HOW SOUND TRAVELS



Compressional Waves

Sounds are vibrations that travel through the air [or another material], produced by vibrating objects. How can we picture the movement of the molecules of the air as a sound travels through? Figure above shows how the vibrations of a turning fork are transmitted through the air. As the prong of the fork moves to the right, it pushes on the air molecules on that side, squashing them together. These molecules push on their neighbours, which become compressed, and which in turn push on their neighbours, and so on.
        It is important to note that the individual air molecules do not travel outwards from the vibrating fork. The air molecules are merely pushed back and forth. It is the vibrations that travel through the air to our ears.
        This picture of how a sound travels also explains why sound cannot travel through a vacuum.  There are no molecules or other particles in a vacuum to vibrate back and forth.
        Figure above also shows another way of representing a sound, as a wavy line rather like the trace on an oscilloscope screen. The crests on the wave match the compressions, and the troughs match the rarefactions. It is much easier to represent a sound as an up-and-down wave like this, rather than a drawing lots of air molecules pushing each other back and forth.
        Here we have used two different models to represent sound:

  1. vibrations travelling through a material - the particles of the material are alternately compressed together and the rarefied as the sound passes through
  2. sound as a wave - a smoothly varying up-and-down line, like the trace on an oscilloscope screen. The first of the these models gives a better picture of what we could see if we could observe the particles of the material through which the sound is passing. The second model is easier to draw. It also explains why we talk about sound waves. The wavy line is rather like the  shape of waves on the sea. There is much more about sound waves [and other waves].
Compression, rarefaction 
Look back to figure above. The areas of the sound wave where the air molecules are close together are called compressions. As the tuning fork vibrates back and forth, compressions are sent out into the air all around it. In between the compressions are rarefactions, areas in which the air molecules are less closely packed together, or rarefied.
         The sound wave has been drawn so that the crests on the wave match the compressions, and the troughs match the rarefactions. Thus the wave represents the changes in air pressure as the sound travels from its source.












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