RESUME EXEL VIII

SOUND

The definition of sound
Sound is a longitudinal wave which spreads in the from of compression and rarefaction.
Sound is produced by an oscillation of matter.
Sound, which is produced by a vibrating object (source of sound), spreads out in air in the form of a longitudinal wave.
If you hit a drum, the membrane of the drum will vibrate. The air molecules surrounding the drum vibrate too, and longitudinal waves spread out to all directions. If the waves reach your ears, you will hear the sound of the drum.
Sound energy can spread out in solids, liquids, and gases (such as, wood, glass, steel, water, and air). Therefore, sound needs a medium. Without a medium to spread out, the sound will not occur.
On the surface of the moon there is no atmosphere, so there is no medium to spread out sound waves.


The sound of the electric bell which is placed in the bell jar can still be heard because there is air in the jar. After the air in the jar is pumped out, there is a vacuum, so the sound of the electric bell cannot be heard. This shows that sound cannot spread out in a vacuum.
So, we can hear a sound if there is a sound source and a medium that transmits the sound from its source to our ears.
Speed of sound depends on two things:
1. temperature of medium
2. kind of medium( solid, liquid and gas )
Air is a medium that is most often passed by sound waves. Sound can spread out through different kinds of media. Liquids and solids are better media than air because the particles in liquids or solids produce stronger interaction than the particles of air do. This causes the movement of sound waves in solids and liquids to be easier than in air.
In the rainy season, we often see a flash of lightning followed by a sound of thunder within a few seconds. This happens because the position of the lightning is far from you. Actually, the flash of lightning and the sound of thunder occur at the same time. But, you hear the sound of the thunder after you have seen the flash. This is fact indicates that the speed of light is faster than the speed of sound. Light travels at the speed of 300,000,000 m/s in a vacuum while the speed of sound is 330 m/s.
The speed of sound in the air depends on its temperature. The higher the temperature of the air, the faster the speed of sound, and vice versa.
The speed of sound can be formulated :

v= s/t

Note : v = the speed of sound ( m/s )
s = distance ( m)
t = time (s )