vibrations or
"overtones" which are going on at the same time as their fundamental
vibrations. It is this difference which lets us tell at once which
instrument is being played.

That brings us to the main idea about musical sounds and about human
speech. The pitch of any complex sound is the pitch of its fundamental
or lowest sound; but the character of the complex sound depends upon all
the overtones or "harmonics" which are being produced and upon their
relative frequencies and amplitudes.

[Illustration: Fig 82]

The organ pipe which ends in the larynx produces a very complex sound. I
can't show you how complex but I'll show you in Fig. 82 the complicated
motion of an air molecule which is vibrating as the result of being near
an organ pipe. (Organ pipes differ--this is only one case.) You can see
that there are a large number of pure notes of various intensities, that
is, strengths, which go to make up the sound which a listener to this
organ pipe would hear. The note from the human pipe is much more
complex.

When one speaks there are little puffs of air escaping from his larynx.
The vocal cords vibrate as I explained. And the molecules of air near
the larynx are set into very complex vibrations. These transmit their
vibrations to other molecules until those in the mouth are reached. In
the mouth, however, something very important happens.

Did you ever sing or howl down a rain barrel or into a long pipe or
hallway and hear the sound? It sounds just about the same no matter who
does it. The reason is that the long column of air in the pipe or barrel
is set into vibration and vibrates according to its own ideas of how
fast to do it. It has a "natural frequency" of its own. If in your voice
there is a note of just that frequency it will respond beautifully. In
fact it "resonates," or sings back, when it hears this note.

The net result is that it emphasizes this note so much that you don't
hear any of the other component notes of your voice--all you hear is the
rain barrel. We say it reinforces one of the component notes of your
voice and makes it louder.

That same thing happens in the mouth cavity of a speaker. The size and
shape of the column of air in the mouth can be varied by the tongue and
lip positions and so there are many different possibilities of
resonance. Depending on lip and tongue, different frequencies of the
complex sound which comes from the larynx are reinforced. You can see
that for you