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When you call your friend who is standing at a distance, your friend can hear your voice. How is it possible for your friend to hear your voice?
He can hear because your sound travels from one place to another. As we saw earlier, the sound is a form of energy, and it needs a medium to travel. It is clear from an experiment that sound cannot travel in a vacuum, and it needs a medium like air. Sound travels in water and solids also. The sound speed is more in solids than in liquids, and it is significantly less in gases.
The sound produced by the boy travels in air medium
Speed of sound:
The speed of sound is the distance travelled by sound in one second. It is denoted by '\(v\)'. It is represented by the expression, \(v = nλ\), where '\(n\)' is the frequency and '\(λ\)' is the wavelength.

The speed of the sound depends on the properties of the medium through which it travels, like temperature, pressure and humidity. In any medium, as the temperature increases, the speed of sound also increases. For example, the sound speed in air is 331m/s at \(0°C\) and 344m/s at \(22°C\).

Vibration produced by tuning fork
We saw that sound travels in a different medium with different speeds. Now let us see how it travels in a medium. When a body vibrates, the medium's particle in contact with the vibrating body is first displaced from its equilibrium position. It then applies a force on the adjacent particle. This process continues in the medium till the sound reaches the ear of the person.
To understand this, let us consider a vibrating tuning fork. When a vibrating tuning fork moves forward, it pushes and compresses the air in front of it, creating a high-pressure region. This region is called a compression (\(C\)). When it moves backwards, it makes a region of low pressure called rarefaction (\(R\)). These compressions and rarefactions produce the sound wave, which propagates through the medium.
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Region A\(-\) Rarefaction
Region B\(-\) Compression