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Sound

NCERT 8th Class (CBSE) Science: Sound

Question: How is sound produced?

Answer: Sound is produced by rapid to-and-fro movements called vibrations.

Question: Distinguish between infrasonic and ultrasonic.

Answer: Difference is as follows:

Infrasonic
Ultrasonic
1. They are sound produced by only 20 Hz which we cannot hear
1. They are sounds produced by 20,000 Hz which also cannot be heard by us.
2. We cannot hear the sound produced by our bones and muscles because they are below 20 Hz.
2. Ultrasonics are used to study the growth of foetus in the uterus of a women.

Question: What do you mean by time period of a vibrating object?

Answer: The time taken for one complete vibration or oscillation is called its time period.

Question:What is the ‘amplitude’ of a vibrating object?

Answer: The maximum distance to which an oscillating or vibrating object moves from its central position is called amplitude. The loudness of sound depends on the amplitude of vibration.

Question: What is the ‘frequency’ of a vibrating object? How is it related to shrillness of sound?

Answer: Sound is produced by rapid to-and-fro movements, called vibrations or oscillations. The number of oscillations per second is called frequency of oscillating body. The SI unit of frequency is hertz (Hz). The higher the frequency, the more shrill is the sound.

Question: Why do women have shriller voices than men?

Answer: Women have shorter vocal cords (about 15 mm long) than men (about 20 mm long). Thus, the frequency of women’s voice is higher than that of men and their voice is found to be shrill.

Question: In what way does loudness of sound depend on amplitude? If the amplitude is increased three times, by how much does the loudness increase?

Answer: Loudness of the sound produced by a vibrating object depends on the amplitude of vibration. If we strike a drum softly, it produces a soft sound. If you strike it hard, its skin vibrates with greater amplitude and a louder sound is produced. This shows that loudness of the sound produced by a vibrating object depends on the amplitude of vibration. Loudness is proportional to the square of the amplitude. Thus, if the amplitude is increased by three times, the loudness increases by nine times.

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