Sound theory: Propagation of sound

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Sound is diffused in the air through multiple collisions between particles. Let's take a loudspeaker as our sound source. The membrane moves backwards and forwards following the amplitude of the electric signal applied to it (for a detailed description see the section on loudspeakers [Sound diffusion systems ] ) and this makes the air particles move by compressing and dilating them.

Sound theory - Compression (C = compression) and dilation (R = rarefaction) of particles in the air

Compression (C= compression) and dilation (R = rarefaction) of particles in the air

Let's now follow the sound diffusing from its source (the loudspeaker). To start off with we'll presume that a compression takes place towards the right followed by a dilation towards the left.

Sound theory - Movement of a loudspeaker

Movement of a loudspeaker

The loudspeaker moves and pushes the air particles on its right (phase a) causing compression to take place. These particles then push their adjacent particles and transfer the energy they have received from the loudspeaker to them. The loudspeaker then moves back again and carries out a compression in the opposite direction, in other words a dilation towards the left takes place (phase b) and in doing so forms an air gap that gets filled up by the air particles that are immediately close by. These particles in turn create other gaps to their right, and so on and so forth. This process allows the particles to transfer their energy to each other by oscillating without physically moving in the sound's direction.

As an example let's consider a cork floating in a pool of water and let's then imagine flinging a stone into it. The cork will bop up and down as the wave generated by the stone diffuses through the water, but it won't be moved in the wave's direction.

If the loudspeaker is driven by a sinusoidal signal, the atmospheric pressure in its immediate proximity will also have a sinusoidal rate as described in the following figure:

Sound theory - Sinusoidal rate of the atmospheric pressure

Sinusoidal rate of the atmospheric pressure


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