## Topic outline

• ### Sound

##### Q38. How does sound of a bird differ from roar of a lion?

Ans. A bird makes a high-pitched sound whereas a lion makes a low-pitched roar. However, the roar of a lion is very loud while the sound of the bird is quite feeble.

##### Q39. Which produces sound of a higher pitch: a drum or a whistle? Why?

Ans. A drum vibrates with a low frequency. Therefore, it produces a low-pitched sound. On the other hand, a whistle has a high frequency and therefore, produces a sound of higher pitch.

##### Q40. What frequency can dogs hear but not humans?

Ans. Some animals can hear sounds of frequencies higher than 20,000 Hz. Dogs have this ability. The police use high frequency whistles which dogs can hear but humans cannot.

##### Q41. Can sound travel through a vacuum? Why or why not?

Ans. Sound cannot travel through a vacuum. A vacuum is an area without any air, like space. So sound cannot travel through space because there is no matter for the vibrations to work in.

##### Q42. How does loudness depend on the amplitude of vibrations?

Ans. Loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. For example, if the amplitude becomes twice, the loudness increases by a factor of 4.

##### Q43. How is sound produced in Sitar?

Ans. When we pluck the string of an instrument, like the sitar, the sound that we hear is not only that of the string. The whole instrument is forced to vibrate, and it is the sound of the vibration of the instrument that we hear.

##### Q44. Why are the voices of men, women and children different?

Ans. The vocal cords in men are about 20mm long. In women these are about 5mm shorter. Children have very short vocal cords. This is the reason why the voices of men, women and children are different.

##### Q45. How can you show that a sounding tabla is vibrating?

Ans. If we put some grains on the membrane of sound producing tabla, the grains will start jumping up and down showing that the tabla membrane is vibrating while producing sound.

##### Q46. How can we distinguish between a man's voice and a woman's voice even without seeing them?

Ans. We can distinguish between a man's voice and a woman voice even without seeing them. This is because the voice of a woman has a higher frequency and is shriller than that of a man.

##### Q47. What can be done along the roads to reduce noise pollution caused by traffic from reaching the residents of the area?

Ans. Trees must be planted along the roads and around buildings to cut down on the sounds reaching the residents, thus reducing the harmful effects of noise pollution.

##### Q48. State one observation from everyday life which shows that sound travels slower than light.

Ans. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. This shows that sound travels slower than light.

##### Q49. What are called inaudible sounds?

Ans. The fact is that sounds of frequencies less than about 20 vibrations per second (20 Hz) and higher than about 20,000 vibrations per second (20000 Hz) cannot be detected by the human ear. Such sounds are called inaudible sounds.

##### Q50. How is pitch related to frequency?

Ans. The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher we say that the sound is shrill and has a higher pitch. If the frequency of vibration is lower, we say that the sound has a lower pitch.

##### Q51. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Ans. Time period = 1/(Frequency of Oscillation)

Frequency of oscillations = 500 Hz

Time period = 1/500 = .002 s