Std 8 General Science · Chapter 15

15. Sound

Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Eight General Science.

Exercises

1Fill in the blanks with appropriate word.

(a) The region in a sound wave with higher pressure and density is called compression and that with low pressure and density is called rarefaction.

Compressions and rarefactions in a sound wave, and the change in air pressure.

(b) Medium is 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 500 Hz.

One compression and one rarefaction make one cycle. So, 1000 ÷ 2 = 500 Hz.

(d) Different sound notes have different frequencies.

(e) In loudspeaker, electrical energy is converted into sound energy.

2Give scientific reasons.

(a) It is essential to change the tension on the vocal cords, as we produce different sound notes from our larynx.

Sound is produced by vibration of vocal cords. Different notes have different frequencies. Tight vocal cords give high frequency. Loose vocal cords give low frequency. Hence, tension on the vocal cords must be changed to produce different notes.

(b) Astronauts on the moon cannot hear each other directly.

Sound needs a material medium to travel. There is no air on the moon. So sound cannot travel from one astronaut to the other. Hence, they cannot hear each other directly.

(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.

Air particles only vibrate at their own places. They pass the energy to the next particles. Thus, only the energy of sound moves forward. Hence, the air itself does not move from one place to the other.

3How 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?

Different notes have different frequencies.

Guitar:

Sound is produced by the vibration of strings.
The tension of the string is changed. A tight string gives a higher note.
The length of the string is changed by pressing it with fingers. A shorter string gives a higher note.

Flute:

Sound is produced by the vibration of the air column inside it.
Holes are closed or opened with fingers. This changes the length of the air column.
A shorter air column gives a higher note.

4How is sound produced in a human larynx and a loudspeaker?

Human larynx:

Two vocal cords are stretched in the larynx. A narrow slit is left between them.
Air from the lungs passes through this slit.
The vocal cords vibrate and sound is produced.

Loudspeaker:

A loudspeaker has a permanent magnet, a coil and a conical screen.
Electric current flows through the coil. The coil moves back and forth.
The screen attached to the coil also vibrates.
It produces sound waves in the air.

In a loudspeaker, electrical energy is converted into sound energy.

5Explain the experiment, with neat diagram, to prove the following: ‘Sound needs material medium for propagation.’

The bell jar experiment — sound needs a material medium to travel.

Apparatus: Bell jar, electric bell, vacuum pump.

Procedure:

Keep an electric bell inside the bell jar. Switch on the bell. Its sound is heard clearly.
Start the vacuum pump. It removes air from the bell jar.
As air is removed, the sound becomes fainter and fainter.
When very little air is left, the sound is very faint.

Conclusion: Sound needs a material medium for propagation.

6Match the following.

Instrument / organ	Sound is produced by
Human larynx	Vibrations in vocal cords
Loudspeaker	Vibrations of screen
Jal-tarang	Vibrations in air column
Tuning fork	Vibrations of metal arms
Sitar	Vibrations in strings

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