## Topic outline

• ### Sound

##### Q52. What is the frequency of a vibrating body whose time-period is 0.05 second?

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

Or Frequency = 1/ Time period

= 1/0.05 second

= 20 Hz

##### Q53. By how much will the loudness of a sound change when the amplitude of vibrations is: (a) doubled? (b) halved?

Ans. (a) If the amplitude of vibrations is doubled, then the loudness will become four times.

(b) If the amplitude of vibrations is halved, then the loudness will become one-fourth.

Ans. I would suggest my parents to buy the house which is three lanes away from the roadside because being away from the road will reduce noise pollution caused by heavy traffic on the road. The intensity of noise decreases with the distance between the source and the listener.

##### Q55. Explain in what way noise pollution is harmful to humans.

Ans. Presence of excessive noise in the surroundings may cause many health related problems. Lack of sleep, hypertension (high blood pressure), anxiety and many more health disorders may be caused by noise pollution. A person who is exposed to a loud sound continuously may get temporary or even permanent impairment of hearing.

##### Q56. What is the frequency of the sound produced when the vocal cords are: (a) tight and thin? (b) loose and thick?

Ans. Muscles attached to the vocal cords can make the cords tight or loose.

(a) When the vocal cords are tight and thin, a sound of high frequency (high pitch sound) is produced.

(b) The frequency produced by tight and thin, a sound of low frequency (low pitch sound) is produced.

##### Q57. Write the loudness of the following in decibels:Normal breathing, Soft whisper (at 5m), Normal conversation, Busy traffic and Average factory

Ans. Normal breathing - 10 dB

Soft whisper (at 5m) - 30 dB

Normal conversation - 60 dB

Busy traffic - 70 dB

Average factory - 80 dB

##### Q58. How do we hear?

Ans. The shape of the outer part of the ear is like a funnel. When sound enters in it, it travels down a canal at the end of which a thin membrane is stretched tightly. It is called the eardrum. The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum vibrate. The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. That is how we hear.

##### Q59. Identify the part which vibrates to produce sound in the following instruments: (a) Dholak (b) Sitar (c) Flute (d) Cymbals (e) Veena (f) Tabla

Ans. Dholak – stretched membrane

Sitar – strings

Flute – air columns

Cymbals – metal plates

Veena – strings

Tabla - stretched membrane

##### Q60. List sources of noise pollution in your surroundings.

Ans. Some sources of noise pollution are as follows:

i.        Running of mixer and grinder

ii.        Blowing of horns of motor vehicles

iii.        Bursting of crackers

iv.        Taking off and landing of aeroplanes

v.        Sounds of loudspeakers

vi.        High volumes in televisions and transistors

##### Q61. What is the difference between noise and music? Can music become noise sometimes?

Ans. Noise

The unpleasant sounds around us are called noise.

Example:

i.        Running of mixer and grinder

ii.        Blowing of horns of vehicles

iii.        Bursting of crackers

Music

The sounds which are pleasant to hear are called music.

Example:

i.        Sounds from musical instruments such as harmonium, sitar, guitar, flute etc.

If a musical sound becomes too loud, it would become noise.

##### Q62. Sketch larynx and explain its function in your own words.OrDescribe how sound is produced by the human voice box (or larynx).

Ans. In humans, the sound is produced by the voice box or the larynx. It is at the upper end of the windpipe. Two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When the lungs force air through the slit, the vocal cords vibrate, producing sound.

Image from NCERT

##### Q63. State the various measures which can be taken to control noise pollution in our surroundings.

Ans. Measures to limit noise pollution

i.        Silencing devices must be installed in air craft engines, transport vehicles, industrial machines and home appliances.

ii.        Use of automobile horns should be minimised.

iii.        The noisy operations must be conducted away from any residential area.

iv.        Noise producing industries should be set up away from such areas.

v.        TV and music systems should be run at low volumes.

##### Q64. How can you show that sound cannot travel through a vacuum?

Ans. Take a metal glass tumbler. Place a cell phone in it. Ask your friend to give a ring on this cell phone from another cell phone. Listen to the ring carefully. Now, surround the rim of the tumbler with your hands. Put your mouth on the opening between your hands. Indicate to your friend to give a ring again. Listen to the ring while sucking air from the tumbler. You will observe that the sound become fainter as you suck. If you had been able to suck all the air in the tumbler, the sound would stop completely.

Actually, sound needs a medium to travel. When air has been removed completely from a vessel, it is said that there is vacuum in the vessel. The sound cannot travel through vacuum.