Propagation and Production of Sound | Science Class 9 PDF Download

PRODUCTION OF SOUND
Perform the following activities to produce sound.

Vibrating Tunning Fork
Activity

1. Take a plastic scale or ruler from your geometry box. Hold it flat on your desk or table with about half its length protruding (stick out from the surface) over the edge. Now bend it down and release it. It will move up and down rapidly (i.e. it will vibrate) and produce the sound at the same time. The sound will last as long as the vibration (i.e. rapid up and down motion) of the scale continues.

 2. Take a tuning fork. Hold it from its stem and strike it with a rubber pad or hammer. You will observe that the prongs of the tuning fork vibrate and at the same time sound is produced (Figure).

3. Place your finger lightly on your throat near the vocal cords as shown in figure. Now say "Ah" for few seconds. You will feel the vibration in your finger as long as you say "Ah".

4. Tie a thin metallic string rigidly at the two ends of a table as shown in figure. Now, pluck the string from the middle and release it. The string begins to vibrate up and down and at the same time, sound is heard.

Conclusion: From these activities, we come to the conclusion that the sound is produced by the vibrating objects or bodies.

Production of Sound in Musical Instruments

 When a drum is beaten, then the skin of drum vibrates and sound is produced.
When the strings of a guitar are plucked and released, they vibrate and produce sound. When air is blown into the flute, pipe, clarinet, sexophone etc., it vibrates in the tube of the instrument and hence sound is produced. Sound is also produced when the birds flap their wings during the flight.

 What is a Wave?
The movement of the disturbance through a medium due to the repeated periodic motion of the particles of the medium about their mean positions is known as a wave.

MECHANICAL WAVE
A mechanical wave is a periodic disturbance which requires material medium (i.e. solid, liquid or gas) for its propagation. In other words, waves that are characterised by the motion of particles of a medium are called mechanical waves

Examples of mechanical waves

(i) Sound waves in air

(ii) Water waves

(iii) Waves produced due to the earthquake (known as seismic waves)

(iv) Waves produced by supersonic jet planes (known as shock waves)

(v) Waves produced in a stretched string.

(vi) Waves produced in a slinky or long spring.

Types of waves
 Waves are of two types : (i) Transverse Wave, (ii) Longitudinal Wave

TRANSVERSE WAVE
If the particles of a medium vibrate or oscillate about their mean positions at right angles to the direction of propagation of the disturbance then the wave is called transverse wave.

Examples : Movement of string of a sitar or violin, membrance of a tabla or dholak, movement of a kink on a rope.
Activity
Describe an activity to show the formation of a transverse wave.
Fix one end of a thin rope and give up and down jerk to the free end of the rope.
The rope oscillates or vibrates up and down as shown in figure. The disturbance travels from the free end to the fixed end but the rope vibrates up and down. This wave is known as transverse wave.

A transverse wave travelling on the surface of water is shown in figure.

 When transverse wave travels through the medium, the shape of the medium changes. At some positions, the particles of the medium rise (or elevate) above their mean positions and at some positions, the particles of the medium go down (or depressed) below their mean positions. 

The point on the elevation of the medium whose distance from the mean position is maximum is known as crest (C). On the other hand, the point on the depression of the medium whose distance from the mean position is maximum is known as trough(T). Thus, crests and troughs are formed when a transverse wave travels through a medium (Figure).

 WAVELENGTH (OR LENGTH OF A WAVE)
The distance between two successive crests or between two successive troughs is known as thewavelength of a transverse wave.

OR
The distance between two successive particles of the medium which are in phase is called wavelength of the wave. It is denoted by l (lambda).

LONGITUDINAL WAVE
If the particles, of a medium vibrate or oscillate to and fro about their mean positions along the direction of propagation of the disturbance then the wave is called longitudinal wave. 

Examples :- Sound wave, Organ pipes, Vibration on resonance appratous 

Activity
Describe an activity to show the formation of longitudinal wave.

Take a slinky or a long spring which can be easily compressed and extended as shown in figure (a). Fix one end of the slinky with a rigid support. Now push the free end of the slinky in the downward direction and release it. It is observed that the slinky begins to move up and down (i.e. "to and fro") as shown in figure (b). The disturbance travels from the free end to the fixed end and the parts of the slinky vibrate along the direction of the propagation the disturbance. This wave is known as longitudinal wave.

When a longitudinal wave passes through a medium, the medium is divided into the regions ofcompressions (C) and rarefactions (R) as shown in figure (b). 

Compression
The part or region of a medium, where the density of the medium is maximum or where the particles of the medium are very close to each other is known as compression. lt is denoted by C.

Rarefaction
The part or region of a medium, where the density of the medium is minimum or where the particles of the medium are far apart from each other is known as rarefaction. It is denoted by R.

PROPAGATION OF SOUND
A vibrating body produces sound. Now we shall study, how the sound travels from one place to another place.
When a body vibrates, then the particles of the medium (say air) around the vibrating body are set into vibrations. The particles of the medium which are very close to the vibrating body are pushed away from the body. These particles of the medium strike against the neighbouring particles. Hence the number of particles of the medium in the region where the displaced particles strike against the neighbouring particles is large. This region is known as compression (C). Since pressure is directly proportional to the number of particles, so the compression is a region of high pressure or high density. When the vibrating body moves backward, a region of emptiness known as rarefaction (R) or a region of low pressure or Low density is created.The displaced particles of the medium rebound into the region of low pressure or rarefaction. At the same time, compression is followed outwards. Therefore, when a body vibrates to produce sound, compressions and rarefactions follow one another as the sound waves travel through the' medium away from the vibrating body. When a sound wave travels through a medium, the particles of the medium simply vibrate about their rest positions and they do not move from one place to another place in the medium.
 Figure represents the regions of compressions (or high pressures) and o rarefactions (or low pressures) as the sound propagates in the medium. 

Sound needs a medium to travel
We have learnt that sound travels from one place to another place when the energy is transferred from one particle to another particle of a medium like air or gas, liquid, solid etc. It means, sound needs a material medium for its propagation. In other words, sound cannot travel through vacuum.

Demonstration to show that sound waves cannot travel through vacuum.
Put an electric bell inside a closed Bell jar connected with a vacuum pump. Initially, air from the jar is not taken out. Connect the electric bell with a battery (Figute). It rings and the sound produced is heard by us.

Now start evacuating the air from a Bell jar using a vacuum pump, we will hear less and less sound. i.e. the loudness of the sound decreases. When there is no air in the Bell jar, we do not hear sound. This activity demonstrates that sound waves require material medium (in this case air) for its propagation.

Sound waves are longitudinal waves
When a sound wave travels through the material medium, then compressions and rarefactions follow one another. The particles of the medium through which a sound wave travels vibrate to and fro about their mean positions parallel to the direction of propagation of the sound wave. Since the wave is known as longitudinal wave, if the particles of the medium vibrate to and fro about their mean positions parallel to the direction of propagation of the wave, therefore, the sound waves are longitudinal waves.