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STD. 8, , SOUND, , I. FILL IN THE BLANKS, (1) The region in a sound wave, with higher pressure and density is called compression and, that with low pressure and density called rarefaction., (2) Medium is needed (or necessary) for generation of sound., (3) 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., (4) Different sound notes have different frequencies., (5) In a loudspeaker, mechanical energy is converted into sound energy., II. Match the columns, 1) Human larynx Vibrations in vocal cords., (2) Loudspeaker - Vibrations of screen, (3) Jaltarang - Vibrations in air column, (4) Tuning fork - Vibrations of metal arms, (5) Sitar - Vibrations in strings, Q. If the air inside the bell jar is totally removed,will we be able to hear the sound of ringing, bell?, Why?, Ans. No. This is because a medium is necessary for generation and propagation of sound., Q. Explain the experiment, with a best diagram, to prove the following:, 1. ‘Sound needs a material medium for propagation’., , Figure 15.3 shows part of the set up. 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

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connected to 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., Soun 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., Q. Give scientific reasons:, , (1) 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., (2) 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., , (3) 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 prod different sound notes from our larynx, Q. 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?

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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., (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., Q. 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?, Ans. If sound waves are generated in air, the state of compression and rarefaction created in, the air moves away from the source, , Q. How is sound produced in the human larynx and a loudspeaker?, Ans. (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 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 15.6 shows the internal construction of the, 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 oil, 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.

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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.