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How is sound wave produced? Do Sound Waves Need a Medium for their Propagation?
If we touch a silent bicycle bell, we do not find anything special in it. Let us now ring the bicycle bell and touch it gently again. We will find that a ringing bicycle bell (which is producing sound) is vibrating, that is, it is moving back and forth continuously through a very small distance.
Now, if we hold this ringing bell tightly with our hand, it stops vibrating and the sound coming from it also stops. It is clear that when the bicycle bell is vibrating, it is producing sound, and when the bicycle bell stops vibrating, the sound also stops. From this discussion we conclude that sound is produced when an object vibrates (moves back and forth rapidly).
In other words, sound is produced by vibrating objects. Whenever we hear a sound, then some material must be vibrating to produce that sound. A vibrating object, which produces sound, has a certain amount of energy which travels in the form of sound waves. The energy required to make an object vibrate and produce sound is provided by some outside source (like our hand, wind, etc.).
The buzzing sound of a bee or mosquito is produced by the vibrations of their wings; the sound in a sitar, veena or guitar is produced by the vibrations of stretched strings (stretched wires); the sound of our voice is produced by the vibrations of two vocal cords in our throat caused by air coming from the lungs; the sound of a drum (or tabla) is produced by the vibrations of its skin (or membrane) when struck; the sound of a flute (bansuri) is produced by the vibrations of air enclosed in the flute tube; the sound of school bell is produced by the vibrations of an iron or brass plate when it is hit by a hammer; and the sound in a radio (or television) is produced by the vibrations of the thin diaphragm of a speaker. In most of the cases, a sound producing object vibrates so fast that we cannot see its vibrations with our eyes.
In the laboratory experiments, sound is produced by vibrating tuning forks. A tuning fork has two hard steel prongs A and B connected at the lower end to a stem or handle H (see Figure). When the prongs of the tuning fork are struck on a hard rubber pad, the prongs vibrate and a sound is produced. If we look at the prongs of the sounding tuning fork, they appear to be blurred as if they were moving back and forth rapidly.
The vibrations of the tuning fork can be shown by touching a small suspended pith ball (cork ball) with a prong of the sounding tuning fork. The pith ball is pushed away with a great force (as shown in Figure). The pith ball can be pushed away only if the prong B moves to the right side (while vibrating) and strikes it hard. This experiment shows that the prongs of a sound producing tuning fork are vibrating.
We can also show the vibrations of the prongs of a sound making tuning fork by performing another simple experiment as follows : We fill water in a beaker up to its brim. Touch the surface of water with the prongs of a sound making tuning fork (which has been struck on a hard rubber pad).
The prongs of sound making tuning fork splash water (see Figure). This shows that the prongs of a sound producing tuning fork are vibrating (moving forwards and backwards rapidly). Please note that we cannot see the ends of a tuning fork vibrating because the vibrations are too fast.
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 (as in a flute),
- By vibrating membranes (as in a drum), and
- By vibrating plates (as in cymbals)
We will now describe how sound waves are produced in air by the vibrating bodies and how sound is transmitted from the sound-producing body to our ears.
Sound Waves in Air
Sound waves in air consist of compressions and rarefactions of air. We will now describe how sound waves are set up in air by a vibrating body.
Suppose the original position of the air layers (when no sound is passing through it) is as shown in Figure (a). We now strike a tuning fork against a rubber pad so that both the prongs start vibrating back and forth continuously and produce sound (To make things simple, we will consider only one prong A of the tuning fork).
Production of sound wave in air
When prong A moves outwards to the right side, it pushes the layer of air in front of it [see Figure (b)]- This air layer pushes the next air layer, and this process goes on. In this way, the layers of air near the prong A are compressed to form a ‘compression’ (which is a region of high pressure).
This compression is passed on to the next layers by the vibrating air layers. Thus, a compression travels towards the right side but the air layers do not move bodily. The air layers only vibrate back and forth at the same place.
Since the tuning fork is vibrating continuously, its prong A now moves to the left side of the original position [see Figure(c)]. Now, when prong A moves inwards to the left side, it leaves a region of low pressure on the right hand side and the air layers move apart to form a ‘rarefaction’.
In this rarefaction, the air layers are farther apart than normal. The rarefaction is passed on to the next layers by the vibrating air layers. Thus, a rarefaction travels towards the right side but the air layers do not move bodily. The air layers keep vibrating back and forth at the same place.
In this way, a compression is followed by a rarefaction, and a rarefaction is followed by a compression, and so on. This process is repeated as long as the tuning fork is vibrating and producing sound. Thus, the net result of a sound producing body (here a tuning fork) is that it sends the waves consisting of alternate compressions and rarefactions in air [as shown in Figure (d)].
When these waves of compressions and rarefactions of air fall on our ears, the ear drums vibrate accordingly and reproduce the sound. And we can hear the sound being produced by the vibrating tuning fork.
The sound waves in air are longitudinal waves. When a sound wave passes through the air, the layers of air vibrate back and forth in the same direction in which the sound wave travels. Please note that the layers of air consist of molecules of gases of air. So, when we say that the air layers vibrate back and forth, we actually mean that the molecules in air layers vibrate back and forth by a small distance.
Propagation of Sound (or Transmission of Sound)
We will now describe how sound reaches from a vibrating body (source of sound) to our ears. When an object vibrates (and makes sound), then the air layers around it also start vibrating in exactly the same way and carry sound waves from the sound producing object to our ears. Suppose a tuning fork is vibrating and producing sound waves in air (see Figure).
Since the prongs of the tuning fork are vibrating, the individual layers of air are also vibrating. For example, the air layers A, B and C in Figure are continuously vibrating through a very small distance on either side of their original undisturbed positions.
Sound travels in the form of longitudinal waves in which the back and forth vibrations of the air layers are in the same direction as the movement of sound wave. For example, in Figure, the back and forth vibrations of the air layers are in the horizontal direction and the sound wave also travels in the same direction (horizontal direction).
Please note that in the transmission of sound through air, there is no actual movement of air from the sound-producing body to our ear. The air layers only vibrate back and forth, and transfer the sound energy from one layer to the next layer till it reaches our ear. This point will become more clear from the following example.
If we turn on a gas tap for a few seconds, a person standing a few metres away will hear the sound of escaping gas first and the smell of gas reaches him afterwards. This can be explained as follows. The sound of gas travels through the vibrations of air layers so it reaches first, but the smell of gas reaches the person through the actual movement of the air layers, which takes more time.
It is clear that the sound is not being transmitted by the actual movement of air from the gas tap to the person, otherwise he would hear and smell the gas at the same time. We will now discuss the amplitude and frequency of a sound wave.
The maximum distance moved by a vibrating air layer on either side of its original position is known as the amplitude of the sound wave (see Figure). The amplitude of a sound wave is usually a very, very small fraction of a centimetre.
The number of complete back and forth vibrations of an air layer in one second is known as the frequency of the sound wave. The frequency of a sound wave is the same as that of the vibrating body which produces the sound. Suppose the frequency of a tuning fork is 256 hertz.
This means that when this tuning fork is vibrating and making sound, then the layers of air make 256 complete vibrations per second (as the sound wave passes through the air). So, the frequency of this sound wave will also be 256 hertz.
Sound Needs A Material Medium To Travel
The substance through which sound travels is called a medium. The medium can be a solid substance, a liquid or a gas. Solids, liquids and gases are called material media (media is the plural of medium). Sound needs a material medium like solid, liquid or gas to travel and be heard. In other words, sound can travel through solids, liquids and gases but it cannot travel through vacuum (or empty space).
Please note that sound waves are called mechanical waves because they need a material medium (like solid, liquid or gas) for their propagation. The sound waves involve the vibrations of the particles of the medium through which they travel.
On the other hand, light waves and radio waves are called electromagnetic waves because they do not need a material medium (like, solid, liquid or gas) for their propagation, they can travel even through vacuum. An electromagnetic wave involves the electric and magnetic fields of the empty space (or vacuum).
From this discussion we conclude that though sound waves cannot travel through vacuum but light waves and radio waves can travel even through vacuum. We will now describe some experiments which will show that sound can travel through solids, liquids and gases, but not through vacuum.
Sound Can Travel Through Solids, Liquids and Gases
If a train is very far away from us, we cannot hear its sound through the air. But if we put our ear to the railway line, then we can hear the sound of the coming train even if it is quite far away. This shows that sound can travel through the railway line which is a solid substance made of steel.
In fact, sound travels about 15 times faster in steel than in air. Let us take another example. The little children in our homes use the toy telephone in which two tins are connected by a thread. If a child speaks into one tin, he can be heard by another child who puts his ear to the other tin. In this case the sound vibrations are transmitted by the thread, which is a solid.
We will now describe an experiment to show that sound can travel through liquids. Let us place a squeaking toy (sound making toy) in a polythene bag and hold it in a bucket full of water. We put our ear to the side of the bucket and squeeze the toy. We can hear the squeak.
This shows that sound can travel through water, which is a liquid. This fact has been used in the detection of submarines hidden under the sea. The sound of the engines of submarines is transmitted through the sea-water and this sound is detected by special hearing-aids called hydrophones.
We will now list some observations which will show that sound can 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 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 these observations show that sound can travel through air, which is a gas.
From the above discussion we conclude that sound can travel through solids, liquids and gases. We will now describe an experiment 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 vessel by removing all the air from it with the help of a suction pump, called vacuum pump. Let us describe the experiment now.
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 experiment.
1. A ringing electric bell is placed inside an airtight glass jar (called bell jar) containing air as shown in Figure (a). We can hear the sound of ringing bell clearly. Thus, when air is present as medium in the bell jar, sound can travel through it and reach our ears.
2. The bell jar containing ringing bell is placed over the plate of a vacuum pump [see Figure(b)]. Air is gradually removed from the bell jar by switching on the vacuum pump. As more and more air is removed from the bell jar, the sound of ringing bell becomes fainter and fainter.
And when all the air is removed from the bell 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 bell 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 bell 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. This happens 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 ringing bell start vibrating back and forth.
The vibrations of air molecules present inside the bell jar are transmitted to the outside air molecules by the glass wall of the bell jar. Due to this, the air molecules outside the bell jar also start vibrating in the same way. When these vibrating air molecules fall on our ears, we can hear the sound of ringing bell.
If, however, there is no air in the bell jar, 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.
Please note that sound can travel through solids, liquids and gases because the molecules of solids, liquids and gases carry the 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 ‘ear’.
The Case of Moon and 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 directly 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. 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 one another. The astronauts who land on moon (or walk in outer space) talk to one another through wireless sets using radio waves. This is because radio waves can travel even through vacuum (though sound waves cannot travel through vacuum).