place, i.e. in the LONGITUDINAL WAVES.
When an object vibrates, it pushes against the air molecules nearby, creating waves of pressure. These waves travel through the air by passing the vibration from one particle to the next until they reach our ears.
Teacher's Tip: Sound moves like a "relay race" of air molecules.
Exam Tip: Mention "mean position" and "longitudinal waves" for a complete answer.

Question 2: What is longitudinal wave ?
Answer: Longitudinal wave: The wave in which the particles of the medium vibrate about their mean positions in the direction of propagation of sound is called longitudinal wave. Such a wave can be produced in solids, liquids as well as gases.
In a longitudinal wave, the vibration of the particles is parallel to the direction the energy is moving. This creates a series of compressions and rarefactions as the wave travels through the material.
Teacher's Tip: Particles move "along" the same line as the wave.
Exam Tip: Emphasize that sound waves in air are always longitudinal.

Question 3: Explain the mechanism of formation of a longitudinal wave when source vibrates in air.
Answer: PROPAGATION OF SOUND IN AIR : When a source of sound vibrates, it creates a periodic disturbance in the medium near it (i.e., the condition of medium changes). The disturbance then travels in the medium in form of waves. This can be understood by the following example.
Example: Take a vertical metal strip with its lower end fixed. Push its upper end to one side and then release it. As it vibrates, i.e. moves alternately to the right and left, sound is produced. Figure shows the steady (or mean position) of the metal strip and normal condition of air layers near the strip.
As the strip moves to the right from a to b in Figure it pushes the particles of air layer in front of it. So the particles of air in this layer come closer to each other i. e., air in that layer gets compressed (or compression is formed at C). The particles of this layer while moving towards right, pushes and compresses the layer next to it, which then compresses the next layer and so on. Thus the disturbance moves forward in form of compression. The particles of the medium do not move with the compression.
As the metal strip starts returning from b to a in Figure after pushing the particles near the strip, the compression C moves forward and the particles of air near the strip return back to their normal positions due to the elasticity of the medium.
When the strip moves to the left from a to c in Figure it pulls the layer of air near it towards left and thus produces a space of very low pressure on its right side. The air layers on the right side of the strip expands in this region thus forming the rarefied layers. ‘This region of low pressure is called a rarefaction R.
By the time the strip returns from c to its mean position a in Figure the rarefaction R moves forward and air layers near the strip return back to their normal position due to the elasticity of the medium.
In this manner, as strip moves to the right and left repeatedly, the compression and rarefaction regions are produced one after the other which carry the disturbance along it with, definite speed depending on the nature of the medium.
One complete to and fro motion of the strip forms one compression and one rarefaction which together constitute one wave. This wave in which the particles of the medium vibrate about their mean positions, in the direction of propagation of sound, is called the longitudinal wave. Thus sound travels in air form of longitudinal waves. Actually the longitudinal waves can be produced in solids, in liquids as well as in gases.
Thus, due to propagation of wave in a medium, the particles of the medium vibrate about their mean positions (without leaving their positions) and they transfer the energy with a constant speed from one place of medium to the other place.
As the source moves back and forth, it alternately squashes and stretches the air around it. This creates a chain reaction of pressure changes that moves through the atmosphere as a sound wave.
Teacher's Tip: Use the metal strip example to show how "pushing" makes a compression and "pulling" makes a rarefaction.
Exam Tip: Use the terms "periodic disturbance" and "elasticity of medium" to describe the process scientifically.

Question 4: Define the following terms : (a) Amplitude (b) Frequency (c) Time period.
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) Frequency (f) or n: Number of oscillations made by a wave in one second is known as its frequency.
(c) Time Period (T): Time taken to complete one vibration is called Time Period, i.e. from A to B.
These three terms are the fundamental measurements used to describe any wave's physical behavior. Amplitude relates to intensity, frequency relates to pitch, and time period is the duration of one cycle.
Teacher's Tip: Amplitude is "how far", Frequency is "how often", and Time Period is "how long".
Exam Tip: Always provide the S.I. units for these definitions (m, {Hz}, s).

Question 5: Obtain relationship between the time period and frequency.
Answer: RELATIONSHIP BETWEEN THE TIME PERIOD (T) and FREQUENCY (f): By definition time period is the time taken to complete 1 vibration. Or In time T, number of vibration= 1.
therefore In 1 second, the frequency or number of vibrations = 1 / T. f = 1 / T Or T = 1 / f
This mathematical relationship shows that frequency is the reciprocal of the time period. If an object vibrates faster (high frequency), the time it takes for one vibration must be shorter (small time period).
Teacher's Tip: Frequency and Time Period are like two sides of the same coin.
Exam Tip: When solving a problem, if frequency is in {Hz} and time is in seconds, you can simply use f = 1/T.

Question 6: Name three characteristics of a musical sound.
Answer: CHARACTERISTICS OF SOUND. Sounds can be distinguished from one another by the following three different characteristics: (i) loudness (ii) pitch or shrillness, and (iii) quality or timbre.
Loudness allows us to hear the sound, pitch gives it a musical note, and quality lets us tell which instrument is playing. Together, these characteristics create the complex sounds we hear in music.
Teacher's Tip: Just remember "LPQ" for Loudness, Pitch, and Quality.
Exam Tip: List all three names clearly to ensure you get full marks for identification.

Question 7: Name the quantity from below which determines the loudness of a sound wave : (a) Wavelength (b) Frequency, and (c) Amplitude.
Answer: Quantity which determines loudness of a sound wave is (c) AMPLITUDE.
Loudness is perceived by how much the air pressure changes as a wave passes by. A wave with a larger amplitude carries more energy and causes our eardrums to move more significantly.
Teacher's Tip: Think of a bigger wave at the beach having more "power".
Exam Tip: If you double the amplitude, the sound doesn't just get twice as loud; it increases by a factor of four!

Question 8: How is loudness related to the amplitude of wave ?
Answer: Greater the amplitude, greater is the loudness.
As the source of vibration moves with more force, it creates waves with higher peaks and deeper troughs. Our brain interprets these higher-energy vibrations as being louder.
Teacher's Tip: Hit a drum harder for a "bigger" wiggle (amplitude) and more noise.
Exam Tip: State that loudness is directly proportional to the square of the amplitude.

Question 9: If the amplitude of a wave is doubled, what will be the effect on its loudness ?
Answer: The loudness of sound is directly proportional to the square of amplitude of wave. Loudness propto (amplitude)². When amplitude is doubled loudness becomes (2)² = 4.
This means that even small changes in amplitude can make a sound seem much louder than before. Doubling the distance a string vibrates will quadruple the intensity of the sound produced.
Teacher's Tip: Squaring a number means multiplying it by itself (2 × 2 = 4).
Exam Tip: Use the mathematical expression L propto A² to justify your answer.

Question 10: How does the wave pattern of a loud note differ from a soft note ? Draw a diagram.
Answer: The amplitude of soft note (faint) has smaller amplitude. The amplitude of loud sound has more amplitude as shown in figure.
A loud note will have much higher peaks and deeper valleys on a graph than a soft note, even if they have the same frequency. The wave looks "stretched" vertically for loud sounds.
Teacher's Tip: Loud waves look "tall", soft waves look "short".
Exam Tip: When drawing, make sure both waves have the same spacing (frequency) but different heights (amplitude).

Question 11: Name the unit in which the loudness of sound is expressed.
Answer: Unit of loudness: decibel ({dB}).
The decibel scale is a logarithmic way of describing how loud sounds are relative to the quietest sound a human can hear. It ranges from 0 { dB} (near silence) up to 140 { dB} (jet engine).
Teacher's Tip: It's named after Alexander Graham Bell, the inventor of the telephone.
Exam Tip: Don't forget the small "d" and big "B" in the symbol {dB}.

Question 12: Why is the loudness of sound heard by a plucked wire increased when mounted on a sound board ?
Answer: A wire mounted on a sound board is plucked, the surface area of vibrating air increases and sends forth greater amount of energy, So the amplitude of vibration is large and louder is the sound.
The thin wire itself doesn't touch much air, so it can't create loud waves. The sound board provides a large flat surface that moves a lot of air at once, boosting the sound's volume.
Teacher's Tip: The sound board "magnifies" the vibration of the wire.
Exam Tip: Mention "increased surface area" as the primary reason for increased loudness.

Question 13: State three factors on which loudness of sound heard by a listener depends.
Answer: THREE FACTOR FOR LOUDNESS OF SOUND : (i) Surface area of sounding body. i.e. is directly proportional to surface area of vibrating body. (ii) On the distance of source of sound, i.e. decreases with distance. (iii) On AMPLITUDE OF WAVE: i.e. increases with amplitude.
Loudness depends on how the sound is made, how far it has to travel, and the size of the object producing it. As sound waves spread out through the air, they lose energy, which is why things get quieter as you walk away.
Teacher's Tip: Think: "Bigger surface, closer distance, bigger wiggle".
Exam Tip: List factors like "Amplitude" and "Distance" as they are the most important.

Question 14: What determines the pitch of a sound ?
Answer: FREQUENCY/i.e. number of vibrations per second determines the pitch. Higher frequency, higher pitch means shrill sound. A low pitch has flat sound.
Pitch is our sensory perception of frequency. High-pitched sounds like whistles vibrate thousands of times per second, while low-pitched bass notes vibrate much more slowly.
Teacher's Tip: Frequency is the "fact", Pitch is what you "hear".
Exam Tip: Remember: High Frequency = High Pitch, Low Frequency = Low Pitch.

Question 15: Name the characteristic of sound related to its frequency.
Answer: Characteristic of sound related to its frequency is PITCH.
Pitch is the property that lets us arrange musical sounds on a scale from high to low. It is entirely dependent on how many cycles the sound wave completes every second.
Teacher's Tip: "P" for Pitch and "F" for Frequency are partners.
Exam Tip: Simply write "PITCH" when asked for the frequency-related characteristic.

Question 16: Name and define the characteristic which enables one to distinguish two sounds of same loudness, but of different frequencies, given by the same instrument.
Answer: Two sounds of same loudness (amplitude) of different frequencies given by same instrument can be distinguished by the characteristics called PITCH.
Pitch allows you to hear the difference between a high string and a low string on a guitar even if you pluck them with the same force. This distinction is based solely on the rate of vibration.
Teacher's Tip: Pitch is what makes a "note" sound high or low.
Exam Tip: Use the word "distinguish" and "frequency" to identify pitch in your answer.

Question 17: Draw a diagram to show the wave pattern of high pitch note and a low pitch note, but of the same loudness.
Answer: [Textbook provides diagram showing Low pitch note with Frequency = 1  s^-1 and High pitch note with Frequency = 2  s^-1.
In these diagrams, the waves will have the same height (amplitude) because the loudness is the same. However, the high-pitch wave will be more "crowded" with more waves packed into the same amount of time.
Teacher's Tip: Same height = same loudness; more wiggles = higher pitch.
Exam Tip: When drawing, keep the peak heights identical but change the horizontal spacing.

Question 18: How is it possible to detect the filling of a bucket under a water tap by hearing the sound standing at a distance ?
Answer: The SOUND BECOMES SHRILLER AND SHRILLER as the water in the bucket rises, the length of air column decreases. So the frequency of the sound produced INCREASES. Thus by hearing the sound from a distance, one can get the idea of water level in the bucket.
As the bucket fills, the empty space for air to vibrate gets smaller and smaller. Short air columns vibrate at higher frequencies, making the pitch of the splashing sound go up.
Teacher's Tip: Small space = High sound. This is like a sliding whistle!
Exam Tip: Mention that "decreasing air column length" leads to "increased frequency".

Question 19: The frequencies of notes given by flute, guitar and trumpet are respectively 400 { Hz}, 200 { Hz} and 500 { Hz}. Which one of these has the highest pitch ?
Answer: The instrument with highest frequency has highest pitch. Hence, trumpet frequency 500 { Hz} has highest pitch.
Since pitch is directly proportional to frequency, the instrument with the highest number of vibrations per second will sound the shrillest. In this group, the trumpet's 500 vibrations per second is the highest value.
Teacher's Tip: Look for the biggest number in {Hz} to find the highest pitch.
Exam Tip: Always state the principle "Higher frequency = Higher pitch" before giving your final answer.

Question 20: Figure shows two jars A and B containing water up to different heights. Which will produce sound of higher pitch when air is blown on them ?
Answer: Jar B which has less air column above water will produce sound of higher pitch. Less air column produces increased frequency.
Blowing air across the jar makes the air inside it vibrate. Jar B has more water and less air, so that small amount of air vibrates much faster than the large amount in Jar A.
Teacher's Tip: Short air column = Shrill sound.
Exam Tip: Compare the "length of air column" in both jars to explain the difference.

Question 21: Two identical guitars are played by two persons to give notes of the same pitch. Will they differ in quantity ? Give reason for your answer.
Answer: No, they have same wave form.
Since the instruments are identical and the notes are the same, the quality of the sound will be the same. Quality only changes if the instrument type or construction is different.
Teacher's Tip: Same instrument + same note = same sound quality.
Exam Tip: Quality (or timbre) depends on the "waveform", so identical sources produce identical waveforms.

Question 22: Two musical notes of the same pitch and same loudness are played on two different instruments. Their wave patterns are as shown in figure. How do they differ in (a) loudness, (b) pitch and (c) quality
Answer: (a) Loudness of each is same as amplitude is same.
(b) Pitch of each is same.
(c) Quality is DIFFERENT as the wave form is different.
The violin and piano produce different-looking waves even when they play the same note at the same volume. These unique shapes represent the "quality" that lets us tell a violin from a piano.
Teacher's Tip: Same height = same volume; same wiggles = same note; different shape = different quality.
Exam Tip: Identify that different "instruments" always lead to different "quality" or "timbre".

Question 23: Which characteristics of sound makes it possible to recognize a person by his voice without seeing him ?
Answer: Characteristic is TIMBRE or QUALITY makes it possible to recognise a person by his voice without seeing him.
Every human vocal cord has a unique structure and way of vibrating, which creates a specific wave shape. This "voiceprint" is why you can recognize your friends or family just by hearing them speak.
Teacher's Tip: Timbre is like a "sound signature".
Exam Tip: Use both terms "Timbre" and "Quality" to show you know they mean the same thing.

Question 24: State the factors that determine (a) the pitch of a note. (b) the loudness of the sound heard. (c) the quality of the note.
Answer: (a) Frequency (b) Amplitude (c) Waveform.
Each major characteristic of sound is tied to a specific physical property of the wave. Pitch is the "speed", Loudness is the "size", and Quality is the "shape".
Teacher's Tip: Just link: Pitch-Freq, Loudness-Amp, Quality-Shape.
Exam Tip: Memorize this set of three for quick and easy marks in short tests.

Question 25: Name the characteristic of the sound affected due to a change in its (a) amplitude (b) wave form (c) frequency.
Answer: (a) Loudness (b) Quality (c) Pitch.
This question asks the same relationship as the previous one but in reverse. It emphasizes that if you change one physical aspect of a wave, a specific sound quality will change as well.
Teacher's Tip: Amplitude = Loudness, Waveform = Quality, Frequency = Pitch.
Exam Tip: Make sure you don't swap them; Frequency always goes with Pitch.

Question 26: Figure shows four waves A, B, C, and D. Name the wave which shows (a)a note from a musical instrument, (b)a soft note (c) a shrill note.
Answer: (a) a note from a musical sound is shown by (D) (b) a soft note is shown by (A) (c) a shrill note is shown by (C).
Wave D has a complex musical shape, while Wave A has the smallest vertical height, making it the softest. Wave C has the most vibrations per second, giving it the highest frequency and therefore the highest pitch or shrillness.
Teacher's Tip: Look for "height" for loudness and "number of wiggles" for pitch.
Exam Tip: When labeling multiple waves, identify the most obvious one (like softest or shrillest) first.

Question 27: How is the pitch of sound in a guitar changed if (a) thin wire is used, (b) wire under less tension is used ?
Answer: (a) Thin wire used: Higher pitch is obtained.
(b) Wire under less tension is used: a low pitch is obtained.
A thin wire is lighter and can vibrate much faster than a thick one, creating a higher frequency. Conversely, a loose (low tension) wire vibrates slowly, resulting in a deeper, lower frequency sound.
Teacher's Tip: Thin = Fast = High Pitch; Loose = Slow = Low Pitch.
Exam Tip: Remember: Pitch increases with tension and decreases with wire thickness.

C. Numerical

Question 1: Two waves of the same pitch have amplitudes in the ratio 1:3 What will be the ratio of their (i) loudness, (ii) pitch ?
Answer: Loudness propto A² (amplitude)².
(i) therefore Ratio of loudness L₁/L₂ = (a₁)² / (a₂)² = (1)² / (3)²= 1/9 = 1:9.
(ii) Two waves of same pitch
therefore Pitch of first wave / Pitch of second wave = 1/1 = 1:1.
Since loudness is based on the square of the amplitude, a triple increase in amplitude leads to a nine-fold increase in loudness. Because they have the same pitch, their frequency ratio remains equal at 1:1.
Teacher's Tip: If amplitude is 3, loudness is 3 × 3 = 9.
Exam Tip: Always show the ratio as "number:number" in your final answer.

Question 2: Two waves have frequencies 256 { Hz} and 512 { Hz}, but same amplitude. Compare their (i) loudness, and (ii) pitch;
Answer: (i) LOUDNESS therefore Amplitude of two waves is same
therefore their loudness is same i.e. in 1:1. (ii) PITCH: Ratio of frequencies 256 { Hz} : 512 { Hz}
therefore Ratio of their pitch = 1:2.
Even though one wave is vibrating twice as fast as the other, their physical "size" (amplitude) is the same, so they sound equally loud. The pitch ratio matches the frequency ratio exactly because pitch is directly proportional to frequency.
Teacher's Tip: Same amplitude = Same volume.
Exam Tip: 256 is exactly half of 512, which makes the ratio 1:2.