c. As the blade returns toward the vertical position, the condensed air modules travel outward (away from the
blade) and an increasing gap occurs between the blade and the area of condensation, as shown in figure 2C. This gap
moves from rarefaction to equilibrium rather rapidly because air molecules rush in to fill the area. By the time the blade
returns to the vertical position, the area immediately to both the right and left of the blade has returned to its original
density.
d. At this point the blade has a lot of velocity, and continues to the left as in figure 2D. It now has caused a
condensation on the left side and a rarefaction on its right. The initial condensation on the right, meanwhile, has moved
still farther from the blade and the initial rarefaction still farther to the left.
e. Now, you can see that at each advance of the blade on either side a crest of condensation is sent outward and
at each retreat of the blade a trough of rarefaction is set up. The energy, crest to crest, was provided by a transfer of the
energy of motion of the blade. This energy, as it continues its outward travel is now called a sound wave! Keep in mind
that the particles which transmit the energy do not go along with it; each collides with its outside neighbor, passing its
energy to the neighbor and returns close to its original position. In figure 2E the blade is back to the vertical position and
normal pressure is restored. By this time, both condensations and rarefactions have moved farther out from the source and
are followed by a new wave as shown in figure 2F. This process and train of waves continue to be sent out as long as the
vibration continues.
5.
Representation of Sound Waves.
a. Sound waves may be represented as a graph by laying out successive groups of relatively compressed air
particles along a path of motion for a certain distance or certain amount of time.
(1) In figure 3 you
can see a portion of figure 2E redrawn, showing the particles making
up several sound waves.
The alternate regions of condensation and rarefaction are moving to
the right (towards the ear)
as described in paragraph 4.
Below this representation is a graph.
On this graph the high parts
and the low parts corresponding to the relative compression of air particles
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