The
Science Notebook
Gilbert Sound - Chapter VII
NOTE: This book was published around 1920 as a
manual to accompany the Gilbert Sound set. The
set and manual were part of the "Boy Engineering" series,
While some of the experiments and activities here may be
safely done as written, some of them may be considered
hazardous in today's world. In addition, some of
the information contained in this book is either outdated
or inaccurate. Therefore, this book is probably
best appreciated for its historical value rather than as a
source of current information and good experiments. If
you try anything here, please understand that you do
so at your own risk. See our Terms of
Use.
Pages 82-88
[82]
Chapter
VII
HOW WE HEAR SOUND
In introducing the Science of Sound I aroused your curiosity by
saying that the ear was the important factor in sound. You may
hardly conceive of such a thing, but it is true, nevertheless, that
if we had no ears, there would be no sound. Noises like the
report of a gun and music from the piano do not travel through the
air as sound, as we are apt to think they do. It is just
Nature's Swinging pendulum - the "to and fro" motions of air
particles. The ear does, however, interpret them as
sounds.
Sometime, if you are around where a cannon is being shot off, put
your fingers in your ears so tightly that you cannot hear. You
will see the flash of the cannon and you will feel the air
disturbance, if you are standing anywhere near it, but you will not
hear the report of the explosion. You will hear no
sound.
(82)
GILBERT
SOUND EXPERIMENTS 83
When your fingers are in your ears, the report of the cannon is just
what it seems to be - a disturbance of the air. (See Figure
54.)
You can very beautifully visualize by comparison the mechanisms
which Nature has provided for hearing and its similarity to the
telephone transmitter. (See Figures 55 and 56.)
Our ear is divided into three parts: the external, middle and
internal ear.
The external ear, like the telephone transmitter, is shell-like in
form. Both the ear and the telephone transmitter serve for
collecting the sound waves, directing them toward the internal
parts. When air waves tap on the ear drum - that is, when
waves of air strike it - they set up a "to and fro" motion,
vibrating and oscillating, and this impulse serves to transmit these
vibrations received on the drum to the internal parts.
Right here is interesting to remark that this "to and fro"
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motion will only be perfect on the drum of the ear when the air on
both sides is of the same degree of density. Nature has well
provided for this by putting a hole which leads from inside of the
throat to just back of the drum which allows the free passage of air
in and out. The short tube which connects to the middle ear
and the throat is called the Eustachian tube . This tube is
about 1 1/2 inches long, and if your hearing apparatus is working as
it should there is a free and ready passage of air at all times in
and out.
Some of us when we have a cold - that is, a cold that affects this
tube - allow inflammation to be set up and clog it up, which does
not permit the free passage of air. Then we have difficulty in
hearing. Sometimes the tube becomes it so affected that we
practically lose our hearing temporarily at least. Therefore
it is important to be very careful with colds and particularly in
the case of throat colds, and you will find that our good friend the
GILBERT
SOUND EXPERIMENTS 85
Doctor will always advise our spraying or gargling the throat to
prevent infection spreading into these tubes.
Now just a word about
THE
MIDDLE EAR
The middle ear is made up of three very fine and minute bones,
one of which is attached to the drum; and the last in the series
which are connected together extends to the opposite side where
there is another membrane, and the two membranes are united together
by means of these bones.
Just one other reference to these bones and that is that there are
some very small muscles that connect and control them. It
is assumed that these little muscles are controlled by the nervous
system and that when we strain our ears in an endeavor to
detect faint sounds they act in some way or other to tighten these
membranes, so that we can detect the faintest sounds.
THE
INTERNAL EAR
The membrane at the base of the middle ear that we have just
referred to closes the middle ear from the internal ear. Very
little is known about the internal ear except that it is a very
complicated mechanism. We do know that it is made up of many
filaments and spiral tubes and that these tubes and filaments are
filled with watery fluid. Into these fluids there extend fine
hair-like nerve ends which are the terminals of the nerves of the
ear, and they unite to form the auditory nerves which go to the
brain to relay the messages that are received.
If you have followed our description you know that sound waves tap
on the ear drum and that the little bones that are attached to it
communicate these vibrations through the chain of bones to the
internal membrane, thereby producing a corresponding vibration into
the watery fluid by pressure and sen-
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sitizing these nerve ends which collect this sound and relay it down
the auditory nerves to the brain. Thus we hear sounds.
You may be asked this question, "How do we know where the sound
comes from?" "How do we determine the direction of it?"
We can best answer this by saying that the generally accepted view
and theory is that we can only acquire this instinct by practice and
experience. The truth of the matter is that location of sounds
is very difficult. We are all more or less familiar with
the ventriloquist, who, as we will describe, can easily fool us as
to the direction of sound. Consequently the theory cannot be
very far wrong that we depend partly upon the loudness of sound to
help us in determining the direction, and partly upon the difference
in the sound waves as we receive them, in turning the head to one
side or the other. With all this fine adjustment we have just
described we never are able to locate sounds from a very great
distance - that is, where they come from. It is for these
reasons that the ventriloquist is able to fool us.
VENTRILOQUISM
Really what we do when we hear a sound is to look around until we
find the motion that makes it and then we see where it comes
from. It is for this reason that it is always much easier when
you can see the lips move. Invariably when a person talks to
us, we unconsciously watch the movements of his lips. People
whose hearing is affected usually learn to read the movements of
lips.
Now the secret of ventriloquism is not in transferring the
sound of the performer's voice from his mouth into that of the
dummy, but in misleading us as to the direction from which the sound
comes. In other words, he makes sentences that do not require
the movement of the lips, but which are produced by the vocal cords
in his throat and for each sound he makes the
GILBERT
SOUND EXPERIMENTS 87
jaws of the dummy are made to move. When we see these
movements, we unconsciously assume that the sound comes from
them. To help him out in this performance the performer
modulates his voice when the dummy is supposed to be talking and
talks in his natural tones when he is talking to the dummy.
(See Figure 57.) In addition to this he gesticulates by
turning his head toward the dummy or to the point where the sound is
expected to come from, and he makes clever use of slight shades in
quality (or timber) and pitch with which you are familiar.
This is all there is to ventriloquism, although it requires great
practice and a person has to be adept to carry on a conversation in
a way that will fool the audience.
We believe this proves to you that the belief that a man can throw
his voice into space is one of pure ignorance. Our idea is not
to deprecate the extraordinary clever and fascinating practice of
the ventriloquism, but to make you realize that it is impossible to
throw the voice in the manner commonly supposed.
DOES
SOUND GO ON FOREVER?
We learn, in the study of chemistry, that no substance is ever lost
or destroyed, though it may change its form, as from a solid
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to a gas, and be seen no more. Likewise, we learn, in the
study of physics, that no energy is ever lost, though it also may
change its form and cease to be felt by the ordinary observer.
Since there really is no sound except as perceived by the ear, we
might say that sound ceases when the ear can no longer record its
presence. Even though we may watch these sound waves with
instruments more sensitive than our ears, in time we will be forced
to admit that they have been smoothed away.
But since sound in the physical sense is really nothing but the
transmission of energy in the form of waves we must realize that
when the waves are smoothed away, the energy that produced them has
not been lost. If we had the necessary knowledge and
instruments, we could trace it in the movement of air particles, in
the heat produced by forced vibrations of various objects, such as
the ear drum, etc. No sound lasts forever as a sound, though
its effects go on forever.
