often do also in my usage of the word. Generally this is not a problem,
unless one is faced with questions like the tree falling in the forest.
The mental experience of sound is believed to be generated by
processes of the nervous system elicited by the physical stimulus
of rhythmic air pressure variation set up by the action of the falling
tree. When the tree falls, it compresses the air molecules (nitrogen
and oxygen, mainly) in the vicinity, producing a transient increase in
their density. This increased density pushes on the nearby molecules,
compressing them. The compression is followed by a rebound of rar-
efaction, or thinning. This variation in air pressure moves out into the
space around the falling tree as a wave of pressure variation, traveling
at the speed of sound.
Another example: when a clapper strikes a metal bell, it sets the bell
into vibration, and the vibration at the surface of the bell alternately
compresses and expands the air in the immediate vicinity. The result
is arhythmic pattern of air pressure variation that moves out into
space as waves of alternating compression and rarefaction (Fig. 15.1).
This can be depicted in graphical form as a sinusoid (Latin sinus =
curve) or sine wave plotting air pressure as a function of time. Figure
15.2 shows a sine wave having a period (the duration of one cycle) of
5 milliseconds. In one second there are two hundred cycles—a fre-
quency of 200 hertz (Hz).
(P»)