NCERT Solutions for Class 8 Science Chapter 13 Sound

Ans : (d) Sound can travel through solids, liquids, and gases. Sound requires a medium to travel through. Solid, liquid and gas provide the medium for sound. Hence, sound can travel through solids, liquids and gases. 

Ans : (c) A man
The voice of an adult man is of lower pitch in comparison to the voices of a baby boy, a baby girl and a woman. Since frequency of a sound is directly proportional to its pitch, man’s voice is of minimum frequency in comparison to a boy, a girl, or a woman's voice. 

Ans : (a) True
Sound requires a medium to travel through. Since vacuum is devoid of any medium, sound cannot travel through it.
(b) False
The number of oscillations per second of a vibrating object is known as its frequency.
Time period is the time required to complete one oscillation.
(c) False
Loudness of a sound is proportional to the square of the amplitude of its vibration.
When the amplitude of vibration of a sound is large, the sound is very loud. The sound is feeble for small amplitude.
(d) True
Humans cannot hear sounds of all frequencies. Humans can hear a sound whose frequency falls in the range of 20 Hz−20,000 Hz. The sound having frequency out of this range is inaudible to humans.
(e) False
The pitch of a sound is proportional to its frequency. As the frequency of vibration increases, the pitch of the sound also increases and vice-versa. A sound is said to be high pitched if its frequency of vibration is high, and is low pitched for a small frequency of vibration.
(f) False
Unwanted or unpleasant sounds are known as noise. Sounds that are melodious and pleasing to ear are known as music.
(g) True
Unwanted or unpleasant sounds are known as noise. If one is subjected to loud unpleasant sound continuously for a long time, then it may cause temporary hearing impairment. 

Ans : (a) Time taken by an object to complete one oscillation is called __time period__.
(b) Loudness is determined by the __amplitude__ of vibration.
(c) The unit of frequency is __hertz (Hz) __.
(d) Unwanted sound is called __noise__.
(e) Shrillness of a sound is determined by the __frequency__ of vibration.
(Shrillness is also called pitch of the sound. Pitch is directly proportional to the frequency of vibration. Hence, shrillness is determined by the frequency of vibration.) 

Ans : Frequency of oscillations is defined as the number of oscillations of a vibrating body per second. It is given by
Frequency = (Number of Oscillations)/Time = 40/4 = 10 Hz
The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.
Time period = 1/(Frequency of Oscillation) = 1/10 = 0.1 sec.