A _tone_ is produced by a periodical motion of the sounding
body--a _noise_ by motions _not_ periodical. We can see and
feel the sounding vibrations of stationary bodies. The eye can
perceive the vibrations of a string, and a person playing on
a clarionet, oboe, or any similar instrument, feels the vibration
of the reed of the mouthpiece. How the movements of the air,
agitated by the vibrations of the stationary body, are felt by
the ear as tone (_Klang_), Helmholtz illustrates by the motion of
waves of water in the following way: Imagine a stone thrown into
perfectly smooth water. Around the point of the surface struck by
the stone there is instantly formed a little ring, which, moving
outwards equally in all directions, spreads to an ever-enlarging
circle. Corresponding to this ring, sound goes out in the air
from an agitated point, and enlarges in all directions as far as
the limits of the atmosphere permit. What goes on in the air is
essentially the same that takes place on the surface of the water;
the chief difference only is that sound spreads out in the spacious
sea of air like a sphere, while the waves on the surface of the
water can extend only like a circle. At the surface the mass
of the water is free to rise upward, where it is compressed and
forms billows, or crests. In the interior of the aerial ocean
the air must be condensed, because it cannot rise. For, “in fact,
the condensation of the sound-wave corresponds to the crest,
while the rarefaction of the sound-wave corresponds to the sinus
of the water-wave.”[7]
The water-waves press continually onwards into the distance, but
the particles of the water move to and fro periodically within
narrow limits. One may easily see these two movements by observing
a small piece of wood floating on water; the wood moves just as the
particles of water in contact with it move. It is not carried along
with the rings of the wave, but is tossed up and down, and at last
remains in the same place where it was at the first. In a similar
way, as the particles of water around the wood are moved by the
ring only in passing, so the waves of sound spread onwards through
new strata of air, while the particles of air, tossed to and fro by
these waves as they pass, are never really moved by them from their
first place. A drop falling upon the surface of the water creates
in it only a single agitation; but when a regular series of drops
falls upon it, every drop produces a ring on the water. Every ring
passes over the surface just like its predecessor, and is followed
by other rings in the same way. In this way there is produced on
the water a regular series of rings ever expanding. As many drops
as fall into the water in a second, so many waves will in a second
strike a floating piece of wood, which will be just so many times
tossed up and down, and thus have a periodical motion, the period
of which corresponds with the interval at which the drops fall. In
like manner a sounding body, periodically moved, produces a similar
periodic movement, first of the air, and then of the drum in the
ear; the duration of the vibrations constituting the movement must
be the same in the ear as in the sounding body.