Production and Propagation of Sound

Objective

This topic gives an overview of; 

  • Sound
  • Sound Produced by a Vibrating Body
  • Sound Produced by Humans
  • Sound Needs a Medium for Propagation
  • Sound through our Ears

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 knocksor 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.

Sound Produced by a Vibrating Body

Touch the school bell when not in use.  Again touch it when producing sound.

Take a metal plate (or a shallow frying pan). Hang it at a convenient place in such a way that it does not touch any wall. Now strike it with a stick. Touch the plate or pan gently with your finger.  You will feel the vibrations. Again strike the plate with the stick and hold it tightly with your hands immediately after striking. You  will still hear the sound. Touch the plate after it stops producing sound. 

Take a rubber band. Put it around the longer side of a pencil box. Insert two pencils between the box and the stretched rubber. Now, pluck the rubber band somewhere in the middle. You will hear sound and the band vibrates.

The to and fro or back and forth motion of an object is termed as vibration. When a tightly stretched band is plucked, it vibrates and produces sound. When it stops vibrating, it does not produce any sound.

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.

Take a hollow coconut shell and make a musical instrument Ektara. You can also make it with the help of an earthen pot. Play this instrument and identify its vibrating part.

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

Musical instruments and their Vibrating Parts

S.No Musical instrument Vibrating part  producing sound
1 Veena Stretched string
2 Tabala Stretched membrane
3 Flute Air-column

Many of you might have seen the manjira (cymbals), the ghatam, and the noot (mudpots) and the kartal. These instruments are commonly used in many parts of our country. These musical instruments are simply beaten or struck.

Take 6-8 metal 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 a pleasant sound. This is your 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.

Sound Produced by Humans

Speak loudly for a while or sing a song, or buzz like a bee. 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 .

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.

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. 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. Now blow through the slit and listen to the sound. Our vocal cords produce sound in a similar manner.

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.

Sound Needs a Medium for Propagation

Take a metal glass tumbler. Make sure that it is dry. Place a cell phone in it. (Remember that the cell phone must not be kept in water.) 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.

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.

Take a bucket or a bathtub. Fill it with clean water. Take a small bell in one hand. Shake this bell inside the water to produce sound. Make sure that the bell does not touch the body of the bucket or the tub.Place your ear gently on the water surface.(Be careful : water should not enter in your ear).You will hear the sound of the bell. It indicates that sound can travel through liquids.

 Let us find out if sound can travel through solids also.

Take a meter scale or a long metal rod and hold its one end to your ear. Ask your friend to gently scratch or tap the other end of the scale.You can also perform the above activity by placing your ear at one end of a long wooden or metallic table and asking your friend to gently scratch the other end of the table.

We find that sound can travel through wood or metal. In fact, sound can travel through any solid. You can perform interesting activities to show that sound can also travel through strings. Recall if you made a toy telephone.

We have learnt so far that vibrating objects produce sound and it is carried in all directions in a medium.

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. To 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. The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. That is how we hear.

We must NEVER put a sharp, pointed or hard thing into our ear. It can damage the eardrum. The damaged eardrum can impair hearing.

Summary

  • Sound is produced by vibrating objects.
  • In human beings, the vibration of the vocal cords produces sound.
  • Sound travels through a medium (gas, liquid or solid). It cannot travel in vacuum.
  • The eardrum senses the vibrations of sound, It sends the signals to the brain. This process is called hearing.

 

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