SOUND IS PRODUCED BY A VIBRATING BODY

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?

Activity 13.1: (A vibrating body produces sound)

*  Take a metal plate (or a pan).

*  Hang it at a convenient place in such a way that it does not touch any wall.

*  Now strike it with a stick (Fig.13.2).

*  Do you hear a sound? Touch the plate or pan gently with your finger.

*  Do you feel the vibrations?

Figure 13.2: Striking a pan

Again strike the plate with the stick and hold it tightly with your hands immediately after striking. Do you still hear the sound? Touch the plate after it stops producing sound. Can you feel the vibrations now?

Activity 13.2: (Sound produced by vibrating body)

*  Take a rubber band.

*  Put it around the longer side of a pencil box (Fig. 13.3).

*  Insert two pencils between the box and the stretched rubber.

*  Now, pluck the rubber band somewhere in the middle.

*  Do you hear any sound? Does the band vibrate?

Figure 13.3: Plucking the rubber band

As you learned in Class VII the to and fro or back and forth motion of an object is called vibration. When a tightly stretched band is plucked, it vibrates and produces sound. When it stops vibrating, it does not produce any sound.

Activity 13.3:  ( Vibrating dish produces waves )

*  Take a metal dish.

*  Pour water in it.

*  Strike it at its edge with a spoon (Fig. 13.4).

*  Do you hear a sound? Again strike the dish and then touch it.

*  Can you feel the dish vibrating? Strike the dish again.

*  Look at the surface of the water.

*  Do you see any waves there? Now hold the dish.

*  What change do you observe on the surface of the water?

*  Can you explain the change?

*  Is there a hint to connect sound with the vibrations of a body?

Figure 13.4: Vibrating dish produces waves in water

We see that a vibrating object produces sound. In some cases, the vibrations are easily visible to us. But in most cases, their amplitude is so small that we cannot see them. However, we can feel them.

Activity 13.4: ( sound produced by musical instruments )

*  Take a hollow coconut shell and make a musical instrument ektara.

*  You can also make it with the help of an earthen pot (Fig. 13.5).

*  Play this instrument and identify its vibrating part.

Figure 13.5: Ektara

Make a list of familiar musical instruments and identify their vibrating parts. A few examples are given in Table13.1. Complete rest of the Table

Table 13.1: Musical Instruments and their Vibrating Parts

Many of you might have seen the manjira (cymbals), the ghatam, and the noot (mudpots) and the kartal. These musical instruments are commonly used in many parts of our country. These instruments are simply beaten or struck (Fig. 13.6). Can you name a few other musical instruments of this type?

You too can make a musical instrument.

Figure 13.6: A few more musical instruments

Activity 13.5:

*  Take 6-8 bowls or tumblers.

*  Fill them with water up to different levels, increasing gradually from one end to the other.

*  Now take a pencil and strike the bowls gently.

*  Strike all of them in succession.

*  You will hear pleasant sounds.

*  This is your jaltrang (Fig.13.7).

Figure 13.7: Jaltrang

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. Similarly, when we strike the membrane of a mridangam, the sound that we hear is not only that of the membrane but of the whole body of the instrument.

Source: This topic is taken from NCERT TEXTBOOK

SOUND PRODUCED BY HUMANS

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).

Figure 13.8: Voicebox in humans

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 from that when they are loose and thick. Let us see how the vocal cords function.

Activity 13.6:

*  Take two rubber strips of the same size.

*  Place these two pieces one above the other and stretch them tight.

*  Now blow air through the gap between them [Fig. 13.9(a)].

*  As the air blows through the stretched rubber strips, a sound is produced.

*  You can also take a piece of paper with a narrow slit and hold it between your fingers as shown in Fig. 13.9 (b).

*  Now blow through the slit and listen to the sound.

*  Our vocal cords produce sound in a similar manner.

Figure 13.9: Working of vocal cords

The vocal cords in men are about 20 mm long. In women, these are about 15 mm long. Children have very short vocal cords. This is the reason why the voices of men, women, and children are different.

Source: This topic is taken from NCERT TEXTBOOK