Sound - Exemplar Solutions 4
NCERT Exemplar Solutions
CHAPTER – 13
Sound
LONG ANSWER QUESTIONS
21. We have a stringed musical instrument. The string is plucked in the middle first with a force of greater magnitude and then with a force of smaller magnitude. In which case would the instrument produce a louder sound?
Ans: The loudness of a sound produced by a vibrating object depends upon the amplitude of the vibrations produced by the sound. The greater the amplitude of the vibrations, the louder will be the sound.
The amplitude of the vibration would be larger when the string is plucked with a force of greater magnitude. The amplitude of vibration would be smaller when the string is plucked with a force of smaller magnitude.
Hence, the instrument will produce a louder sound when the string is plucked in the middle with a force of greater magnitude.
22. How is sound produced and how is it transmitted and heard by us?
Ans: Production of sound: Sound is produced by rapid to and fro movements called vibrations. The back and forth movements of molecules in an object produce vibrations of sound.
Transmission of sound: The vibrating object causes the adjacent molecules of the medium to vibrate with the same frequency. These molecules then transfer the vibrations to the neighbouring molecules and so on. The vibrations are carried (transmitted) in all possible directions through the medium.
Hearing: The vibrations transmitted through the medium reach our outer ears and strike a tightly stretched thin membrane called the eardrum. The eardrum starts vibrating with the same frequency and then sends the signals to the auditory nerve of our nervous system. The sound is heard by us when the brain receives the signal through the auditory nerve.
23. An alarm bell is kept inside a vessel as shown in Fig. 13.1. A person standing close to it can distinctly hear the sound of alarm. Now if the air inside the vessel is removed completely how will the loudness of alarm get affected for the same person?
Ans: The loudness of the sound of the alarm will decrease as the air is removed slowly from the vessel. If the air inside the vessel is removed completely, there will be a vacuum inside the vessel. The sound will not be able to travel through the vacuum and the person standing close will not be able to hear the sound of the alarm at all.
24. The townhall building is situated close to Boojho’s house. There is a clock on the top of the townhall building which rings the bell every hour. Boojho has noticed that the sound of the clock appears to be much clearer at night. Explain.
Ans: The noise level is quite low at night. Therefore, the sound of the clock appears much clearer at night than that during the day.
25. Suggest three measures to limit noise pollution in your locality.
Ans: Measures to limit noise pollution could be as follows:-
i) Use of horns in the locality should be minimised. Persons of the locality can be sensitised to reduce noise.
ii) Double glazed glass can be used in windows to keep out noise.
iii) Trees should be planted along the roads and around the buildings.
iv) Silencers should be installed in transport vehicles and industrial machines.
(Any three measures; any other reasonable measure would be acceptable)
i) Use of horns in the locality should be minimised. Persons of the locality can be sensitised to reduce noise.
ii) Double glazed glass can be used in windows to keep out noise.
iii) Trees should be planted along the roads and around the buildings.
iv) Silencers should be installed in transport vehicles and industrial machines.
(Any three measures; any other reasonable measure would be acceptable)
