The
Science Notebook
Gilbert Sound - Chapter IV
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the information contained in this book is either outdated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 35-51
[35]
Chapter
IV
TRANSMISSION OF SOUND - CONCLUDED
What do we mean when we say sound travels? What is it that
travels through the air, water or steel so readily? There was
a time when it was quite generally supposed that electricity was an
invisible substance which flowed through wires. You are surely
enough of a scientist to see very readily that, in the transmission
of sound, as in that of electricity, no substance actually travels
from place to place, as from tuning fork to the ear. What is
transmitted may be very easily demonstrated.
Experiment No. 14. Take
a
metal tube or make of paper or cardboard tube by rolling some paper
or cardboard around a stick and then removing the stick. Make
this about 6 or 8 feet in length and at least 3 inches in
diameter. Seal one end by means of a thin rubber dam tightly
stretched across the opening, holding it in place with a rubber
band, string or wire. At the other and make a cone with
an inch opening. Place a little candle a few inches from the
cone so that the flame from the candle will be just opposite the
hole in the end of the cone. Your apparatus is now ready
for the experiment. (See Figure 21.)
Make a noise at the end of the tube just beyond the rubber diaphragm
with any vibrating body, such as hitting two pieces of metal
together or two blocks of wood, and watch the flame of the
candle. If you have followed the directions, you will find
that when the vibration - that is, the sound - is produced the flame
will bend away from the opening. Now this should
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GILBERT BOY ENGINEERING
be conclusive proof to you in view of the fact that the tube is
sealed up by means of a rubber dam that air cannot pass through
it. Therefore, if you reason as a scientist should reason, you
will come to the conclusion that the flicker of the candle flame is
due to energy which has being transmitted from the origin of the
noise down the tube to the flame. The energy produced at the
point of origin of the sound strikes the rubber diaphragm, causing
it to vibrate and thereby to transmit a certain amount of energy on
to the flame.
MANOMETRIC
FLAMES
Experiment No. 15. The
manner
in which this "sound energy" is transmitted may be beautifully shown
by an experiment which you can perform for yourself. By using
a little thought and ingenuity you can rig up the necessary
apparatus, the ideal form of which is shown in the accompanying
illustration. (See Figure 22.)
A wooden or metal box about 3 inches square is divided into two
chambers by means of a thin rubber diaphragm placed fairly near to
one side of the box. A stream of gas is admitted to the small
chamber, from which it passes out again to a small jet, making an
even, pointed flame. The small and of a mega-
GILBERT
SOUND EXPERIMENTS 37
phone is fitted into the other end of the box. About 2 1/2
feet away place a mirror, preferably so arranged so that it can be
rapidly revolved. For the revolving type, it is best to take
four
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GILBERT BOY ENGINEERING
mirrors and fasten them to the four sides of a square box.
When the room is darkened, you are ready for the experiment.
With the flame burning steadily, start the mirror revolving or - to
produce the same result - turn the eye quickly, throwing the line of
sight across the image of the flame in the mirror. You will
see the flame reflected as a straight band of light.
Now hold a tuning fork, mounted on a sounding box, near the large
end of the megaphone and start it vibrating by means of a violin bow
or cork hammer. When the mirror is revolved, you will see the
flame reflected as a series of sharp points of light. See
figure. As the vibration of the tuning fork dies out, the
points on the band of light in the mirror become shorter until, when
the fork stops vibrating, the straight band of light is again seen
in the mirror.
The one thing which this experiment shows more clearly than anything
else is the fact that "sound energy" is transmitted in regular
pulsations, which are called waves - air waves or sound waves.
As a prong of the tuning fork moves forward toward the megaphone, it
pushes the air particles next to it ahead,
GILBERT
SOUND ENGINEERING 39
condensing them. (See Figure 24.) The fork then suddenly
changes direction and moves backward, leaving a partial vacuum or a
rarefaction behind it. The air particles which have been
condensed now rush back to fill this rarefied space; but in the
meantime they have acted upon the air particles next to them,
thereby setting up a series of condensations and rarefactions which
eventually reach the rubber diaphragm, causing it to vibrate.
As the diaphragm vibrates, it causes the pressure of the gas in the
chamber to change rapidly and these changes in gas pressure cause
the flame to flicker up and down. The changes in the height of
the flame are so rapid that the eye cannot detect them unless they
are separated by one of the two methods previously described.
There are many examples of the transmission of energy by means of
wave motion. We have all seen a smooth pond of water thrown
into ripples (wave motion) by throwing into it a stone. (See
Figure 25.) Anyone living near the water knows the tremendous
force of waves breaking up on the shore. We all know the sting
the hand gets if, when holding a piece of steel, it is hit with a
hammer at the other end. The jar of a door may be felt all
over the house. An explosion miles away may rattle the windows
and even break them. A huge tidal wave
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GILBERT BOY ENGINEERING
may be caused by the eruption of a volcano. In 1886 Krakatoa
exploded, producing air waves that passed around the earth three
times and a tidal wave that passed clear across the Pacific.
Hold one end of a piece of rope about 16 feet long and ask a friend
to hold the other end. Strike the rope sharply a few inches
from your hand. You will see a wave run along the length of
the rope and cause a sudden jerk at the other and. You may
substitute for the rope a coil spring about 8 feet long, which may
be made by winding wire on a piece of gas pipe.
Attach one end of this coil to a hook on the wall and hold
GILBERT
SOUND EXPERIMENTS 41
the other end in your hand, stretching it out to some length.
(See Figure 26.) Strike this coil with the hand and you will
notice that a wave will run to the fixed end and return, when a
sudden jerk will be felt by the hand. This is a reflex or
return wave which demonstrates that the wave transmits the force of
the blow. In other words this vibration, or oscillation, that
is set up in one part of the wire is transmitted to the other parts
in the form of waves.
WATER
WAVES
Let us study the action of water waves. Take a bowl of water
and drop into it a ball and watch it produce a series of
waves. Here you will see a form of motion. Now if a cork
is thrown upon these waves you will observe that it rises, moves
forward with the crest of the water, or wave, and then it sinks and
moves backward, repeating this action with each wave motion that
follows it; but the cork doesn't move from its original
position. This demonstrates conclusively that the water itself
does not move with the wave but that the motion is passed along from
one mass of water to the next.
To make this clear to you let us assume, and the theory is, that
water is made up of molecules (small particles). Note the
illustration (Figure 27.) which should convey to you a series of
balls suspended by strings so that they merely touch each
other. If one of these balls is touched a wave motion is
produced and still the balls will remain in their same
position.
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GILBERT BOY ENGINEERING
AIR
WAVES
The air waves we explained in the experiment with the revolving
mirror are like the water waves produced by dropping the ball in the
water in one respect - that is, they both go out in all directions
from the point of original vibration. But air waves are
different from water waves in regard to the type of vibration.
In the case of water waves the motion of the water is in a circular
direction (remember the bobbing cork).
In air or sound waves, the air particles vibrate back and forth in
the same direction in which the waves are traveling. In water
waves we have not only this but also the up and down motion.
(See Figure 28.) In airwaves we have a longitudinal
GILBERT
SOUND EXPERIMENTS 43
vibration, which was explained by the ball and elastic in Chapter
I.
The length of water waves is measured from the crest of one wave to
the crest of the next. The length of sound waves is measured
from the center of one condensation to the center of the next.
(See Figure 29.)
Experiment No. 16. You
can
show the similarity between water waves in the transverse and
vibration of the pendulum, tuning fork, etc., in the following
manner:
Prepare a strip of glass with a few drops of kerosene and sprinkle
some flour over it. Attach a whalebone with the a hole bored
near the end of it to a block screwed to a board. (See Figure
30.) Place the glass between two strips attached to the board and
underneath the whalebone. Attach a bristle to the whalebone in
a position so that it just touches the glass. By vibrating the
whalebone and pulling the strip of glass out in a uniform movement
you can trace these vibrations in the form of waves.
Experiment No. 17. A
fine way of demonstrating the "to
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GILBERT BOY ENGINEERING
and fro" motion which takes place in compressional waves (of which
sound waves are an example) is as follows:
Take a stiff coil spring about 2 1/2 or 3 feet long and attach each
end firmly to hooks or nails on the wall so the spring is somewhat
stretched out. (See Figures 31.) Grasp the spring 3 or 4
inches from one end and draw it toward that end, producing a
condensation of the spring between your hand and the hook. Now
quickly release your hold of the spring so as
GILBERT
SOUND EXPERIMENTS 45
not to start it vibrating up and down. Place one finger
lightly anywhere on the spring. What happens?
If you have carefully followed the preceding explanation of
longitudinal waves you will readily understand that the rapid back
and forth motion which you feel with your finger is due to a series
of pressure waves which were set up along the spring when the
condensation at one end was released.
SYMPATHETIC
VIBRATIONS
Now that you have seen to your entire satisfaction that the
transmission of sound is nothing more or less than the transmission
of energy in the form of waves, you will understand one of the most
interesting phases of the Science of Sound.
THE
MONOCHORD OR SONOMETER
More than 2,000 years ago a Greek scientist, by the name of
Phagoras, invented an instrument called a monochord. It was
with this instrument that he discovered many things about sound
produced by strings when vibrating. You should construct a
monochord for yourself, as many of the experiments to follow will
required its use.
This is not a difficult piece of apparatus to rig up, and the
illustration (Figure 32) will probably help you more than a
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GILBERT BOY ENGINEERING
detailed explanation. The essential features are the sounding
box, the strings (mandolin, violin or wire strings), the sliding
bridge and the tightening screws.
Experiment No. 18. By
using the tightening screws, get two of the strings of the monochord
to the same note as the tuning fork you are using. Now set the
fork into a strong vibration, using method No. 3, described on page
18, and place it on the box between the two strings of the
monochord, as shown in the illustration (Figure 33).
After two or three seconds, stop the vibration of the fork by
placing your fingers on the prongs. You will be astonished now
by hearing the two strings singing to you with some note that the
tuning fork gave.
We believe that by this time you are enough of a scientist to reason
out that the air waves from the fork, which you set in vibration
with the violin bow, have traveled to the two strings and, having
enough strength, have set them into similar vi-
GILBERT
SOUND EXPERIMENTS 47
bration. This means that when a sounding body is near another
that has the same rate of vibration, the waves from the first will
set the second body into vibration.
Experiment No. 19. You
may
use two tuning forks, instead of a tuning fork and the monochord
strings, and demonstrate the same phenomenon. When two forks
are used, they must be of exactly the same rate of vibration.
If you have two forks that vibrate at slightly different rates, you
can get them the same by putting a small piece of wax on one prong
of the fork which vibrates more rapidly. You may have to try
several times before you get just the right amount of wax to make
the two forks give exactly the same tone. Place each fork on a
block and set them on a table or large box, about 18 inches
apart. (See Figure 34.) Set one of the forks in vibration and,
after a few seconds, dampen it with the fingers as described in the
preceding experiment. Now, as before , you will hear the note
of the fork you struck being sounded by the other fork which was not
struck.
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GILBERT BOY ENGINEERING
Experiment No. 20. By
using two strings of the monochord you will be able to get a result
similar to that obtained with the two tuning forks. First, by
means of adjusting screws, tighten the strings so that they give the
same note. Now hold one of the strings with the fingers of
your left hand so it cannot vibrate, and pluck the other string as
you would in the case of a mandolin. After a few seconds take
your hand away from the string you have been holding and dampen the
string which you plucked. You should be able to hear a clear
distinct note being sounded by the string which you first
held.
Now compare the sympathetic vibration of the tuning forks with
things in every-day life. The act of pushing a swing is a
simple illustration. You exert your energy at regular
intervals, which are the same as the natural rate at which the
swing moves to and fro. Bridges have a natural rate of
swinging to and fro. People who frequently walk across the
Brooklyn Bridge get into the habit of adapting their stride to the
swing of the bridge. It has been said that, were it not for
offsetting influences, you could strike the Brooklyn Bridge (or any
other large bridge) with a hammer at the natural rate at which the
bridge swings and in time cause it to swing so violently that it
would topple over.
The swinging notion of bridges has long been recognized in military
maneuvers. The next time you see a large body of men cross a
bridge or viaduct you will notice that the officer in charge will
command his men to "break step" - that is, to walk out of
step. This is because the "measured tread" of a large body of
men is liable to cause the bridge to vibrate or swing to such an
extent as to become a unsafe.
The phenomenon of sympathetic vibration explains why things jingle
when we play the piano. Vases on the parlor table, picture
frames on the wall, cut glass in the cupboard, knives and forks all
have their natural rates of vibration. When
GILBERT
SOUND EXPERIMENTS 49
sound waves of the same rate of vibration are sent out from the
piano and strike them, they vibrate just as the second tuning fork
did in our last experiment.
BREAKING
A GLASS WITH THE VOICE
Experiment No. 21. A
very striking experiment showing the strength of sound waves caused
by the human voice can be admirably illustrated by an experiment or
demonstration that oftimes is made by great singers. Probably
you have heard of singers having such strong voices that the
vibrations produced from their throat would actually break glasses,
and by the following experiment you can demonstrate that this is
possible with your own voice. Take a very thin, sensitive
cut-glass goblet and set it in vibration by hitting it with a cork
hammer or by rubbing it with your moistened finger if you are adept
in doing this; this should cause the glass to emit loud and musical
tones. After you have set the tumbler in vibration, place the
opening close to your mouth and shout into it as loud as you
possibly can. At the same time try to imitate the same
tone or key that is emitted from the glass. This will render
double the amplitude of the vibrations and the glass will break into
pieces.
A famous bass singer by the name of Labache always demonstrated this
feat to show the strength of his voice much to the admiration of his
friends and, although he had a wonderful voice, he also knew the
secret of doing the trick that we have given you here.
FORCED
VIBRATIONS
It's sometimes happens that one vibrating body will set another body
into vibration even though not possessing the same natural rate of
oscillation. Set a tuning fork in vibration and place the
lower end of it on a light wooden box that will readily
vibrate.
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GILBERT BOY ENGINEERING
You will find that the fork sets the box in vibration and this
increased volume of vibration produces a loud sound which lasts for
a comparatively short time.
The reason that the tuning fork stops vibrating so quickly and the
noise subsides so soon is because the vibration of the box requires
an extra amount of work from the fork and its energy is soon used
up. You will notice that the sound dies out very
quickly. The same principle is at work in the case of the
swing. You can cause a swing to move to and fro faster or
slower than its natural rate, but it is hard work and you soon
become tired out.
As an example of forced vibration, let me tell you how to do a dandy
trick. This is a very mystifying trick and one that you can
perform at any time without any prepared apparatus. It is an
exceptionally fine trick to perform at a dinner or house
party.
Hold a fork in your left hand, as shown in Figure 35, with your
wrist on the edge of the table and the handle of the fork
GILBERT
SOUND EXPERIMENTS 51
free from touching the table. Snap the prongs of the fork with
your thumb and index finger and you will get a ringing sound from
the fork. The trick now is to carry the sound with your right
hand over to the glass on the table and throw the sound from your
hand into the glass. As you lean forward to throw the sound
into the glass you press the handle of the fork on the table
unnoticed by your audience. This changes the faint ringing
sound of the fork into a loud sound short duration, and it appears
to your audience that you really carried the sound from the fork to
that glass.
The reproducer of a phonograph, the metal discs in the telephone
transmitter and receiver, the rubber diaphragms used in Experiments
No. 14 and No. 15 are all familiar instances of the forced
vibrations and you can doubtless think of other examples
yourself. The transmission of energy in the form of waves is
the important fact demonstrated in each case.
