NCERT Book Class 8 Science Sound

SOUND

How do you come to know that a ‘period’ is over in your school? You come to know easily that someone is at your door when he knocks or you hear the sound of the doorbell. Most of the time you can make out that someone is approaching you by just hearing the foot steps. You might have played a game called hide and seek. In this game a person is blind-folded and has to catch the remaining players. How is the blindfolded person able to guess which player is closest to her? Sound plays an important role in our life. It helps us to communicate with one another. We hear a variety of sounds in our surroundings. Make a list of sounds you hear in your surroundings.

In the music room of your school you hear the sounds made by musical instruments like flute, tabla, harmonium etc. (Fig 13.1). How is sound produced? How does it travel from one place to another? How do we hear sound? Why are some sounds louder than others? We shall discuss such questions in this chapter.

13.1 Sound is Produced by a Vibrating Body

Touch the school bell when not in use. What do you feel? Again touch it when producing sound. Can you feel it vibrating? The to and fro or back and forth motion of an object is termed as vibration as you learnt in Class VII When a tightly stretched band isplucked, it vibrates and produces sound. When it stops vibrating, it does not produce any sound.

13.2 Sound Produced by Humans

Speak loudly for a while or sing a song, or buzz like a bee. Put your hand on your throat as shown in Fig. 13.8. Do you feel any vibrations? In humans, the sound is produced by the voice box or the larynx. Put your fingers on the throat and find a hard bump that seems to move when you swallow. This part of the body is known as the voice box. 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 (Fig.13.8). When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose.

When the vocal cords are tight and thin, the type or quality of voice is different Can you think of an explanation? Is it possible that the decreasing amount of air in the tumbler had something to do with decreasing loudness of the ring? Indeed, 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. Does sound travel in liquids? Let us find out.

13.4 We hear Sound through Our Ears

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. It performs an important function. T understand what the eardrum does, let us build a tin can model of the eardrum. The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum vibrate (Fig. 13.16). The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. That is how we hear. You have already learnt in earlier classes about the oscillatory motion and its time period.

The number of oscillations per second is called the frequency of oscillation. Frequency is expressed in hertz. Its symbol is Hz. A frequency of 1 Hz is one oscillation per second. If an object oscillates 20 times in one second, what would be its frequency? You can recognise many familiar sounds without seeing the object  producing them. How is it possible? These sounds must be different to enable you to recognise them. Have you ever thought what factors make them different? Amplitude and frequency are two important properties of any sound. Can we differentiate sounds on the basis of their amplitudes and frequencies?

Exercises

1. Choose the correct answer :

Sound can travel through

(a) gases only (b) solids only

(c) liquids only (d) solids, liquids and gases.

2. Which of the following voices is likely to have minimum frequency?

(a) Baby girl (b) Baby boy

(c) A man (d) A woman

3. In the following statements, tick T against those which are true, and F against those which are false:

(a) Sound cannot travel in vacuum. (T/F)

(b) The number of oscillations per second of a vibrating object is called its time period. (T/F)

(c) If the amplitude of vibration is large, sound is feeble. (T/F)

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)

(e) The lower the frequency of vibration, the higher is the pitch. (T/F)

(f) Unwanted or unpleasant sound is termed as music. (T/F)

(g) Noise pollution may cause partial hearing impairment. (T/F)

4. Fill in the blanks with suitable words.

(a) Time taken by an object to complete one oscillation is called ................... .

(b) Loudness is determined by the .........................of vibration.

(c) The unit of frequency is..........................

(d) Unwanted sound is called ......................... .

(e) Shrillness of a sound is determined by the ................. of vibration.

5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

6. 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?

7. Identify the part which vibrates to produce sound in the following instruments:

(a) Dholak (b) Sitar (c) Flute

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

9. List sources of noise pollution in your surroundings.

10. Explain in what way noise pollution is harmful to humans.

11. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

12. Sketch larynx and explain its function in your own words.

13. 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. Can you explain?

Please refer to attached file for NCERT Class 8 Science Sound