What produces sound by vibrating?
By learning Physics Topics, we can gain a deeper appreciation for the natural world and our place in it.
The sensation felt by our ears is called sound. Sound is a form of energy. Sound is that form of energy which makes us hear. We hear many sounds around us in everyday life. At home we hear the sounds of our parents talking to us. We also hear the sounds of telephone bell, radio, television, stereo-system, mixer-grinder and washing machine. At school we hear the sounds of our teachers, classmates, and the school bell. We hear the sounds of scooters, motorcycles, cars, buses and trucks on the roads. And the sound of a flying aeroplane is heard from the sky.
At a music concert, we hear the sounds produced by various musical instruments like sitar, veena, violin, guitar, tanpura, piano, harmonium, flute, shehnai, tabla and cymbals, etc. In a garden we hear the sounds of chirping of birds and rustling of leaves of the trees in breeze. In rainy season, we hear the sound of rain drops falling on the roof and the sound of thundering of clouds. In a factory we hear the different types of sounds made by various big and small machines. And at night, when most of the sounds cease (stop), we can still hear the faint sounds of the ticking of a clock and the buzzing of a mosquito.
Each sound is special to the object which produces it. For example, the ticking of a clock, the click of an electric switch and the whirr (sound of rotation) of a ceiling fan are the characteristic sounds produced by a clock, an electric switch and a ceiling fan, respectively. We rarely forget a sound having heard it once and quickly recognise this sound when we hear it again. We can even recognise a person from the sound of his voice even without seeing his face. For example, we can usually tell from the voice which singer is singing a particular song on radio even though we do not see the face of the singer. Thus, every sound has a unique ‘quality’ which distinguishes it from other sounds. Sound plays an important role in our daily life. It helps us to communicate (or talk) with others. We will now discuss how sound is produced. This is called production of sound.
Sound Is Produced By Vibrating Objects
When an object moves ‘backwards and forwards’ (to-and-fro) rapidly, we say that the object ‘vibrates’ or that the object is ‘vibrating’. Sound is produced when an object vibrates (moves to-and-fro rapidly). In other words, sound is produced by vibrating objects. So, whenever we hear a sound, then some object must be vibrating to produce that sound.
The energy required to make an object vibrate and produce sound is provided by some outside source (like our hand, wind, etc.). In some cases, the vibrations of a sound producing object are quite large which we can see with our eyes. But in other cases, the vibrations of the sound producing object are so small that we cannot see them easily, we have to feel the vibrations of such an object by touching it gently with the fingers of our hand. We will now give some examples to show that sound is produced by vibrating objects.
1. Let us ring a bicycle bell and touch it gently with our fingers. We will find that a ringing bicycle bell (which is producing sound) is shaking back and forth continuously. We say that the bicycle bell is vibrating. Now, if we hold the ringing bicycle bell tightly with our hand, it stops vibrating, and the sound also stops coming. From this we conclude that sound is produced by a vibrating bicycle bell. When the bicycle bell vibrates, it produces sound; and when the bicycle bell stops vibrating, the sound also stops coming from it. In general, we can say that a body must vibrate to produce sound. The sound of a school bell is produced by the vibrations of iron or brass plate when it is hit by a hammer.
2. Stretch a rubber band and tie it tightly between two nails fixed on a table [as shown in Figure (a)]. In this position the rubber band is not vibrating and hence not producing any sound.
Let us pluck the stretched rubber band in the middle with our finger. The rubber band starts vibrating and produces sound. We can see the vibrations of the stretched rubber band. The rubber band vibrates (moves backwards and forwards rapidly) between the positions A and B [see Figure (b)]. Thus, sound is produced when a stretched rubber band vibrates. Now, if we hold the vibrating rubber band tightly with our hand, the rubber band stops vibrating and the sound being produced by it also stops.
3. Sitar is a musical instrument. It has many stretched strings (stretched wires). If we pluck the string (wire) of a sitar in the middle, the sitar makes a sound. If we now put our finger gently on the sitar string, we can feel the string vibrating. Thus, sound is produced when a sitar string vibrates. The sound of a veena is also produced by the vibrations of stretched strings.
4. When we talk, we make sound. This sound is made by the vibrations of two vocal cords present in our voice box fixed in the throat. This can be shown as follows: Let us hold the fingers of our right hand gently on our throat and talk to one of our friends (or sing a song). When we are talking (making sound) our fingers feel that something is moving or vibrating inside the throat. Actually, when we talk, air from the lungs passes up the wind-pipe.
This air makes the vocal cords in our voice box to vibrate rapidly. And vibrating vocal cords produce the sound (of our talk). Thus, sound is produced when our vocal cords vibrate. Please note that the vibrations of our vocal cords are small, so we can only feel them with our hands. We cannot see these vibrations with our eyes. Mosquitoes and bees make a buzzing sound by vibrating their wings very, very rapidly.
5. If we blow across the mouth of an empty test-tube, then a whistling sound is produced [see Figure (a)]. This sound is produced by the vibrations of air present in the test-tube. Thus, sound is produced when the air column enclosed in a tube vibrates. The sound of a flute (bansuri) is produced by the vibrations of air column enclosed in the flute tube. The sound of a bursting balloon is produced by the vibrations of air enclosed in the balloon (when it escapes).
6. If we hit the stretched membrane (or skin) of a tabla, the membrane starts vibrating and produces a sound. Now, if we put a few small pebbles on the membrane of this sound producing tabla, the pebbles will start jumping up and down showing that the tabla membrane is vibrating while producing sound. Thus, sound is produced when the membrane (or skin) of a tabla vibrates. When we strike at the membrane (or skin) of a drum, it vibrates to produce sound [see Figure (b)], Thus, sound is produced when the stretched membrane (or skin) of a drum vibrates.
If we switch on a transistor radio, the thin cone of its speaker vibrates and produces sound. We can feel these vibrations if we place our fingers gently on the cone of the speaker. So, in a radio or television, the thin cone of the speaker vibrates ‘backwards and forwards’ rapidly and produces sound. Thus, the sound of a radio (or television) is produced by the vibrations of the cone of speaker.
From the above discussion we conclude that: Sound can be produced by the following methods:
- by vibrating strings (as in a sitar),
- by vibrating air columns (as in a flute),
- by vibrating membranes (as in tabla), and
- by vibrating plates (as in a bicycle bell).
We will now discuss how sound reaches from a sound producing object to our ears. This is called transmission of sound or propagation of sound.
Propagation of Sound
Sound is produced by the vibrations of an object. When an object vibrates back and forth in air, then the molecules of air close to this object also start vibrating back and forth with the same frequency. These vibrating air molecules pass on their motion to the next layer of air molecules due to which they also start vibrating back and forth. This process goes on and on. And ultimately, all the air molecules around the sound producing object start vibrating back and forth (just like the vibrating object). When the vibrating air molecules fall on our ears, the ears feel these vibrations as sound. Thus, when an object vibrates (and makes sound), then the air around it also starts vibrating in exactly the same way and carries sound to our ears through the vibrations of its molecules. And we say that a sound wave travels from the sound producing object to our ears, through the air.
Sound Produced By Humans
The human beings produce sound by using the voice box which is called ‘larynx’. Voice box (or larynx) is situated in our throat at the top of the wind-pipe (or trachea). The human voice box (or larynx) contains two ligaments known as vocal cords. The vocal cords are a kind of strings. Sound is produced by the vibrations of vocal cords (see Figure). The vocal cords are attached to muscles which change the tension (stretching) in the cords and the distance between the cords.
(i) Normally, the muscles of vocal cords are completely relaxed due to which the vocal cords are separated and loose so that air from the lungs passes through them without producing any sound [see Figure], Thus, when we are not talking (or singing), the two vocal cords are far apart with a lot of gap between them.
(ii) When we want to speak, the muscles of vocal cords contract due to which the two vocal cords become stretched and close together leaving only a narrow slit between them [see Figure]. The lungs pass a current of air between the two vocal cords. This air makes the vocal cords vibrate. And the vibrating vocal cords produce sound. Thus, when we talk or sing (or make any other sound), we actually make our vocal cords vibrate. And vibrations of vocal cords by expelled air produce vocal
We can demonstrate the working of vocal cords to produce sound as follows : Take two rubber strips of the same size. Place these two rubber strips one above the other. Hold the two ends of the rubber strips in your hands and stretch them tight. Keep the stretched rubber strips in front of your mouth and blow air through the thin gap between them (see Figure). As the air blows through the stretched rubber strips, a sound is produced. This sound is produced by the vibrations of stretched rubber strips when air rushes through the thin gap between them. Our vocal cords produce sound in a similar way.
When we talk or sing, then the frequency of sound produced by us changes continuously. The changes in frequency of sound while talking or singing are brought about by the action of muscles attached to the vocal cords in the voice box. When the muscles attached to vocal cords contract and stretch, the vocal cords become tight and thin, and a sound of high frequency is produced. On the other hand, when the muscles relax, the vocal cords become loose and thick, and a sound of low frequency is produced.
The vocal cords of a man are about 20 mm long. The vocal cords of a woman are about 5 mm shorter than man. Due to the shorter vocal cords, the frequency (or pitch) of a woman’s voice is higher than that of a man. Small children have very short vocal cords due to which the frequency (or pitch) of their voice is very high. This is why their voice is shrill. So, it is due to the different frequencies (or different pitch) caused by the different lengths of their vocal cords that the voices of men, women and children are different.
Sound Needs A Medium For Propagation
The substance through which sound travels is called medium. The medium can be a solid substance, a liquid or a gas. And transmission of sound is called propagation of sound. So, by saying that sound needs a medium for propagation, we mean that sound needs a solid, liquid or gas for transmission. In other words, sound needs a material medium like solid, liquid or gas to travel and be heard. Sound can travel through solids, liquids and gases but it cannot travel through vacuum (or empty space). Sound can travel through solids, liquids and gases because the molecules of solids, liquids and gases carry sound waves from one place to another (through their vibrations). Sound cannot travel through vacuum because vacuum has no molecules which can vibrate and carry sound waves. So, a material medium like air, water, wood, etc., is necessary for the transmission of sound from the ‘source of sound’ to our ‘ears’. In other words, sound needs a medium for propagation. We will now describe some activities which will show that sound can travel through solids, liquids and gases, but not through vacuum.
Sound Can Travel Through Solids, Liquids and Gases
Let us press our ear on to one side of a wooden bench and ask a friend to tap or scratch the other end of the bench lightly. We will hear the sound of tapping or scratching through the wooden bench quite loudly. This means that sound can travel through wood, which is a solid. Now take a metre scale made of metal and hold its one end close to your ear very carefully. Ask your friend to scratch the other end of metre scale lightly. You will be able to hear the sound of scratching through the metallic metre scale quite loudly (though other persons around you cannot hear the same sound of scratching). This means that sound can travel through a metal, which is a solid. In general, we can say that sound travels through solid substances. We can also make a toy telephone as follows to show that sound travels through solids.
Take two open tin-cans, each having a small hole at the centre of its bottom. Also take about 20 metres long thick thread. Pass one end of the thread into the hole of one tin-can and hold it inside the can by tying it to a pin. Similarly, pass the other end of thread into the hole of second tin-can and tie it to another pin. The two tin-cans connected by the thread are now held by two children and taken as far as possible so that the thread gets stretched tightly (as shown in Figure).
Now, if one child speaks into one tin-can, he can be heard by the child at the other end who puts his ear to the other tin-can. For example, in Figure, the child at the left end is speaking into the toy telephone and the child at the right end (who is 20 metres away) can hear his sound clearly. In this case of toy telephone, sound made by the child while speaking, travels through the thread, which is a solid substance.
The above activities show that sound can travel through solids like wood, metal and thread. In fact, sound can travel through all the solid substances like metals (iron, steel, etc.), wood, bricks, stone and glass, etc. We will now describe an activity to show that sound can also travel through liquids. We will use water as liquid in the following activity.
Place a squeaking toy (sound making toy) in a polythene bag and immerse it in a bucket full of water. Now, if we put our ear to the side of this bucket and press the toy, we can hear the sound of squeaking toy clearly. In this case the sound of squeaking toy comes to our ear through water contained in the bucket. This shows that sound can travel through water, which is a liquid. In general, we can say that sound can travel through liquids. Dolphins and whales which live in the sea can communicate (or talk) with one another under water because sound travels through sea water (which is a liquid).
The following observations will show that sound can also travel through gases. When the telephone bell rings in our home, we can hear its sound even from a distance. In this case, the sound of ringing telephone bell travels to us through the air in the room, which is a gas (or rather a mixture of gases). When we talk to a person standing near us, then the sound of our talk travels to the other person through the air around us. The sounds of radio, television, motor cars, buses, trains, aeroplanes, and the chirping of birds, all travel through the air and reach our ears. In fact, most of the sounds which we hear in our everyday life, reach us through the air. All the above observations show that sound can travel through air, which is a gas. In general, we can say that sound can travel through gases.
From the above discussion we conclude that sound can travel through solids, liquids and gases. We will now describe an activity to show that sound cannot travel through vacuum. The word ‘vacuum’ means empty space’. Even air is not present in vacuum. Thus, when there is no air in something, we say there is vacuum. A vacuum can be created in a glass jar by removing all the air from it with the help of a suction pump called vacuum pump.
Sound Cannot Travel Through Vacuum
A material medium (like air) is necessary for the transmission of sound. Sound cannot travel through vacuum (or empty space). This can be shown by the following activity.
1. A ringing electric bell is placed inside an airtight glass jar containing air as shown in Figure (a).
We can hear the sound of ringing bell clearly. Thus, when air is present ¡n the glass jar, sound can
travel through it and reach our ears.
2. The glass jar containing ringing bell is placed over the plate of a vacuum pump [see Figure (b)]. Air is gradually removed from the glass jar by switching on the vacuum pump. As more and more air is removed from the glass jar, the sound of ringing bell becomes fainter and fainter. And when all the air is removed from the glass jar, no sound can be heard at all (though we can still see the clapper striking the bell) [see Figure (b)]. Thus, when vacuum is created in the glass jar, then the sound of ringing bell placed inside it cannot be heard. This shows that sound cannot travel through vacuum (and reach our ears).
3. If air is now put back into glass jar, the sound of ringing bell can be heard again. This shows that
air is necessary for the sound to travel from the ringing bell to our ears.
We can explain the above observations as follows : When clapper hits the bell, the bell vibrates (and makes sound). The vibrating bell makes the nearby air molecules to vibrate back and forth. These vibrating air molecules make the next layer of air molecules to vibrate, and so on. In this way, ultimately all the air molecules around the tinging bell start vibrating back and forth. When these vibrating air molecules fall on our ears, we can hear the sound of ringing bell. If, however, there is no air between the ringing bell and the ear, then the vibrations of the ringing bell cannot reach our ears and hence we cannot hear the sound of ringing bell. So, when there is vacuum in glass jar, there are no air molecules to carry sound vibrations.
The Case of Moon (or Outer Space)
The moon has no air or atmosphere at all. It is all vacuum (or empty space) on the surface of moon. Sound cannot be heard on the surface of moon because there is no air on the moon to carry the sound waves (or sound vibrations). So, we cannot talk to one another directly on the moon as we do on earth, even though we may be very close. If an astronaut talks to another astronaut on the moon, he would see the lips moving but no sound will be heard at all. This is because sound cannot travel through the vacuum which exists on the surface of moon. Similarly, there is no air (or any other gas) in outer space to carry sound waves. It is all vacuum in outer space due to which sound cannot be heard in outer space. Thus, the astronauts who land on moon (or walk in outer space) are not able to talk directly to each other. The astronauts who land on moon (or walk in outer space) talk to each other through wireless sets using radio waves. This is because radio waves can travel even through vacuum (though sound waves cannot travel through vacuum).