Physics - Representing sound waves
Representing sound waves
Sound waves can be represented using two different types of approach. The
more commonly used is the 'pressure variation' graph. On the vertical axis
the value of air pressure is graphed in terms of how much the air pressure
changes from the usual atmospheric pressure values (or normal atmospheric
pressure). What is of interest here is the variation in pressure rather
than the actual air pressure values. Pressure variation can be plotted
against the distance from the sound source or for a set recording position.
Alternatively, pressure variation is often graphed against time. Imagine
that a speaker is producing a 100 hertz (Hz) tone. If we could photograph
the positions of the air molecules carrying the sound wave at any
particular instant it would look like the diagram below.
Graph points above the horizontal axis represent situations where the air
pressure was higher than normal, that is, areas of compression. Points
below the horizontal axis represent areas of lower-than-normal pressure or
areas of rarefaction.
Pressure variation versus time graphs are not easy to visualise, but
imagine yourself to be an air molecule a set distance from a sound source
such as a speaker. At one moment you would be experiencing peak
compression. Gradually the pressure would drop to normal atmospheric
pressure and an instant later you would be experiencing very low pressure
or a rarefaction. Again the cycle would continue from compression to
rarefaction to compression to rarefaction as the sound waves travelled
through you position.
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