propagation of a disturbance

happened? The first spring is disturbed from its
equilibrium length. Since the second spring is
connected to the first, it is also stretched or
compressed, and so on. The disturbance moves
from one end to the other; but each spring only
executes small oscillations about its equilibrium
position. As a practical example of this situation,
consider a stationary train at a railway station.

Different bogies of the train are coupled to each
other through a spring coupling. When an
engine is attached at one end, it gives a push to
the bogie next to it; this push is transmitted from
one bogie to another without the entire train
being bodily displaced.

Now let us consider the propagation of sound
waves in air. As the wave passes through air, it
compresses or expands a small region of air. This
causes a change in the density of that region,
say δρ, this change induces a change in pressure,
δp, in that region. Pressure is force per unit area,
so there is a restoring force proportional to
the disturbance, just like in a spring. In this
case, the quantity similar to extension or
compression of the spring is the change in
density. If a region is compressed, the molecules
in that region are packed together, and they tend
to move out to the adjoining region, thereby
increasing the density or creating compression
in the adjoining region. Consequently, the air
in the first region undergoes rarefaction. If a
region is comparatively rarefied the surrounding
air will rush in making the rarefaction move to
the adjoining region. Thus, the compression or
rarefaction moves from one region to another,
making the propagation of a disturbance
possible in air.