What Is Sound? Vibration And The Propagation Of Sound

Have you ever used a musical instrument like a guitar, drum or a violin? When you strike the strings of a guitar, we hear a sound. Same with the drum, when we hit a drum we hear a ‘thump’ sound. Also how is one instrument able to create a wide variety of sounds? How does this happen? What is sound? How do we make sound? How is sound produced and about propagation of sound ?

What is Sound?

Sound is a form of energy, just like electricity, heat or light. Let’s examine some sources of sounds like a bell. When you strike a bell, it makes a loud ringing noise. Now instead of just listening to the bell, put your finger on the bell after you have struck it. Can you feel it vibrating? This is the key to sound. It is even more evident in guitars and drums. You can see the wires vibrating every time you pluck it. When the bell or the guitar stops vibrating, the sound also stops.

This to and fro motion of the body is termed as Vibration. You can see examples of vibrations everywhere. Vibrating objects produce sound. Some vibrations are visible, some aren’t. If you pull and then release a stretched rubber band, the band moves to and fro about the central axis and while doing so it also produces a sound. The sound moves through a medium by alternatively contracting and expanding parts of the medium it is travelling through.  A medium is essential for the propagation of sound. Sound cannot travel through vacuum due the fact that there are no molecules that can be compressed and expanded in space. Our voice too is produced by the vibration of strings known as the vocal chords which is inside the Adam’s apple. When you make a sound, its vibration travel through air and when it reaches your brain through your ears, it is interpreted as sound. In this case propagation of sound takes place through air medium. How your brain and ear decode pressure variation in sound waves into sound is fascinating. If you want to know more, please visit the article on The Human Ear.

Propagation Of Sound

Propagation of sound:

Sound is pictorially represented by a continuous succession of peaks and valleys. The distance between two consecutive peak or trough is termed as the wavelength of the wave or the period. The number of cycles per unit time is termed as the frequency of that sound. Frequency is measured in cycles per second, or Hertz. The faster an object vibrates, i.e. the higher the frequency, then the higher the pitch of the sound. The difference between a man’s voice and woman’s must be clearly evident to you. The voice of a man has a lower frequency which contributes to the deepness or the bass in the voice. Women, in contrast, have a voice with higher frequency resulting in a higher shrillness or pitch (will be discussed in later articles).

Propagation Of Sound

Let an electric bell be enclosed within a bell jar which is placed over a disc. Through a hole in the disc, air can be removed by using a vacuum pump. On gradually removing the air, the sound of the bell becomes feebler and eventually becomes inaudible when all the air is removed from the tumbler. Why so?

Let’s understand this using a real life example. If you hear a very loud sound, what do you do? You cover your ears. How do you think that helps? When you cover your ears, you shut off the air inside your ears from the rest of the atmosphere. The sound waves travelling around you are now unable to get through to your ear or the intensity of the sound you hear is greatly reduced. Your blocking your ears creates a discontinuity in the medium due to which the flow of sound energy is disturbed. Through this we can make a very important observation; Sound waves rely on the medium for propagation. The propagation of sound wave is not possible through vacuum. The medium here can be gas, liquid or solid. The speed of sound when it is travelling through a medium depends on the type of medium. The speed of sound when travelling through air is 343 m/s or 1,235 km/h.

Practise This Question

State true or false.
Particles in a longitudinal wave move perpendicular to the direction of the wave propagation.