Gilbert Sound - Chapter III
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the information contained in this book is either outdated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
Pages 25 -34
TRANSMISSION OF SOUND
Now that we have traced sound to its beginning, it is natural to
ask, "What carries this sound to our ears and how?" The simple
question of what carries sound brings us to a lot of interesting and
important experiments .
A very simple experiment that you may not be able to do, but may be
fortunate enough to see done in a laboratory, proves conclusively
that sound is transmitted by our old and important friend, the air
or the atmosphere. We have proved that its source is a
vibrating body and by this experiment we can prove that it is
carried from the point of vibration to the ear by the air.
Experiment No. 9. An
electric bell is suspended in a jar. (See Figure 17.)
Now if an electric current sets the bell in vibration, sound is
produced. If, by means of an air pump, the air in the jar is
pumped out, still keeping the bell in vibration, gradually the sound
becomes fainter and fainter until a vacuum is created, when the
sound will be entirely silenced. Therefore, sound will,
evidently, not be carried in a vacuum, but it will travel through
SOUND BE CARRIED BY ANY OTHER SUBSTANCE THAN AIR?
We will answer this and then you can demonstrate it yourself by a
series of experiments that are quite interesting. THE SOUND OF A VIBRATING BODY CAN BE
TRANSMITTED BY ANY SUB-
26 GILBERT BOY ENGINEERING
STANCE - GASEOUS, LIQUID OR
SOLID. We have just demonstrated to you that they are
transmitted by air, which is a gas, and that this is the most common
method for sound transmission.
By liquid. We are all
familiar with the distinctness of sounds made under water.
Every boy that has gone in swimming has at some time or other hit
two stones together under water and noticed how the sound has been
transmitted so distinctly. Therefore sounds can be transmitted
by liquids - water is a good example.
By Solids. You have
probably tried the stunt, which most boys are familiar with and
which I often tried when a boy, of listening for the sound of a
train through the rail by putting your ear upon the rail before the
train had come into sight and long before you could hear it through
the air. Or you have noticed the effect of a man striking a
blow on a rail quite a way up the track. Put your ear to
the track and notice how soon the sound is heard and, when you take
your ear away, it takes quite a few seconds longer for it to reach
your ear through the air.
SOUND EXPERIMENTS 27
CHIMES WITH A SILVER SPOON
Experiment No. 10. The
following is an extremely pleasing, interesting and novel experiment
in sound transmission:
Take a silver spoon and tie a string to it, as per
illustration. (See Figure 18.)
Wrap one end of the string around one finger and the other end
around another finger and place them in the ears. Then let the
spoon hit the edge of the table and a very beautiful sound will be
heard in the ears, just like the chimes of the church
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This is a very fascinating experiment and one worth while, as you
will be amazed by the results. Those standing around you hear
practically no sound at all, while the sound waves will be
transmitted through the string to your ears in a most fascinating
By substituting a tuning fork for the spoon, you will be able to get
the same result in an even greater degree. The tone may be
heard for much longer time than when the spoon is used.
Experiment No. 11. A
very satisfactory method of proving that sound travels more readily
through solids than through the air is as follows:
Cause a tuning fork to vibrate by hitting one prong against a
SOUND EXPERIMENTS 29
table top and place it firmly on the top end of a ruler, the bottom
end of which is resting on a box cover. (See Figure 19.)
The vibrations of the tuning fork in this case are transmitted down
the ruler, causing the entire box cover to vibrate and giving a
loud, clear tone.
Experiment No. 12. The
same effect can be obtained, except in lesser degree, by using
a piece of string instead of the ruler. Tie one end of the
string around the tuning fork and hold the other end firmly against
a box cover with the thumb of your left hand. (See Figure
19A.) Holding the tuning fork in your right hand, set it in
vibration and then draw the string tight. By placing your ear
close to the end of the string which is pressing against the box,
you will be able to hear the tone of the tuning fork quite
distinctly. The fact that this sound is being transmitted down
the string may be proved by lifting the end of the string off the
box, in which case the sound is not nearly so loud. The reason
for this is that when the string
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is pressing against the box cover, the entire cover vibrates just as
in the case of the preceding experiment, causing a louder
tone. When the end of the string does not press against the
box cover, the resonance of the box does not come into play and
therefore the sound is not so well.
The old Indian method of listening for the advance of an enemy on
horseback was to place his ear on the ground. Put your ear to
a telegraph pole and you can hear the hum of the wires, whereas you
cannot hear it through the air. Take a long stick of wood or,
better still, a section of a gas pipe and place your ear at one end
and have someone lightly scrape the other end with a piece of metal;
and although with the ear away from the piece of wood or pipe
the scraping cannot be heard, it is quite audible while the ear is
at the end. These are all familiar instances of the
transmission of sound through solids.
TO MAKE A TOY TELEPHONE
Experiment No. 13.
Take two tin boxes. Pierce the center of each of the boxes
and, through the holes, put a stout cord or string and stretch it as
per illustration. Now this is a very simple kind of a toy
telephone, and yet is demonstrated quite conclusively and
effectively how nicely sounds will travel through solids - that is,
down the string by means of vibrations (the "to and fro"
motion). This "to and fro" motion, which is caused by the
vibration on one of the tin cans, sets the string in vibration which
transmits the sound from one to the other. In this way two may
talk much further apart and much more distinctly than they could
through the air. (See Figure 20.) Any boy can rig up a
telephone without very much expense and have great fun with
it. You can elaborate on this apparatus considerably and make
quite a unique telephone.
From the foregoing experiments you can readily see that some
SOUND EXPERIMENTS 31
substances are better conductors of sound than others. Both
water and iron or steel are very much better conductors than
Most of the sounds we hear, however, come through the air, and even
though we receive sounds which have traveled through solids as we
have described, these sounds must travel a little way through the
air to reach our ears. Is it not strange that Nature has
provided the air as a principal medium for the carrying sound when
it is such a poor conductor?
SOUNDS CAN COME THROUGH THICK WALLS
By this time you can readily understand why it is that sounds can be
relayed through thick walls, if such a wall is a good conductor of
sound. If a wall is constructed of materials that are good
sound conductors - such as wood - the sound will be relayed through,
regardless of its thickness. Sometimes sounds cannot be heard
through walls, owing to the fact that they have
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been deadened. What is meant by deadening a floor or
wall? It is a very common practice in offices or any place
where privacy is necessary to put some substance like felt or
certain kinds of paper that are non-conductors between the walls or
floors and thereby prevent sound from passing through. They
are then called sound-proof.
FAST DOES SOUND TRAVEL?
We have already had proven to us by various experiments in
transmitting sound by steel rails, cast iron pipe, wood and through
water, that the speed with which the sound waves travel depends very
much up on the medium.
A very interesting experiment on the velocity of sound was recorded
in a demonstration made at Lake Geneva. An explosion was
produced by powder at the same time that a bell was struck under the
lake, and an observing station some eight miles away accurately
measured the time from the flash as seen by the eye to the sound as
recorded through the water. This is of interest because it
shows the simplicity and the accuracy with which the velocity of
sound in different matters can be recorded.
Noises of every variety, whether musical or discordant, high or low,
move through the atmosphere over the surface of the earth at a
velocity of 1,090 feet a second, 765 miles an hour at 0°
centigrade. Just think how fast a racing auto goes.
Sound travels through the air about ten times as fast. Light
travels at the rate of about 670,000,000 miles an hour, or over one
million times as fast as sound.
The velocity of sound and the rate at which it travels is of great
importance in measuring the time of certain things. Take, for
example, the timing of an athlete in a hundred-yard dash.
The next time you see a college track meet watch the timers.
SOUND EXPERIMENTS 33
You will find that they are very careful to watch for the flash of
the pistol. The man who knows how to time a race knows that
the timing by the flash is the only accurate way. The poor
timer is the one who listens for the sound. Their stopwatches
are set the instant the flash is seen and not when the pistol is
heard. The timer who does not start his watch until he hears
the sound of the gun gives the runner credit for more speed than he
This all explains to us why it is we see the steam from a factory
whistle before we actually hear the noise. It also explains
why we see the lightning and then hear the thunder afterward.
In reality they have both occur at the same time; but it takes sound
so much longer to travel than light it is easy to understand why we
see the flash of lightning long before we hear the clap of
thunder. The next time you witness a thunder storm watch for a
flash of lightning; then count the number of seconds between the
flash and the time at which you hear the clap of thunder and divide
this by five, and it will tell you how many miles away the lightning
is. This is a good experiment in itself.
DETERMINES THE VELOCITY OF SOUND
There are many things that enter into the velocity at which sound
will travel, and particularly important are the elasticity and
density of the medium through which the sound may be passing.
In other words, the more elastic the medium the greater will be the
velocity of sound traveling through it and the denser the medium the
less will be the velocity. Therefore, through warm air, sound
waves will move more rapidly than through cold air. Ask
yourself why this is. You should know that warm air is
expanded air because heat expands it. Consequently it is not
so dense. While sound at zero temperature on
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a centigrade thermometer travels at the rate of only 1,090 feet
per second, as the temperature increases, at each degree the
velocity of sound will increase two feet per second.
Through other mediums sound travels much faster. Through water
it travels at a velocity of 4,708 feet per second; through
solid like tin, for instance, at a velocity of 8,107 feet per second
and through solids like iron or glass and certain woods sound
attains a velocity of 18,530 feet per second. Why these
Perhaps the answer to this question had better be left until we have
finished our next series of experiments and learned how sound