Class 8 science chapter 15 Sound Questions and Answers


Question 1:Fill in the blanks with approriate word:

a. The region in a sound wave,  with higher pressure and density is called................ and that with low pressure and density is called........... .

b. Medium is ....................... 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.................. .

d. Different sound notes have different.............. .

e. In loudspeaker................energy is converted into..................energy.

Answer :
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.

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 (One compression and rarefaction together makes a cycle of sound wave).

d. Different sound notes have different frequencies.

e. In loudspeaker electrical energy is converted into sound energy.


Question 2: Give scientifcic reasons.
a. It is essential to change the tension on the vocal cords, as we produce different sound notes from our larynx.
Answer : When the vocal cords are tight and thin the type of equality of voice is different and it is essential to change the tension on the vocal cords as we produce different sound notes from larynx.

b. Astronauts on the moon can not hear each other directly.
Answer : 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.
Answer : in sound production, only the transfer of energy of the vibrating particles takes place and not the transfer of particles themselves. So, yes 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 from one place to the other.


Question 3: How are differnt sound notes generated in musical instruments like guitar, which uses strings for sound generation, and flute, which uses blown air for sound generation ?
Answer :

  1. In stringed instruments (like guitar): Different notes are generated in these instruments when the frequency of vibration of string is changed. It can be changed by changing the tension in string: 
  2. If the string of the instrument is under high tension, then the pitch of the note produced on plucking the string will be high i.e. the note will be shriller because frequency of vibration of the string will be high.
  3. thickness of string: If the string of the instrument is thin, then the pitch of the note produced on plucking the string will be high as frequency of vibration of the string will be high.
  4. position of plucking of string: If a string stretched between two fixed ends is plucked closer to one of the fixed ends, then the pitch of the sound note produced will be high.



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

When a sound signal passes through the voice coil, a magnetic field is produced around the coil causing the diaphragm to vibrate. The cone structure amplifies these vibrations, pumping sound waves into the surrounding air and towards your ears. That's how sound comes from a loudspeaker.

In humans sound is produced by the vioce box or larynx. Two vocal cords are stretched across the voice box leaving a narrow slit. When the lungs force air through the slit, it vibrates and produces sound.


Question 5: Explain the experiment, with neat diagram, to prove the following:
'Sound needs material medium for propagation.
Answer :

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

  1. A bell jar
  2. A vacuum pump connected to the jar through a tube
  3. An electric bell inside the jar and is connected to the electric supply through the lid of the bell jar.


At the start of experiment,  the  vacuum pump is off and the bell-jar contains air. If the key to electric  bell  is now switched on, the bell start ringing and the ringing can be heard from outside  the  bell  jar.  Now, if  the  vacuum  pump is switched on, it  starts removing  the air from the bell  jar. As the quantity of air inside the bell jar decreases, the level  of ringing sound heard outside also decreases. If the pump is operated for sufficiently long period, the  quantity  of air like inside the bell jar will be low. At this point the level of ringing sound will also be very low.    This  experiment  demonstrates  that  sound generation  and propagation  needs  a medium. If the  air inside  the  bell  jar is totally  removed.


Question 6 : Match the following

Answer :
1) Human larynx - Vibraions 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.


#Can you Recall
1. How is sound produced ?
Answer : A vibrating object will produce sound waves in the air. For example, when the head of a drum is hit with a mallet, the drumhead vibrates and produces sound waves. The vibrating drumhead produces sound waves because it moves alternately outward and inward, pushing against, then moving away from, the air next to it.

#Use your Brain Power 
1. 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?
Answer: Compression and rarefactionare produced because of the disturbance in medium caused bysound wave. Sound wave propagates because of compression and rarefaction of the particles of the medium. When an object starts vibrating, it creates disturbance in medium.


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