[Illustration: FIG. 173.--Waves in a coiled wire.]
Sound waves may be said to consist of a series of condensations and
rarefactions, and the distance between two consecutive condensations
and rarefactions may be defined as the wave length.
257. How One Sounding Body produces Sound in Another Body. In
Section 255 we saw that any object when disturbed vibrates in a manner
peculiar to itself,--its natural period,--a long-roped hammock
vibrating slowly and a short-roped hammock vibrating rapidly. From
observation we learn that it requires but little force to cause a body
to vibrate in its natural period. If a sounding body is near a body
which has the same period as itself, the pulses of air produced by the
sounding body will, although very small, set the second body into
motion and cause it to make a faint sound. When a piano is being
played, we are often startled to find that a window pane or an
ornament responds to some note of the piano. If two tuning forks of
exactly identical periods (that is, of the same frequency) are placed
on a table as in Figure 174, and one is struck so as to give forth a
clear sound, the second fork will likewise vibrate, even though the
two forks may be separated by several feet of air. We can readily see
that the second fork is in motion, although it has not been struck,
because it will set in motion a pith ball suspended beside it; at
first the pith ball does not move, then it moves slightly, and finally
bounces rapidly back and forth. If the periods of the two forks are
not identical, but differ in the slightest degree, the second fork
will not respond to the first fork, no matter how long or how loud the
sound of the first fork. If we suppose that the fork vibrates 256
times each second, then 256 gentle pulses of air are produced each
second, and these, traveling outward through the air, reach the silent
fork and tend to set it in motion. A single pulse of air could not
move the solid, heavy prongs, but the accumulated action of 256
vibrations per second soon makes itself felt, and the second fork
begins to vibrate, at first gently, then gradually stronger, and
finally an audible tone is given forth.
|