December 28, 2015


1. Introduction - What is Sound?

Sound is a sensation that is always all around us. We hear sounds when objects strike or any disturbance suddenly happens in air. But if we close our ears, we cannot hear any sound. Sounds are of different kinds. Some sounds are loud, other sounds are low or soft. Some sounds have a higher pitch. A scream is at a high pitch. Girls have a higher pitch of sound than boys. We know and understand pitch directly by our sensation of it i.e. we can directly know what is at a higher pitch and what is at a lower pitch. Gradation of pitch is after all the musical notes – “Sa”, “re”, “ga”, “ma”, “pa”, “dha”, “nee”.

Sounds coming from far are heard faintly and those coming from near are heard more clearly. There is also a quality of a sound. We can recognize our friend’s voice by the quality of his voice.

Now these facts seem straight forward but they are actually wondrous facts – in nature. They actually raise many questions. We normally do not ask these questions because we take these facts for granted having got used to them from childhood. We feel they are like that because they are like that.

But these facts suggest many deep questions.

What is sound? This question means that, what is happening really between the source of sound and our ear? When we clap our hands (for example), we hear a distinct sound but the real question is, what happened quickly between the point of clapping and our ear? It is a wnder that something happens, isn’t it? What is it? To put the question in another way, what is the physical basis of our sensation of sound.

The next deep question that can be asked about the facts given before is, when is a sound heard loudly and when softly?

The third question that can be asked is when is the pitch high and when is the pitch low.

You see, something invisible is happening between the source of the sound and our ear.

What is it?

Let us now proceed to answer this question.

The ancients asked these same questions and suspected that air has something to do with propogation of sound.

There is the famous bell jar experiment that can be done.

Take a jar and put a bell with connection in it. When the top is not sealed and the switch is on, the bell rings and we can hear the ringing of the bell.

Now, remove the air inside the jar using a vacuum pump and seal the top. Again put the switch on, we see the bell shaking i.e. ringing but we don’t hear anything.

Yes! Air is part of the process of transmission of sound to the ear.

But the question still remains, what is the role that air is playing? What happens to the air?

In the 10th century a remarkable fact came into light that an inch of any material – solid, liquid or gas is made up of crores & crores of atoms or combination of atoms called molecules.

Keeping this in mind, it is obvious that something internal is happening that causes the disturbance that reaches our ear and causes in us the sensation of sound.

One can understand this by realizing that sound propogation is invisible and something must be happening inside and in the air..

Now the gases in a region, or liquid or a solid contains atoms / molecules in close proximity. Of course they are closer in solids than in liquids and closer in liquids than in gases but atoms are close.

Now atoms when brought together have the property of elasticity. They are like a balls with springs attached in the middle. When we shout, (lets say) or 2 things strike, the molecules immediately beside the source get disturbed and get compressed. But since the molecules are connected like springs and are elastic, they snap back but in going back to their original position, they do not come back exactly to the original position but go an extra distance. This pushes the next immediate air that gets compressed and that too tries to come back to the original position and goes an extra distance and compresses the next immediate air and so on. A wave of compressions (more pressure) and what is called rarefactions (less pressure, molecules more apart) occur.

The above is only for one hit of the source. But the source is continuously giving pressure, so a series of waves is sent out as long as the source is vibrated.

The wave travels out and it is energy and due to the pressure increase loses ultimately it’s energy due to getting converted to heat. Hence we cannot hear sounds (or only very faintly) at a distance.

2. Pitch and loudness depends on frequency and amplitude

Let us try to fully grasp the above point and what it implies.

There are 2 things that are important properties of sound as mentioned earlier – loudness and pitch.

Let us now understand both these facts with our deeper understanding of sound as a series of compressions and rarefactions.

Lets take loudness first. When we speak softly, we are putting less pressure an air so the (maximum) compression (or rarefaction) is less.

When we speak loudly, we are putting more pressure on air so the (max) compression or rarefaction is more. So, the greater the compression from normal state, greater is the volume or loudness!

Take a spoon and bang on the table lightly, the sound is low! Bang it hard, the sound is louder. Again the same principle applies. If the pressure is more, compression is more and hence loudness is more.

A tyre or balloon bursts i.e. gives a loud sound because the air suddenly shoots out, the pressure is more and hence the compression (max) is more and hence the ear grasps that as a loud sound and gives us a loud sensation.

Wherever there is a loud sound there is high pressure i.e. high (max) compression and hence high volume.

The maximum compression is called by the technical word amplitude.

So more amplitude – distance from mean position, the greater is the loudness of the sound.

The next question is with respect to pitch. What about pitch? What determines pitch? How do we relate the deeper nature of sound (as having compressions and rarefactions) with the pitch.

Why do girls have high pitch and boys lower pitch. Why does “Nee” note have higher pitch than “re” note.

The answer is that when there are more vibrations per second i.e., the number of successive compression and rarefaction in one second is more, the pitch is more.

This number of successive compressions and rarefactions per second is called by the name frequency.

Hence if frequency is more, the pitch is high.