Solution-Class 8-Science-Chapter-15-Sound -Maharashtra Board

a. The region in a sound wave, with higher pressure and density is called compressionand that with low pressure and density is called rarefaction.
b. Medium is required 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 500 Hz.

d. Different sound notes have different frequencies.
e. In loudspeaker electricalenergy is converted into soundenergy.

• Different sound is produced when frequency of vibration of the source of sound is changed. In humans, vocal cords in larynx vibrate and produce sound.
• The frequency of vibration of vocal cords is directly dependent on the tension produced on it.
• The higher the tension on the chords, higher frequency of the voice will be produced.
• Therefore it is essential to change the tension on the vocal cords to produce different sound notes from our larynx.

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.

• If sound waves are generated in air, the state of compression and rarefaction created in the air moves away from the source.
• Particles in the air, vibrating about their mean position and transfer their energy to the other layer of particles near to them.
• The air does not move away from the source because air is elastic and the force due to the vibration varies periodically with time.
• So, in sound production, only the transfer of energy of the vibrating particles takes place and not the transfer of particles themselves from one place to the other.
• Therefore we can say that as the sound wave propagates from one place to the other in air, the air itself is not required to move

(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.

• Different notes are produced in these instruments by varying the length of the air column in them.
• The length of the air column can be increased or decreased by closing or opening the more number of holes present in these instruments.

(1) Sound production in thehuman 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.).

• 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).

(2) Production of sound in loudspeaker
A loudspeaker consists of following parts:

• An electric coil wound on a permanent magnet.
• A conical shaped screen of the speaker connected to the coil.

• When variable current flows through the coil, magnetic field is produced around it due to electromagnetism and it behaves like an electromagnet.
• Because of this, the coil is repelled and attracted by the permanent magnet alternately.
• The screen attached to the coil moves back and forth due to the attraction and repulsion of the coil and hence, produces sound.

Bell jar experiment can prove that sound needs material medium for its propagation.
Construction

• A bell jar
• A vacuum pump connected to the jar through a tube
• An electric bell inside the jar and is connected to the electric supply through the lid of the bell jar

Working :

• 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.