Chapter 15 - Sound
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: compression, rarefaction
b. Medium is ……………for generation of sound.
Ans: necessary
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: 500Hz
d. Different sound notes have different ……..
Ans: frequencies
e. In a loudspeaker, ……..energy is converted into …………energy.
Ans: electrical, sound
2. Give scientific reasons :
a. It is essential to change the tension on the vocal cords, as we produce different sound notes from our larynx.
Ans:
(i) Different sound notes have different frequencies.
(ii) In humans, when the lungs force air through the slit between the vocal cords, sound is produced.
(iii) The frequency of this sound depends upon the tension of the vocal cords, which are controlled by muscles in the larynx.
Hence, it is essential to change the tension on the vocal cords, as we produce different sound notes from our larynx.
b. Astronauts on the moon can not hear each other directly.
Ans:
(i) Sound requires a material medium for its propagation.
(ii) The moon does not have an atmosphere. Since there is no medium between the astronauts on the moon, direct sound propagation between them is not possible.
Hence, astronauts on the moon 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.
Ans:
(i) A sound wave propagates in the form of compression and rarefaction.
(ii) When a compression is formed, compressed air molecules transfer their energy to air molecules in the next region without leaving their positions.
(iii) As a result, air molecules in the next region get compressed and transfer energy further in the same manner.
(iv) Meanwhile, a rarefaction forms and moves forward in a similar way.
(v) As a combined effect, air molecules propagate sound waves without actual movement.
Hence, during the propagation of a sound wave in air from one place to another, air itself is not required to move.
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:
(i) Musical instruments like the guitar use strings to produce sound. In such string-based instruments, the frequency of vibration of the string is changed by changing the tension on the string and/or by changing the vibrating length of the string. This results in the generation of different notes.
(ii) In musical instruments like the flute, the holes are opened or closed to change the length of the vibrating air column in the flute. The frequency of the waves, therefore, changes, resulting in the production of different notes. Also, by changing the way air is blown, different notes can be generated.
4. How is sound produced in a human larynx and a loudspeaker?
Ans:
(A) Sound production in humans:
(i) In humans, sound is produced in the larynx (voice box), situated at the upper end of the windpipe.
(ii) Two vocal cords are stretched across the larynx, leaving a narrow slit between them for the passage of air.
(iii) When the lungs force air through the slit, the vocal cords vibrate, producing sound.
(iv) Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of sound produced is different than the sound produced when the vocal cords are loose and thick.
(B) Sound production in loudspeaker:
(i) A loudspeaker consists of a permanent magnet with a coil wound around it.
(ii) When current flows through the coil, a magnetic field is generated around the coil.
(iii) Due to this magnetic field, the coil moves back and forth.
(iv) The frequency and amplitude of the movement of the coil depend on the variation in the current flowing through the coil.
(v) Due to the motion of the coil, the conical screen attached to the coil also starts moving back and forth.
(vi) This back and forth movement of the loudspeaker screen gives rise to sound waves in the air.
5. Explain the experiment, with neat diagram, to prove the following:
‘Sound needs a material medium for propagation.’
Ans:
Aim: To verify that a material medium is essential for the propagation of sound waves with the help of the bell jar experiment.
Procedure:
(i) Take an inverted glass jar and place it on a smooth surface. Using a tube, connect the jar to a vacuum pump. As shown in the illustration, place an electric bell in the jar.
(ii) At the beginning of the experiment, the vacuum pump is switched off, and the bell-jar contains air. Now switch on the key to the electric bell. The bell starts ringing, and the ringing can be heard outside the bell jar.
(iii) Take the air out of the glass jar with the help of a vacuum pump.
Observation:
When air is steadily taken out of the glass jar, the sound of the bell goes on decreasing, and after a certain time, the sound becomes inaudible. However, vibrations from the gong of the bell can be seen from outside.
Conclusion:
(i) A material medium is required for the propagation of sound waves.
(ii) Sound cannot travel in a vacuum.
6. Match the following:
| A | B |
|---|---|
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 |
Ans:
| A | B |
|---|---|
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 |