Table of Contents
I. "To and Fro" Motion
- Page 7
The laws of the pendulum - Example of the swing -
Concentration of thought to produce sound - Longitudinal
II. Origin of Sound
- Page 16
Sound produced by vibrating body - Example of the bell, wine
glass, vocal cords, etc. - Vibration of air columns - Musical
III. Transmission of Sound
- Page 25
What carries sound - The toy telephone - Why sounds can come
through thick walls - The velocity of sound
IV. Transmission of Sound
- Page 35
How sound travels - Sound explained as the transmission of
energy in the form of waves - Manometric flames - Other types of
weight motion - Air waves and water waves compared and
contrasted - Sympathetic vibration - Breaking a glass with
the voice - Forced vibrations - Carrying sound from a fork
to a glass
V. Intensity, Pitch and Quality
- Page 52
The monochord or sonometer - The factors which determine the
intensity of sound - The quality of sounds explained -
Overtones - Nodes and loops of string and of air columns -
Why a good piano makes better music than a cheap one - The
fiddle string that can be made to laugh or cry - Nodes and loops
of vibrating plates, and of a vibrating wine Glass.
VI. Reflection, Refraction, Interference and Resonance
- Page 67
Echoes - Acoustic properties - Refraction of sound above by a
toy balloon - Fog signals - Resonance - The seashell - Table
rapping - The human voice and the cause of different types of
language - Interference - Beats.
VII. How We Hear Sound
- Page 82
The human ear explained - Ventriloquism - Do sounds go on
VIII. Modern Inventions
- Page 89
The invention of the telephone - Comparison of the telephone
transmitter and human ear - The phonograph - The function of the
reproducer and resonating devices.
My sole aim in writing this book is to bring the Science of
Sound down to your understanding so that you can have a whole
lot of genuine fun - the kind of fun I liked when I was a boy -
in doing these intensely interesting and scientific
experiments which I am about to explain.
This book will not make you the smartest boy in your class at
school, but it will teach you a lot of things that perhaps the
smartest boy in school does not know, and you will stand for
leadership among boys because you will know some things that
most boys and even grown up people know very little about.
We are living in the age of the world's greatest discoveries and
inventions, and you should realize the importance of the fact
that the world's biggest scientific and engineering problems are
yet in front of us. Just think we do not even know what
electricity is as yet, and not one of the recent great
discoveries or inventions such as wireless telegraphy or the
telephone are anywhere near perfected. The problems of
food production, soil fertilization, automatic machines of every
description and kind, are problems in their mere infancy and
they hold in store abundant possibilities for the scientist who
will perfect and work out the things that are of so much
importance to the whole civilized world. This book is an
honest endeavor to kindle that scientific enthusiasm which every
boy has, in the hope that I may start many of you on the way to
the goal of achievement to furnish the world with new
discoveries and new inventions; to unravel many unknown
scientific and a chemical wonders of our earth and life.
In my book on Weather I said that we live in an Ocean of Air and
without this Ocean of Air the earth would be a world of
Now sound is conveyed to the ear through air. If either
the ear or the air were destroyed we would be buried in the
When I tell you that there is really no sound or voice, does it
not arouse your curiosity? Do you know that hearing is
just feeling with the ear? That, in reality, the thing
which we call sound, which we think of as noise or as a musical
tone, is just an impression on the brain.
The tree does not hear anything. It may feel the pressure
of the wind, but to the tree, the world is a world of
silence. The only reason that we interpret sound as
meaning anything is because our ears record such sounds as noise
What do we mean when we say we hear sound? What is the
difference between noise and music? Why does a piano
play? Why do sounds go through the wall? What makes
the wind whistle? What makes an echo? Why does a sea
shell make a noise like the waves of the ocean? Let us see
if we can find the answer to these questions and at the
same time have a lot of fun doing other interesting experiments
in Natural Magic or the Science of Sound.
GILBERT SOUND EXPERIMENTS
MOTION, "TO AND FRO"
Before I can lead you into the interesting experiments of sound,
you will have to have patience enough to carry out the few
simple experiments outlined in this chapter, as these are so
necessary to enable you to understand the "whys and wherefores"
of the experiments that are to follow.
Just one little story I must tell you here; first, because it is
interesting and second because it describes how a boy scientist,
by careful observation and reasoning many hundred years ago, led
up to some of our important laws of physics.
This great scientist, known as Galileo, born in Pisa Italy, in
the year of 1564, was destined to be one of the greatest
philosophers and inventors that the world has ever known.
As a boy he was bent upon a scientific career. He took up
his study at the University of Pisa. On a certain
afternoon when he was still a young boy he had occasion to visit
the great Cathedral of Pisa. While he was there, he saw
the watchman lighting a lamp that hung from the ceiling of the
cathedral. In order to light the lamp it was necessary for
him to draw it down toward him and when finished lighting it, he
let go of the lamp and it swung back and forth. This
attracted the attention of Galileo and, having a scientific bent
of mind, he
8 GILBERT BOY ENGINEERING
became interested in the movements of the lamp. At first
the lamps swung a long arc back and forth and then, as you all
know, its motion grew less and less. But what you probably
do not know, and the thing that struck him as intensely
interesting, was that these "to and fro" movements or
oscillations, whether the lamp was swinging wide or short were
always made in exactly the same time.
He then went home and reasoned it out. He made a piece of
apparatus that was similar to the lamp - that is, with a thread
and weight attached to it he made what is now called a
pendulum. He was thereby able to duplicate the swinging
motion of the lamp and to measure it accurately. In this
way he discovered the laws of "to and fro" motion - that is, of
the pendulum and its vibration.
Now do a little observing for yourself and see if you can
compare this "to and fro" motion of the pendulum with things in
Nature . Go out of doors and observe things that are
taking place all about us.
What is the effect of the wind upon the trees? Do we not
find that they are swaying backward and forward? The
telegraph wires that hang on the poles are swinging to and fro,
likewise the waves of the sea. In fact you are going to
find as we go along with our experiments that all of these
motions that we see about us, and the sounds that come to our
ears and the waves that carry the telephone messages and the
telegraph and wireless messages, are all result of this "to and
Make a pendulum apparatus of your own by means of a plumb bob
attached to a thread. The length of the thread from where
it is supported above to the center of the plum bob is the
length of the pendulum.
SOUND EXPERIMENTS 9
Experiment No. 1.
Swing the plumb bob backward and forward. The movement
from one side of the arc to the other and back again is known as
a complete vibration - that is, from 1 to 3 and back to 1
again. (See Figure 1.) The movement of the plumb bob
from one side of the arc to the other is known as a simple
vibration - that is, from 1 to 3; the movement of the plumb bob
from the center or point of rest to one side - that is, from 2
to 3 is the amplitude of vibration. The period of
vibration is the time required to make a simple vibration.
THINGS TO KNOW AND NOTICE ABOUT THE MOVEMENTS OF THE
That the motion of
the palm bob is most rapid at the center - that is, at position
That the pendulum
or plum bob comes to a point of rest at the center (like the old
cat dies) and oscillation or vibration ceases.
the pendulum not keep swinging? The truth of the matter is
that it could not stop itself. It is the force of gravity
and friction of the air which, little by little, stops the
pendulum and finally brings it to a point of rest. That
is, the plumb bob comes to rest at a point as near to the center
of the earth as it can; because gravity draws it in that
direction into a vertical position or point No. 2.
GILBERT BOY ENGINEERING
Hold the pendulum in your hand and cause it to swing first high,
then low. Ask some of your friends whether the period of
the pendulum is greater when swinging high or low.
Ninety-nine out of hundred will tell you that it is greater when
swinging low than when you give it a long swing. The
reason they give you this answer is because they are not
scientific. They do not know what you are going to know
after you make the next experiment; that is what Galileo
discovered hundreds of years ago.
Experiment No. 2.
Take a watch and count the number of oscillations that the
pendulum makes in ten seconds. Start the pendulum swinging
from different positions, giving it first a long swing then a
short swing. Determine the time of the vibrations when it
is swinging high and swinging low and you will be surprised to
find that the pendulum will always vibrate in the same period of
time, no matter how long the swing or how short it is.
This is the most wonderful and important law of the
Experiment No. 3.
Try plumb bobs of different weights - one of wood, one of lead;
that is, one light and one heavy but of the same size.
Count the motions as in the above experiment and determine the
time of vibration, and you will find that you have worked
out the second great law of the pendulum - that is,
it makes no difference whether the weight is light or heavy; as
long as the pendulums are of the same length both will vibrate
in the same time.
told you that the friction of the air had an influence on the
stopping of the pendulum. Consequently, if the plum bob is
large, there would be more friction and naturally it would come
to rest quicker. But if you could carry this experiment on
in a vacuum (where there is no air), it would not make any
difference whether the weight was large or small - the vibration
would be in the same time.
SOUND EXPERIMENTS 11
Experiment No. 4.
The third great law of the pendulum is arrived at by taking
several weights of the same size and weight and swinging them on
different lengths of thread. (See figure 2.) You may
then determine the number of vibrations per second for each one
and prove the third law of the pendulum, which is that the
shorter the pendulum the more rapidly it vibrates or
These principles that we have just illustrated with the pendulum
are intensely interesting to us because, first, they are going
to help us in understanding and appreciating the science of
sound and, second, they are of great importance in the movements
of the clock.
Speaking as a scientist, the movements of the pendulum after all
have a great deal to do with their every-day life, for we set
our time for going to bed and rising in the morning by its
actions; it is the guardian angel of the trains that run by
In another book on scientific experiments with motion, we will
take you into some interesting fun and study on the pendulum
GILBERT BOY ENGINEERING
action of the clock and we will give you the apparatus for
actually duplicating the clock and putting it together yourself
and understanding the principles that make this instrument
In addition to the three great laws of the pendulum you should
notice one more thing about its manner of swinging. Have
you ever pushed a swing in the park or in your back yard?
When the swing is at rest, you give it a shove and it starts
swinging, but not very high. As a starts forward the
second time, you give it another shove and its swings
higher. By exerting a moderate amount of energy, you can
soon get the swing going so high you can hardly reach it,
provided you push it just as it is starting forward each
time. You would not think of pushing the swing when it is
coming toward you. If you did you would never get it going
to any great extent.
We this in mind, you will now be able to understand a very
mystifying trick which I'll explain to you.
CONCENTRATION OF THOUGHT TO PRODUCE SOUND
Get a series of different size bottles. Through the center
of the cork of each bottle make a hole, through which you pass a
thread and to the end of the thread attach a little lead
weight. Now each of these pendulums, formed by the thread
and weight, swings at different lengths. If you have
carefully read the first part of this chapter, you will notice
that the shorter pendulum will vibrate faster than the longer
one; and if you bear in mind the action of a swing that we have
just been talking about, you will understand the working of this
very mysterious and magical feat.
See Figure 3 as to how to arrange these bottles on the
table. You now ask your friends about the table to select
any one of the bottles and concentrate their minds on that one
SOUND EXPERIMENTS 13
watch it intently. Then each one is to place his hands
upon the table, and by force of their thought (?) you will cause
the little bottle to produce a sound .
Now what you do is to set the table and vibration by very slight
movements of your hand which should be made unnoticed by your
audience. Keeping your eye on the pendulum in the bottle
that has been selected, make these movements so they are at the
same rate as those of the pendulum; gradually, by keeping these
motions of the table going, which by practice can be accom-
GILBERT BOY ENGINEERING
plished unnoticed, the pendulum can be made to swing more and
more until the little weight touches the glass and produces a
The mysterious part of it is that the pendulums in the other
bottles, although they may vibrate or oscillate to some extent,
will not swing back and forth enough to produce any noise.
Different lengths have different rates of vibration and
therefore do not get the benefit of the movement or the
vibrations of the table, as that is timed to suit the pendulum
in the bottle selected to make the noise.
Your audience can now select another bottle and you can, without
being noticed, change the vibration of the table to correspond
with the next bottle selected and produce the same mysterious
ANOTHER KIND OF "TO AND FRO" MOTION
We have described the more general laws of the pendulum and its
vibration; but after all, the kind of "to and fro" motion
demonstrated in the next experiment is even more closely related
to the Science of Sound and should be thoroughly
Experiment No. 5.
an elastic or long rubber band to a ball such as a return
ball. (See Figure 4.) Now, if the rubber band is
attached above to some definite place, you can pull the ball
directly down and let it go. What happens? The
SOUND EXPERIMENTS 15
ball vibrates up and down - that is, in line with the elastic
which is holding it. This is called longitudinal
vibration. Here again you will find that whether the range
of vibration - that is, the amplitude - is great or small, the
vibrations will be at the same rate.
The laws of the pendulum and various kinds of vibration that we
have given you are going to be mighty interesting and valuable
to you as you go on into the Science of Sound, for there you
will also find "to and fro" motion playing the leading part.