Sound

The sound is the form of energy, which is produced by vibration of molecules of a body. Sound travels in the form of a wave. Like light, it also undergoes reflection, refraction etc.

Wave:

The path of energy, through which the energy travels is called a wave. The wave can also be defined as the periodic disturbance of the particles of the medium. The waves can be classified into two types’ mechanical waves and electromagnetic wave. Mechanical waves can be propagated only in material medium whereas electromagnetic waves do not require any material medium for propagation.

Wave in water surface

when a stone is dropped into a pool of water, circular waves spread out on all directions on the surface from the center of disturbance. There are circular regions called crest, where the water level is above the normal level and there are other circular regions, called trough, where the water level is below the normal level. Water waves on the surface of water are the examples of wave motion.

Waves in rope:

If a rope held horizontally, is fixed at one end and the free end is moved up and down at right angle, a disturbance consisting of a series of crests and troughs travel along the rope.

Sound wave:

A wave of compression and rarefaction, by which sound is propagated in an elastic medium such as air is known as sound wave.

Disturbance set up in a medium is called a wave and the propagation of disturbance, without involving transfer of any material within it, is called wave motion.

 Factors influencing the velocity of sound:

1. Density: The velocity of sound decreases with increase in the density of the medium.
2. Temperature: The velocity of the sound increases with increase in temperature.
3. Humidity: The velocity of sound in air increases with increase in humidity.
4. Pressure: The velocity of sound is independent o pressure.
5. Direction of air flow: If the sound propagation take place in the direction of air flow then its velocity increases.

Types of wave motion are:

Waves can be classified into two types on the basis of the nature of the vibration in particles of the medium. They are transverse wave and longitudinal wave.

Comparison of transverse and longitudinal wave are as follows:

Transverse waves	Longitudinal waves
Vibrations of particles of a medium are perpendicular to the direction of propagation of wave.	Vibrations of particles of a medium are along the direction of propagation of wave.
These waves can be generated in solids and liquids.	These waves can be generated in solids, liquids and gases.
A complete wave consists of a crest and trough	A complete wave consists of a compression and rarefaction.
It can travel in vacuum.	It cannot travel in vacuum.
Speed of this wave is more.	Speed of this wave is less.

 

Noise Pollution:

The sound which produces an unpleasant effect is called noise. It is a unwanted sound.

Effects of sound pollution are as follows:

• Hearing loss, harsh effects in ears
•  Mental illness
• Causes the different disease like, increase in sugar level, blood pressure etc

Ways to reduce the sound pollution are as follows:

•  By using the sound absorbing materials
• Shifting the noisy source away from the sources to the affected regions

Reflection of Sound:

When sound travels from one medium to another medium, it strikes the surface of the medium and bounces or return back in some other direction, this phenomenon of returning back of sound is called the reflection of sound. Echo and reverberation are the products of reflection of sound.

Echo: The repetition of sound, which is reflected from walls of large room or distant surface is called echo.

Conditions required for echo are as follows:

The distance between the source of sound and reflecting body should be more than 17m
The reflector should be hard and of large area.
 

Reverberation: If the distance between the source of sound and reflecting body is less than 17m, original sound and reflected sound mix and the sound is prolonged. This process is known as reverberation. It is heard in an empty room or newly built building.

Refraction of Sound:

Refraction is the process of bending of waves when they pass from one medium to another where their speed is changed or different. Refraction of sound is not important as the refraction of light but the refraction of sound is an interesting phenomenon in sound. As we know that sound in heard clearer at night than in day- time. This is due to the refraction of the sound wave.

During the day- time, the temperature of the air near the land is more than the temperature of the air above it. As we go up, the layer of air behave as a denser medium and hence the ray of sound diverges upwards from the source and as a result less sound is heard by the listener. This phenomenon is just opposite during the night.

The layer of air near the land or ground behave as a denser medium and the upper layer as a rarer medium. Here, the ray diverging upwards from the source is refracted away from the normal and undergoes total internal reflection and travels downwards. As a result, sound is heard more distinct and clear by the listener.