Sound | Class 8 | Science | Chapter 15 | Maharashtra State Board
Explore the fascinating world of sound. Learn how sound is produced, its propagation, the role of mediums, frequency, and the science behind musical instruments. Dive into engaging experiments, diagrams, and applications for better understanding.
Questions & Answers
1. Fill in the blank with appropriate word:
a. The region in a sound wave, with higher pressure and density is called ………….. and that with low pressure and density is called ……….. .
Ans.
The region in a sound wave, with higher pressure and density is called compression and that with low pressure and density is called rarefaction.
b. Medium is …………… for generation of sound.
Ans.
Medium is needed (or necessary) for generation of sound.
c. The total number of compressions and rarefactions produced per second in a sound wave is 1000. The frequency of the sound wave is ………… .
Ans.
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.]
d. Different sound notes have different ………………. .
Ans.
Different sound notes have different frequencies.
e. In a loudspeaker, …………… energy is converted into ………….. energy.
Ans.
In a loudspeaker, mechanical energy is converted into sound energy.
2. Give scientific reasons:
a. It is essential to change the tension in the vocal cords, as we produce different sound notes from our larynx.
Ans.
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.
b. Astronauts on the moon cannot hear each other directly.
Ans.
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.
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.
Ans.
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.
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?
Ans.
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.
4. How is sound produced in the human larynx and a loudspeaker?
Ans.
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).
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.
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.]
5. Explain the experiment, with neat diagram, to prove the following:
‘Sound needs a material medium for propagation.’
Ans.
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.
6. Match the following:
Column 'A' | Column 'B' | |
1. | Human larynx | Vibrations of metal arms |
2. | Loudspeaker | Vibrations in air column |
3. | Jal-tarang | Vibration in vocal cords |
4. | Tuning fork | Vibrations in strings |
5. | Sitar | Vibrations of screen |
Ans.
Column 'A' | Column 'B' | |
1. | Human larynx | Vibration in vocal cords |
2. | Loudspeaker | Vibrations of screen |
3. | Jal-tarang | Vibrations in air column |
4. | Tuning fork | Vibrations of metal arms |
5. | Sitar | Vibrations in strings |
E X T R A
1. Select the correct option and write the completed statements:
1. Sound waves cannot travel through a ………… .
(a) solid
(b) liquid
(c) gas
(d) vacuum
Ans.
Sound waves cannot travel through a vacuum.
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
Ans.
If the frequency of a sound wave is 512 Hz, the number of rarefactions produced per second is 512.
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
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 decreases.
2. Find the odd one out and give the reason:
1. Sound waves, Waves on the surface of water, Waves on a stretched string, Light waves.
Ans.
Light waves. Light waves can travel in a vacuum. Other waves need a medium for propagation.
2. Sitar, Violin, Guitar, Flute.
Ans.
Flute. It is not a string based instrument. Others are string based instruments.
3. State whether the following statements are True or False.
(If a statement is false, correct it and rewrite it.)
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.
Ans.
True.
2. In Jaltarang, the frequency of sound produced is independent of the height of the air column inside the glass cup.
Ans.
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.)
4. Answer the following questions in one sentence each:
1. What is sound?
Ans.
Sound is a form of energy which produces sensation of hearing in our ears.
2. What is vibration?
Ans.
Vibration is a rapid, to and from periodic motion of an object about its mean position.
3. What is compression?
Ans.
As a sound wave travels through air, the region in which air is at high pressure and high density is called compression.
4. What is rarefaction?
Ans.
As a sound wave travels through air, the region in which air is at low pressure and low density is called rarefaction.
5. What is the relation between the frequency of vibration and the vibrating length (or height) of the air column?
Ans.
The greater the vibrating length (or height) of the air column, the less is the frequency of vibration of the column.
6. For a particular string (given material and radius), what are the factors on which its frequency of vibration depends?
Ans.
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.]
7. Name an instrument in which a stretched skin is used to produce musical sound.
Ans.
Tabla.
5. Answer the following questions:
1. Explain the production and propagation of sound in air.
Ans.
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.
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.
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