Maharashtra Board Class 8 Science Chapter 15 Sound Solutions

Std 8 Science Chapter 15 Sound Question Answer Maharashtra Board

Class 8 Science Chapter 15 Sound Question Answer Maharashtra Board

1. Fill In The Blank With Appropriate Word:

Question a. The region in a sound wave, with higher pressure and density is called ........ and that with low pressure and density is called .........
Answer: The region in a sound wave, with higher pressure and density is called compression and that with low pressure and density is called rarefaction.
In simple words: In a sound wave, areas where particles are crowded together, increasing pressure and density, are compressions. Areas where particles are spread out, reducing pressure and density, are rarefactions.

🎯 Exam Tip: Understanding the terms 'compression' and 'rarefaction' is fundamental to describing sound wave propagation and is often tested in definitions or fill-in-the-blank questions.

 

Question b. Medium is ................ for generation of sound.
Answer: Medium is needed (or necessary) for generation of sound.
In simple words: Sound requires a medium like air, water, or solids to travel because it propagates through the vibration of particles.

🎯 Exam Tip: This question tests a core concept of sound - its dependence on a medium. Be prepared to explain why sound cannot travel in a vacuum.

 

Question c. The total number of compressions and rarefactions produced per second in a sound wave is 1000. The frequency of the sound wave is ................
Answer: The total number of compressions and rarefactions produced per second in a sound wave is 1000. The frequency of the sound wave is 500 Hz. [Note: Total number: 100. .. 500 compressions and 500 rarefactions are produced per second.]
In simple words: A single cycle of a sound wave consists of one compression and one rarefaction. If 1000 total compressions and rarefactions are produced per second, it means 500 complete cycles, making the frequency 500 Hz.

🎯 Exam Tip: Remember that one full oscillation (or cycle) of a sound wave includes one compression and one rarefaction. Frequency is the number of *cycles* per second.

 

Question d. Different sound notes have different ................
Answer: Different sound notes have different frequencies.
In simple words: The pitch or characteristic 'note' of a sound is determined by its frequency; higher frequency means a higher pitch.

🎯 Exam Tip: Connect the term 'sound note' with 'frequency' and 'pitch' as they are directly related. This is a basic but important concept.

 

Question e. In a loudspeaker, ................ energy is converted into ................ energy.
Answer: In a loudspeaker, mechanical energy is converted into sound energy.
In simple words: Loudspeakers convert the electrical signals into mechanical vibrations of a cone, which then produce sound waves.

🎯 Exam Tip: Energy transformations are key in physics. For a loudspeaker, the primary conversion is from electrical to mechanical (vibration) and then to sound energy.

2. Give Scientific Reasons:

 

Question a. It is essential to change the tension in the vocal cords, as we produce different sound notes from our larynx.
Answer: Different sound notes correspond to different frequencies. The frequency of sound depends on the tension in the vocal cords. Hence, it is essential to change the tension in the vocal cords, as we produce different sound notes our larynx.
In simple words: Our vocal cords vibrate to produce sound. By adjusting the tension in these cords, we change their vibration frequency, which in turn creates different pitches or sound notes.

🎯 Exam Tip: When explaining 'scientific reasons,' clearly state the principle (frequency dependence on tension) and then link it to the observed phenomenon (different sound notes).

 

Question b. Astronauts on the moon cannot hear each other directly.
Answer: Two astronauts on the moon talking to each other directly, will be unable to listen to each other, even if they are very close to each other. The moon does not have atmosphere. Since there is no medium which is necessary for generation and propagation of sound, between the astronauts, direct sound propagation between them is not possible. Therefore, the astronauts use some technology like the one used in our cell-phones to communicate with each other. The waves used in cell-phone do not need any medium for propagation.
In simple words: Sound needs a medium (like air) to travel because it's a mechanical wave. The Moon lacks an atmosphere, creating a vacuum, so sound cannot propagate directly between astronauts.

🎯 Exam Tip: This question highlights the crucial role of a medium for sound propagation. Emphasize the absence of an atmosphere on the Moon as the primary reason.

 

Question c. As the sound wave propagates from one place to the other in air, the air itself is not required to move from one place to the other.
Answer: Propagation of sound through air occurs due to energy transfer by vibrating air molecules in one region to those in the adjacent region away from the source. The molecules simply vibrate about their mean positions. Hence, the air itself is not required to move from one place to the other.
In simple words: Sound waves are longitudinal waves where air molecules vibrate back and forth in place, transferring energy to neighboring molecules without themselves moving long distances.

🎯 Exam Tip: Distinguish between the movement of energy (sound wave propagation) and the movement of the medium's particles (vibration about mean position). This is a common misconception.

 

Question 3. How are different sound notes generated in musical instruments like guitar, which uses strings for sound generation, and flute, which uses blown air for sound generation?
Answer:1. Guitar: It is a string based, flat-backed instrument. It has usually six strings, the strings are plucked or strummed with the fingers or a small piece of plastic, wood, etc. It has a flat sounding board with a circular sound hole in the centre. Also it has a fretted fingerboard. Its frequency range is more than three octaves. The frequency of vibration of the string is changed by changing the tension in the string or changing the vibrating length of the string. As the tension is increased, the frequency increases. As the vibrating length is increased, the frequency decreases. This is how different sound notes are generated. [Note: The acoustic guitar has hollow body and six or twelve strings while the electric guitar usually has solid body and six strings. The electric bass guitar has four strings.]
2. Flute: It is a wind instrument where air is blown against the edge or rim of the blowing hole. The frequency of the sound produced depends upon the length of the vibrating air column in the tube. The greater the length of the vibrating air column, the less is the frequency of the sound produced. This is how different sound notes are produced. The flute has six or seven or eight holes to generate sounds of different frequencies. Different notes can be generated also by changing the way of air-blowing.
In simple words: Guitars produce different notes by changing string tension and vibrating length, which alters frequency. Flutes produce notes by changing the length of the vibrating air column inside the instrument, also altering frequency.

🎯 Exam Tip: When describing musical instruments, focus on the vibrating part (strings, air column) and the physical factors that change its frequency, such as tension or length.

 

Question 4. How is sound produced in the human larynx and a loudspeaker?
Answer:1. Sound production in the human larynx: In the humans, sound is produced in the voice box called the larynx. It is located at the upper end of the windpipe. Two vocal cords (chords) are stretched across it with a narrow slit between them for the passage of air (Fig. 15.5). When the lungs force air through the slit, the cords start vibrating. The frequency of the sound produced depends upon the length and thickness of the cords, and the tension in the chords. The frequency increases with the increase in tension and the more the length or the thickness of the cord, the less is the frequency. Muscles attached to the cords can make the cords tight (more tension) or loose (less tension).
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र मानव स्वरयंत्र (larynx) की आंतरिक संरचना को दर्शाता है, जिसे आवाज का बक्सा भी कहते हैं। इसमें एपिग्लॉटिस (एक फ्लैप जो निगलने के दौरान वायु-मार्ग को बंद करता है), स्वर रज्जु (vocal chords) जो हवा के गुजरने पर कंपन करते हैं, श्वासनली (trachea) और अन्नप्रणाली (esophagus) जैसी महत्वपूर्ण संरचनाएँ शामिल हैं, जो ध्वनि उत्पादन में सहायक होती हैं। 2. Sound produced in a loudspeaker: Figure shows the internal construction of a loudspeaker. Here, a coil is wound around a permanent magnet. The conical screen of the loudspeaker is attached to the coil.
In simple words: In the human larynx, vocal cords vibrate as air passes through them, with muscle adjustments changing their tension and length to produce different sounds. In a loudspeaker, an electrical current creates a magnetic field that moves a coil and attached conical screen, causing vibrations that generate sound.

🎯 Exam Tip: For both the larynx and loudspeaker, emphasize the core mechanism of vibration (vocal cords, conical screen) and how changes in physical properties (tension, electrical current variation) lead to different sound characteristics.

 

ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक लाउडस्पीकर की आंतरिक संरचना को दर्शाता है। इसमें एक स्थायी चुंबक (permanent magnet) के चारों ओर एक कॉइल (coil) लिपटी हुई है, जिसके साथ एक शंकु के आकार की स्क्रीन (screen) जुड़ी हुई है। जब माइक्रोफ़ोन से विद्युत धारा कॉइल में प्रवाहित होती है, तो चुंबक के साथ उसकी क्रिया के कारण कॉइल आगे-पीछे गति करती है, जिससे स्क्रीन कंपन करती है और ध्वनि उत्पन्न होती है। When a current is passed through the coil, a magnetic field is produced. Its interaction with the permanent magnet results in the back and forth motion of the coil. The frequency and the amplitude of the motion of the coil depends on the variation in the current through the coil. As the coil moves, the conical screen also moves back and forth. The vibrations of the screen produce sound waves in air. Very loud sound can be produced by changing the current. [Note: If you gently touch the vibrating screen, you can feel the vibrations.]
In simple words: (Already covered above as part of the main Q4 answer)

🎯 Exam Tip: (Already covered above as part of the main Q4 answer)

 

Question 5. Explain the experiment, with0 a neat diagram, to prove the following: 'Sound needs a material medium for propagation.'
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक बेल जार प्रयोग को दर्शाता है जिसका उपयोग यह सिद्ध करने के लिए किया जाता है कि ध्वनि को संचरण के लिए एक माध्यम की आवश्यकता होती है। एक विद्युत घंटी को वैक्यूम-टाइट बेल जार के अंदर रखा गया है, जो एक वैक्यूम पंप से जुड़ा है। जब हवा निकाली जाती है, तो घंटी की आवाज धीमी होती जाती है, जिससे पता चलता है कि ध्वनि को यात्रा करने के लिए हवा जैसे माध्यम की आवश्यकता होती है। Figure shows part of the set up used in this experiment. What is not shown is the electric circuit in which the electric bell is connected. A vacuum tight bell jar contains an electric bell connected to a power supply through the lid of the jar. The jar is placed on a smooth horizontal surface such as that of glass. Initially the vacuum pump is off and the jar contains air. The circuit containing the bell is completed using the key or the switch so that the bell starts ringing. This can be heard outside the jar. Then the vacuum pump is switched on so that it starts removing the air from the jar. We find that the level of ringing sound heard goes on decreasing as the quantity of air in the jar becomes less and less. When the pump is operated for a sufficiently long time interval, the quantity of air in the jar becomes so less that the level of ringing sound becomes very low; sound is hardly audible. But we can see the striker in the bell hitting the gong. By extrapolation, we conclude that sound generation and propagation needs a medium.
In simple words: The bell jar experiment shows that as air is removed from a jar containing a ringing bell, the sound becomes fainter. This proves that sound requires a material medium (like air) to travel, as it cannot propagate in a vacuum.

🎯 Exam Tip: Clearly describe the setup, initial observation (sound audible), the action taken (removing air), and the final observation (sound fades). Conclude by linking the observations to the need for a medium.

6. Match The Following:

 

Question a.

Human larynx	Vibrations of metal arms
Loudspeaker	Vibrations in air column
Jal-tarang	Vibrations in vocal cords
Tuning fork	Vibrations in strings
Sitar	Vibrations of screen

Answer:

Column I	Column II
Human larynx	Vibrations in vocal cords
Loudspeaker	Vibrations of screen
Jaltarang	Vibrations in air column
Tuning fork	Vibrations of metal arms
Sitar	Vibrations in strings

In simple words: This matching exercise connects various sound-producing instruments or organs with the specific parts that vibrate to create sound.

🎯 Exam Tip: For matching questions, it's helpful to identify the vibrating component for each item. Understanding how each instrument produces sound will ensure accuracy.

Can You Recall?

 

Question a. How is sound produced?
Answer: Sound is produced by the vibrations of an object such as a stretched membrane or a string under tension.
In simple words: Sound is generated when an object vibrates, causing disturbances that travel through a medium.

🎯 Exam Tip: The core idea of sound production is always vibration. Ensure you can provide examples of vibrating objects.

Class 8 Science Chapter 15 Sound Important Questions And Answers

Select The Correct Option And Write The Completed Statements:

 

Question 1. Sound waves cannot travel through a ................ (a) solid (b) liquid (c) gas (d) vacuum
Answer: (d) vacuum
In simple words: Sound waves need particles to vibrate and transmit energy, so they cannot travel through a vacuum where there are no particles.

🎯 Exam Tip: This is a fundamental concept. Remember that sound is a mechanical wave, requiring a medium for propagation, unlike electromagnetic waves.

 

Question 2. If the frequency of a sound wave is 512 Hz, the number of rarefactions produced per second is ................ (a) 256 (b) 512 (c) 1024 (d) 128
Answer: (b) 512
In simple words: Since each complete sound wave cycle (frequency) includes one rarefaction, a frequency of 512 Hz means 512 rarefactions are produced per second.

🎯 Exam Tip: Understand that frequency directly corresponds to the number of compressions *or* rarefactions per second, as each cycle has one of each.

 

Question 3. In the experiment to show that a medium is necessary for propagation of sound, as the quantity of air inside the bell jar decreases, the level of ringing sound heard outside ................ (a) increases (b) decreases (c) fluctuates randomly (d) changes at regular intervals
Answer: (b) decreases
In simple words: As air is removed from the bell jar, the medium for sound transmission diminishes, causing the sound intensity to decrease.

🎯 Exam Tip: This question tests your understanding of the bell jar experiment. The key takeaway is the inverse relationship between the quantity of air (medium) and the audibility of sound.

Find The Odd One Out And Give The Reason:

 

Question 1. Sound waves, Waves on the surface of water, Waves on a stretched string, Light waves
Answer: Light waves. Light waves can travel in a vacuum. Other waves need a medium for propagation.
In simple words: Light waves are electromagnetic waves that don't need a medium to travel, unlike sound waves and water/string waves which are mechanical and require a medium.

🎯 Exam Tip: Differentiate between mechanical waves (requiring a medium) and electromagnetic waves (not requiring a medium) to correctly identify the odd one out.

 

Question 2. Sitar, Violin, Guitar, Flute.
Answer: Flute. It is not a string based instrument. Others are string based instruments.
In simple words: The flute is a wind instrument where sound is made by vibrating an air column, whereas sitar, violin, and guitar are string instruments that produce sound from vibrating strings.

🎯 Exam Tip: Classify instruments based on how they produce sound (e.g., string, wind, percussion) to correctly identify the one that doesn't fit the group.

State Whether The Following Statements Are True Or False. (If A Statement Is False, Correct It And Rewrite It.)

 

Question 1. If the vibrating length of a stretched string is kept constant and the tension in the string is increased, the frequency of the sound produced increases.
Answer: True.
In simple words: For a string, increasing its tension makes it vibrate faster, thus increasing the frequency and pitch of the sound it produces.

🎯 Exam Tip: Remember the relationship between tension and frequency for vibrating strings: higher tension leads to higher frequency.

 

Question 2. In Jaltarang, the frequency of sound produced is independent of the height of the air column inside the glass cup.
Answer: False. (In Jaltarang, the frequency of sound produced depends on the height of the air column inside the glass cup; the more the height of the air column, the less is the frequency of the sound produced.)
In simple words: In a Jaltarang, the height of the air column in the cups directly affects the frequency of the sound produced; a taller column vibrates slower, producing a lower frequency.

🎯 Exam Tip: For wind instruments or air columns, the length of the vibrating air column is inversely proportional to the frequency. A longer column gives a lower pitch.

Answer The Following Questions In One Sentence Each:

 

Question 1. What is sound?
Answer: Sound is a form of energy which produces sensation of hearing in our ears.
In simple words: Sound is a type of energy that travels as vibrations through a medium, allowing us to hear.

🎯 Exam Tip: A concise definition of sound should mention it as a form of energy and its role in hearing.

 

Question 2. What is vibration?
Answer: Vibration is a rapid, to and from periodic motion of an object about its mean position.
In simple words: Vibration is the quick back-and-forth movement of an object around its central resting point.

🎯 Exam Tip: Key terms for vibration are "rapid," "to and from," and "mean position."

 

Question 3. What is compression?
Answer: As a sound wave travels through air, the region in which air is at high pressure and high density is called compression.
In simple words: Compression is a region in a sound wave where air particles are squeezed together, resulting in higher pressure and density.

🎯 Exam Tip: Define compression by linking it to high pressure and high density in the medium.

 

Question 4. What is rarefaction?
Answer: As a sound wave travels through air, the region in which air is at low pressure and low density is called rarefaction.
In simple words: Rarefaction is a region in a sound wave where air particles are spread apart, leading to lower pressure and density.

🎯 Exam Tip: Define rarefaction by linking it to low pressure and low density in the medium, contrasting it with compression.

 

Question 5. What is the relation between the frequency of vibration and the vibrating length (or height) of the air column?
Answer: The greater the vibrating length (or height) of the air column, the less is the frequency of vibration of the column.
In simple words: For an air column, a longer vibrating length results in a lower frequency of vibration, thus producing a lower-pitched sound.

🎯 Exam Tip: This is an inverse relationship. Understand that longer lengths (of strings or air columns) generally produce lower frequencies (lower pitch).

 

Question 6. For a particular string (given material and radius), what are the factors on which its frequency of vibration depends?
Answer: For a particular string (given material and radius), the frequency of vibration of the string depends upon its vibrating length and the tension in the string. [Note: in the above case, frequency is proportional to the square root of the tension and inversely proportional to the length.]
In simple words: The frequency of a string's vibration depends on how long it is (vibrating length) and how tightly it's stretched (tension).

🎯 Exam Tip: List the two primary factors: vibrating length and tension. It's good to also know the inverse relationship with length and direct relationship with tension (square root).

 

Question 7. Name an instrument in which a stretched skin is used to produce musical sound.
Answer: Tabla.
In simple words: The Tabla is a percussion instrument that uses stretched animal skin (membrane) to produce sound when struck.

🎯 Exam Tip: Identify instruments based on their primary sound-producing component. For stretched skin, percussion instruments are the key category.

Answer The Following Questions:

 

Question 1. Explain the production and propagation of sound in air.
Answer: Let us consider production of sound by using a tuning fork. A tuning fork is made of a metal. It has a stem and two prongs (Fig). Figure (a) shows the state of air in the absence of propagation sound through it. Equally spaced vertical lines show that the average distance between the air molecules is the same everywhere. It means average pressure and average density in the regions A, B and C are the same.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक ट्यूनिंग फोर्क और वायु में ध्वनि के उत्पादन तथा संचरण की प्रक्रिया को दर्शाता है। शीर्ष चित्र एक ट्यूनिंग फोर्क (prong और stem के साथ) दिखाता है। नीचे के चित्र वायु अणुओं की स्थिति को दर्शाते हैं: (a) सामान्य वायु दाब; (b) जब शूल बाहर की ओर बढ़ते हैं तो संपीड़न (compression) होता है; (c) जब शूल अंदर की ओर बढ़ते हैं तो विरलन (rarefaction) होता है, जिससे ध्वनि तरंगें उत्पन्न होती हैं। Suppose the tuning fork is held vertical, its stem is fixed in a stand (not shown in the figure) and its prongs are struck lightly using a light hammer with a piece of rubber at the top. The prongs then start vibrating. Figure (b) shows what happens when the prongs move away from each other. The air outside the prongs is compressed (layers in the region A). The pressure and density increase in this region. Figure (c) shows what happens when the prongs move close to each other. The air molecules near the prongs move away from each other resulting in lower pressure and lower density. This region is called rarefaction. Meanwhile, the compression produced earlier moves forward, i.e., away from the prongs because the air molecules in this region transfer their energy to the air molecules in the region B producing a compression there. The periodic formation of compression and rarefaction results in propagation of sound waves away from the prongs. Eventually, these sound waves reach our ears, the ear-drum vibrates, and we get a sense of hearing a sound as the specific signals reach the brain.
In simple words: Sound is produced by vibrations, like a tuning fork. As the fork vibrates, it creates alternating regions of high pressure (compressions) and low pressure (rarefactions) in the air. These compressions and rarefactions travel through the air, transferring energy from one molecule to the next, thus propagating the sound.

🎯 Exam Tip: When explaining sound production and propagation, clearly describe how vibrations create compressions and rarefactions, and how these regions transmit energy without the medium's particles moving long distances.

Use Your Brain Power:

 

Question a. If sound waves are generated in air, what moves away from the source? Is it the air itself or the state of compression and rarefaction created in the air?
Answer: If sound waves are generated in air, the state of compression and rarefaction created in the air moves away from the source. [Note: The air does not move away from the source because air is elastic and the force due to the tuning fork varies periodically with time.]
In simple words: When a sound wave travels through air, it is the energy, represented by the patterns of compression and rarefaction, that moves away from the source, not the air molecules themselves. The molecules just vibrate in place.

🎯 Exam Tip: Emphasize that in wave propagation, energy is transferred, not matter. For sound, the disturbance (compressions/rarefactions) moves, while air particles oscillate locally.

 

Question 2. Explain the term frequency of a sound wave.
Answer: Sound travels in the form of waves. A wave consists of cycles of alternate compression and rarefaction. The number of cycles produced in the air (or any other medium) per second is called the frequency of the sound wave. It is expressed in hertz (Hz). 1 hertz = 1 cycle per second or 1 vibration per second or 1 oscillation per second.
In simple words: Frequency of a sound wave is how many complete cycles (one compression and one rarefaction) pass a point each second, measured in Hertz (Hz). It determines the pitch of the sound.

🎯 Exam Tip: Provide a clear definition of frequency, including its unit (Hertz) and its relation to cycles/vibrations per second. Mentioning its effect on pitch adds value.

Sound travels in the form of waves. A wave consists of cycles of alternate compression and rarefaction. The number of cycles produced in the air (or any other medium) per second is called the frequency of the sound wave. It is expressed in hertz (Hz). 1 hertz = 1 cycle per second or 1 vibration per second or 1 oscillation per second.

ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक ध्वनि तरंग में संपीड़न (उच्च दबाव) और विरलन (कम दबाव) के चक्रों को दर्शाता है। यह दर्शाता है कि कैसे ध्वनि तरंग के माध्यम से हवा के कण आगे-पीछे कंपन करते हैं, जिससे दबाव और घनत्व में परिवर्तन होता है, जो ध्वनि के प्रसार को जन्म देता है।

[Note: The Sl unit of frequency, the hertz, is named in honour of Heinrich Hertz (1857-94), German physicist.]

 

Question 3. State the factors on which the frequency of a tuning fork depends.
Answer: The frequency of a tuning fork depends on the dimensions of the prongs, i.e., length and thickness, and the material used for making the fork.
In simple words: The size, shape, and material of a tuning fork determine how fast it vibrates, which in turn sets its unique sound frequency.

🎯 Exam Tip: Remember the three main factors-dimensions (length, thickness) and material-as these are key to understanding a tuning fork's frequency.

Try This:

 

Question a. Take 6 - 7 glass cups. Arrange them in a line and fill them with water with gradually increasing water level from one end to other. Take a pencil and strike the cups sequentially. The sound generated by each cup will be different. Why is it so?
Answer: When a cup is struck, waves are set up in the air column above the water level in the cup. The frequency of the generated wave depends on the height of the air column inside the glass cup. Since the water level in each glass is different, the height of the air column in each glass is also different. Therefore, the frequency of sound generated by each glass cup will also be different. So, the sound generated is different.
In simple words: In a Jaltarang, different water levels create air columns of varying lengths in the cups. These different lengths cause the air to vibrate at different frequencies when struck, producing distinct musical notes.

🎯 Exam Tip: Focus on the relationship between the length of the air column and the frequency of sound produced; this is a fundamental concept in wave physics.

Use Of ICT:

Download videos of Jaltarang from youtube and send them to your friends by email.

 

Question b. An 'app' for measurement of sound frequency may be available on cellphones. With the help of your teacher, use the app to measure the frequency of the sound generated from each glass cup. Do you observe any relation between the frequency of generated sound and the height of the air column in the glass cup?
Answer: Yes. The greater the height of the air column in the glass cup, the less is the frequency of the sound produced.
In simple words: Yes, there's a clear relationship: taller air columns produce lower frequency sounds, and shorter ones produce higher frequency sounds.

🎯 Exam Tip: This experiment demonstrates the inverse relationship between the length of a vibrating air column and the frequency of the sound it produces, a key principle in understanding wind instruments.

 

Question c. This is your simple 'Jaltarang'! Can this experiment be performed with stainless steel pots of different size?
Answer: Do it. See what happens as the stainless steel pot itself starts vibrating producing a sound. Also see how this sound is damped by water.
In simple words: Yes, the Jaltarang concept can be adapted using different sized stainless steel pots. The sound would depend on the pot's vibration and how water affects it.

🎯 Exam Tip: Consider how the material and size of the vibrating object, along with the damping effect of water, influence the sound's pitch and resonance in similar experiments.

Do You Know?

 

Question a. What are the frequencies of musical notes 'sa', 're', 'ga', 'ma', 'pa', 'dha', 'ni' in the 'madhya saptak'?
Answer:

Note	Frequency (Hz)
sa	256
re	280
ga	312
ma	346
pa	384
dha	426
ni	480

[Note: The diatonic scale:
do: 256 Hz, re: 288 Hz, mi: 320 Hz, fa: \(341 \frac{1}{3}\) Hz, sol: 384 Hz, la: \(426 \frac{2}{3}\) Hz, ti: 480 Hz
\( \frac{288}{256} = \frac{9}{8} \), \( \frac{320}{256} = \frac{5}{4} \), \( \frac{341}{256} = \frac{4}{3} \)
\( \frac{384}{256} = \frac{3}{2} \), \( \frac{426}{256} = \frac{5}{3} \), \( \frac{480}{256} = \frac{15}{8} \)
(ratio of integers). These ratios are called intervals.
Also, \( \frac{320}{288} = \frac{10}{9} \), \( \frac{341 \frac{1}{3}}{320} = \frac{16}{15} \), \( \frac{384}{341 \frac{1}{3}} = \frac{9}{8} \)
\( \frac{426 \frac{2}{3}}{384} = \frac{10}{9} \), \( \frac{480}{426 \frac{2}{3}} = \frac{9}{8} \)
(ratio of integers).
\( \frac{384 \text{ Hz}}{256 \text{ Hz}} = \frac{3}{2} \)
Such a simple fraction is pleasing to the ear and is called a musical interval. Frequency ratios 2: 1, 3: 2, 4 : 3 are examples of musical interval.
In simple words: Musical notes have specific frequencies; for example, 'sa' is 256 Hz. The relationships between these frequencies, expressed as simple ratios, are known as musical intervals.

🎯 Exam Tip: Understanding the fixed frequencies of musical notes and their integer ratios helps in grasping the mathematical basis of music and harmony.

Observe And Discuss:

 

Question 1. Identify the instruments and discuss sound production in them. [Students should see these musical instruments.]
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र विभिन्न भारतीय और पश्चिमी वाद्य यंत्रों को दर्शाता है, जिनमें सितार, शहनाई, बांसुरी, तबला, संतूर, बिगुल, मैंडोलिन और सैक्सोफोन शामिल हैं। छात्र इन चित्रों का उपयोग विभिन्न प्रकार के संगीत उपकरणों को पहचानने के लिए कर सकते हैं।
Answer:
1. Sitar
2. Shehnai
3. Flute
4. Tabla
5. Santoor
6. Bugle
7. Mandoline or Mandolin
8. Saxophone.

(i) Sitar, Santoor and Mandoline or mandoline (String instruments):
Vibrating strings produce the sound. The greater the tension in the string, the higher is the frequency of the sound. The greater the vibrating length and the diameter of the string, the lower is the frequency of the sound.

(ii) Flute, Saxophone and Shehnai (Wind instruments):
Vibrating air column produces the sound. The greater the length of the column, the lower is the frequency of the sound.

(iii) Tabla (Percussion instrument):
Vibrating skin produces the sound. The greater the tension in the skin, the higher is the frequency of the sound. The greater the thickness of the skin, the lower is the frequency of the sound.
In simple words: Different instruments produce sound through vibrations: strings (like Sitar) vibrate based on tension and length, air columns (like Flute) vibrate based on their length, and stretched membranes (like Tabla) vibrate based on their tension and thickness.

🎯 Exam Tip: Classify instruments by their sound-producing mechanism (strings, air columns, membranes) and understand how factors like tension, length, and thickness influence the frequency and pitch of the sound.

Try This:

'Apps' for generation of different sound notes (sound note generator app) may be available on cellphones. With the help of your teacher, using such an app, generate sound notes listed in the table.

Note	Frequency (Hz)
sa	256
re	280
ga	312
ma	346
pa	384
dha	426
ni	480

Vocal cords are 20 cm in length in male, about 15 cm in female and even smaller in children. Therefore the voice of male, female and children are different. [Note: Vocal cords are 20 mm in length in male, about 15 mm in female and even smaller in children. Reference NCERT textbook Std. VIII, published in 2017.]

 

Question 1. Produce a sound 'bho ... bho ...' just like a dog-barking and 'meow ... meow...' just like a mewing cat. Carefully notice the tension on the vocal cords, when you produce these sounds. Do you feel that the tension on the vocal cords changes when you produce these two different sounds?
Answer: Yes.
In simple words: Yes, the tension in your vocal cords changes when you produce different sounds like a dog's bark versus a cat's meow, allowing for variation in pitch and tone.

🎯 Exam Tip: This highlights how vocal cord tension directly impacts the frequency and characteristic sound produced, a fundamental aspect of voice modulation.

 

Question 2. An 'app' may be available on cell-phones to measure the loudness of sound in decibel. With the help of your teacher, use the app to measure the sound level of a sound from a loudspeaker at some public place. Measure the sound level at different distances from the loudspeaker. Do you observe some relation between the distance from the loudspeaker and the sound level?
Answer: Yes. As the distance increases, the sound level decreases.
In simple words: Yes, sound level decreases as you move further away from the source because sound energy spreads out over a larger area, making it less intense at greater distances.

🎯 Exam Tip: Understand that sound intensity follows an inverse square law, meaning sound level significantly drops as the distance from the source increases, a key concept for sound propagation.

Always Remember:

We should take care that others are not disturbed when we study sound and its production. Sound pollution is a major cause affecting the environment and social health. Hence we should find ways to avoid sound pollution.

8th Std Science Questions And Answers:

• Human Body and Organ System Class 8 Questions And Answers
• Introduction to Acid and Base Class 8 Questions And Answers
• Chemical Change and Chemical Bond Class 8 Questions And Answers
• Measurement and Effects of Heat Class 8 Questions And Answers
• Sound Class 8 Questions And Answers
• Reflection of Light Class 8 Questions And Answers
• Man-made Materials Class 8 Questions And Answers
• Ecosystems Class 8 Questions And Answers
• Life Cycle of Stars Class 8 Questions And Answers

Class 8