What Sound Needs to Travel?

 

SOUND NEEDS A MEDIUM TO TRAVEL

 

Since sound is a mechanical wave, it needs a material medium for its propagation and cannot travel through vacuum. This can be demonstrated by the following experiment.

 

Let us consider an electric bell, E contained in a bell-jar. The electric bell is connected to a battery, B through a key, K as shown in Fig. 6.3. If we insert the plug in the key K, the electric circuit is closed and a sound is heard. When we go on taking air out of the bell-jar, the sound produced by the bell goes on getting fainter and fainter. If the process of evacuating the jar is continued so that a near perfect vacuum is created within it, we shall hear practically no sound though the hammer H of the bell will be seen to strike the bell and create sound. This sound is not heard as there is no medium in the jar to carry the sound to the listener.

 

Thus, a material medium is essential for the propagation of sound.

 

We have so far learnt that :

 

(i) to produce sound, we must supply energy for the vibration of the source and

(ii) for the onward transmission of this energy, i.e., sound, we must provide a material medium. When we say that sound is travelling from a vibrating source, all that we understand is that energy (IE. sound) emitted by the source is being transmitted through the medium in the direction of sound When this eneres falls on the ear drum of a listener, it produces in him the sensation of hearing. Though vibrations of any frequency are able to carry energy through the medium, yet only those vibrations which lie in the frequency range of 20 Hz to 20 kHz produce sensation of hearing when they fall on the ear. By sound, we therefore, normally mean that portion of vibratory energy which produces in us the sensation of hearing. Thus,

 

Sound is a form of energy which is emitted by a vibrating source and transmitted through a material medium producing in us the sensation of hearing.

 

The waves that carry sound energy are called sound waves.

 

NOTE

 

Some interesting consequences of the role of medium are as follows.

1. Due to the absence of atmosphere (i.e., a material medium), two astronauts cannot talk to each other on the Moon as they do on the Earth.

2. For similar reason, one cannot hear a bomb explosion on the а Moon as it has is no atmosphere.

3. We can be heard in an adjoining room even when we are talking in another room with doors tightly shut. This is due to the reason that even though the doors of the rooms are shut, these are connected to each other through air, walls and the material of shut doors, i.e., a material medium is there to carry sound from one room to the other.

 

PRODUCTION OF COMPRESSIONS AND RAREFACTIONS NEAR A SOURCE OF SOUND

 

As said earlier, a source of sound puts the particles of the medium into vibratory motion. Though the medium does not bodily move from the source to the listener, there occur changes in its density and pressure sound propagates through it. Air is the most common medium through which sound travels and it does so the help of intervening layers of air. Let us consider a vibrating tuning fork as a source of sound and confine our attention to its right hand prong only.

(i) When the right hand prong moves from left extreme (L) towards the right extreme (R), it compresses the layer of air in front of it. As a result of this, the pressure (as well as density) of this layer increases. This layer (or regions of compressed air is called a compression. Now, this compression compresses the layers next to it and thus a pulse of compression travels towards right as shown in Fig. 6.4 (a).

 

(ii) When the prong moves from its right extreme (R) to the left extreme (L), the air in front of the prong expands (i.e., gets rarefied). As a result of this, pressure (as well as density) of this layer decreases. This region of rarefied air is called a rarefaction. It follows the earlier compression, which by that time has moved forward as shown in Fig. 6.4 (b). Thus, in one complete vibration of the prong (i.e., from L to R and back from R to L), one compression and one rarefaction are formed.

 

(iii) As long as the prong continues to vibrate, compressions and rarefactions are sent out in regular succession. These compressions and rarefactions travelling towards right and alternating with each other, constitute a sound wave as shown in Fig. 6.4 (c).

 

(iv) The left hand prong sends sound wave towards the left in a similar way in the form of compressions and rarefactions.

 

(v) A compression is formed due to an increase in pressure and consequently an increase in density of the medium. Conversely, a rarefaction is formed due to a decrease in pressure and consequently a decrease in density. We know that a sound wave propagates as a series of compressions and rarefactions. Thus, a sound wave can be considered as propagation of pressure or density variations in the medium.  

 

SOUND WAVES ARE LONGITUDINAL WAVES

 

In order to understand the nature of longitudinal waves, let us take a slinky AB (a slinky is a toy in the form of a long flexible spring which can be very easily extended or compressed) and arrange it in the horizontal position with its end B fixed. Initially, when the slinky is neither compressed nor stretched, there is a fixed distance between its loops.

 

(i) When the free end A of the slinky is pushed forward slightly, a few loops near it are compressed. This region where the loops of the slinky are closer to each other than the normal distance is called a compression. The compression (C) so formed travels along the slinky till it reaches B.

 

(ii) If the free end of slinky is pulled outwards, a few loops near it are pulled away from each other. This region where the loops of the slinky are farther apart than the normal distance is called a rarefaction. The rarefaction (R) so formed also travels along the slinky and follows the compression produced earlier.