What Is Sound?

 Light and sound are both waves. However, the former can travel through a vacuum while the latter cannot. So what is sound and how does it propagate as a wave?

Sound is actually a pressure wave. When an object vibrates, it creates a mechanical disturbance in the medium in which it is directly adjacent to. Usually, the medium is air. The medium then carries the disturbance in the form of oscillating and propagating pressure waves.
The frequency of the waves are dependent on the frequency of the vibrating source. If the frequency of the vibrating source is high, then the sound wave will also have a high frequency. The sounds that we hear, from the voice of the person right next to you, to the music coming from your iPod earphones, to the crashing noise of shattered glass, all come from a vibrating source.
As the sound waves propagate through a medium, the pressure at a localized region in the medium alternates between compressions and rarefactions (or decompressions). Thus, if at one instant, a region in the medium experiences compression, the regions adjacent to it along the line of propagation are expected to be experiencing rarefactions.
Then as time progresses, the region in question undergoes a rarefaction while those adjacent to it undergo compressions. Therefore, if no medium exists, then the compressions and rarefactions cannot occur.
Now, how does one hear sounds? Remember how a source has to vibrate to produce a sound wave, and how a vibrating medium (e.g. air) has to exist to allow the sound wave to propagate? In the same manner, the receiver of the sound has to have something that can vibrate in order to ‘interpret’ the sound carried by the vibrating medium.

In the case of our ears, our eardrums serve as the receivers. When the vibrating air reaches our eardrums, it causes our eardrums to vibrate as well. The eardrums then transmit these vibrations to tiny bones in the middle ear, and so on until they reach the inner ear where the oscillating pressures are converted into electrical signals and sent to the brain.
Our ears are sensitive to vibrations between 20 to 20,000 Hz. Normally, frequencies that are higher or lower than the range provided cannot be processed by our auditory system. Young kids however, are able to hear slightly higher frequencies. That means, the range over which we are sensitive to diminishes as we grow older.

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