. A copy can be obtained by sending ten cents to the
Commissioner of Public Documents, Washington, D. C.

[Footnote 6: Cf. "Within the Atom," Chapter X.]

LETTER 16

THE HUMAN VOICE

DEAR SIR:

The radio-telephone does not transmit the human voice. It reproduces
near the ears of the listener similar motions of the air molecules and
hence causes in the ears of the listener the same sensations of sound as
if he were listening directly to the speaker. This reproduction takes
place almost instantaneously so great is the speed with which the
electrical effects travel outward from the sending antenna. If you wish
to understand radio-telephony you must know something of the mechanism
by which the voice is produced and something of the peculiar or
characteristic properties of voice sounds.

[Illustration: Fig 75]

The human voice is produced by a sort of organ pipe. Imagine a long pipe
connected at one end to a pair of fire-bellows, and closed at the other
end by two stretched sheets of rubber. Fig. 75 is a sketch of what I
mean. Corresponding to the bellows there is the human diaphragm, the
muscular membrane separating the thorax and abdomen, which expands or
contracts as one breathes. Corresponding to the pipe is the windpipe.
Corresponding to the two stretched pieces of rubber are the vocal cords,
L and R, shown in cross section in Fig. 77. They are part of the larynx
and do not show in Fig. 76 (Pl. viii) which shows the wind pipe and an
outside view of the larynx.

[Illustration: Fig 77]

When the sides of the bellows are squeezed together the air molecules
within are crowded closer together and the air is compressed. The
greater the compression the greater, of course, is the pressure with
which the enclosed air seeks to escape. That it can do only by lifting
up, that is by blowing out, the two elastic strips which close the end
of the pipe.

The air pressure, therefore, rises until it is sufficient to push aside
the elastic membranes or vocal cords and thus to permit some of the air
to escape. It doesn't force the membranes far apart, just enough to let
some air out. But the moment some air has escaped there isn't so much
inside and the pressure is reduced just as in the case of an automobile
tire from which you let the air escape. What is the result? The
membranes fly back again and close the opening of the pipe. What got
out, then, was just a little puff of air.

The bellows are working all the whi