The
Science Notebook
Gilbert Sound - Chapter
VI
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the information contained in this book is either outdated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 67-81
[67]
Chapter
VI
REFLECTION, REFRACTION,
INTERFERENCE AND RESONANCE
The reflection of sound is very interesting. In the experiment
with the spring we demonstrated that in striking the spring the
waves traveled to one end and, returning, the recoil was felt by the
hand. Sound does exactly the same thing.
ECHO
An echo is nothing more than a reflected sound. Throw a ball
against the side of a building so that it strikes at right
angles. You know the ball will come straight back to
you. Now if you hit the wall at any angle other than straight
against the side, it will glance off. This is exactly what
sound waves do when they hit a building or a hill. If they hit
at right angles the sound will be reflected back to you, and this is
called an echo. We know that sound waves may act like the ball
striking something at an acute angle and glancing off, in which case
they will not revibrate back and do not make an echo.
MULTIPLE
ECHOES
When you make a sound between two hills the echo may be repeated
many times - that is, it rebounds from one hill to the other, etc.,
producing a number of echoes.
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REVIBRATIONS
All of us at one time or another have shouted down into a well or
empty barrel and you know the confused rumbling sounds that you hear
as a result. This peculiar sound you hear is due to the sound
waves which strike the walls of the well or barrel and are bounced
back and forth between them. The walls are so close together
that the sound waves follow one another very closely and make a
confusion of sounds.
Experiment No. 35. Make
two
paper tubes about 3 feet long and 3 or 4 inches each in diameter, or
use two glass tubes about 6 inches long and 5/8 of an inch in
diameter. Arrange them as in Figure 46. Under No. 1 tube
place a watch. In
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SOUND EXPERIMENTS 69
position No. 2 place a piece of glass; at position No. 3 place the
other tube and at the end of this tube put your ear where you may
listen. With your ear away from the tube the ticking of the
watch cannot be heard, but if the ear is placed at the tube, as per
illustration, the ticking of the watch will be distinctly
heard. In other words, the sound waves are passing up tube No.
1, hitting the glass at No. 2 and reflected through tube No. 3 to
the drum of the ear where, in turn, they are recorded by relays to
the brain.
ACOUSTIC
PROPERTIES OF BUILDINGS
You often hear people speak of the acoustic properties of
buildings. By this they refer to the science of arranging a
building so that the sounds made in any part of it may be heard all
over the building. Some of these buildings are called
"whispering galleries." Probably one of the best illustrations
in the United States is the Tabernacle in Salt Lake City.
This tabernacle was built without a single metal nail. The
whole structure being of wood the shape of a bowl. Although it
is some 200 feet long, the acoustic properties are so fine that a
pin dropped in one end of the building can be heard 200 feet
away. The sounds in a building of this kind are repeatedly
reflected from point to point, which causes them to travel around
the walls and be heard in any part of the building.
REFLECTION AND REFRACTION OF SOUND
WAVES
Whenever sound waves meet the surface between two media, both
reflection and refraction take place.
Experiment No. 36. Fill
a
toy balloon with carbon dioxide, which will have to be done for you
in a laboratory. If you take the balloon that has been filled
with carbon dioxide and place it
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between your ear and a watch, then move it back and forth you will
find a position where it will amplify the sound. (See Figure
47.) The cause of this is almost identical with the action of
the convex lens on light waves. (See the Gilbert book on
"Light.") The sound waves are converged to a focus just as
light waves or with a convex lens.
Now exactly the opposite results can be obtained by a repetition of
the experiment only with hydrogen gas. Instead of the sound
waves being converged to a focus, the hydrogen gas diffuses them in
exactly the same way as light waves are diffused in passing through
of concave lens. This refraction of sound is due to a change
in velocity and has been caused by that part of the sound waves that
passes through the balloon. Sound travels through air faster
than through carbon dioxide gas but slower than through
hydrogen gas.
FOG
SIGNALS
A good example of reflection and refraction may be gotten in a boat
out in a fog. It is almost impossible for anyone who has not
had experience to locate where a sound comes from in a fog.
For instance, a fog horn may not be heard at all by a vessel
near the shore and even in danger. Even experienced mariners
cannot be absolutely sure of themselves, because the sound is
reflected and refracted on account of the different
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SOUND EXPERIMENTS 71
temperatures of the air during these times. We have already
learned that sound travels at different velocities in different
temperatures of air. If the lower portion of a horizontal
moving sound wave is in warmer air than the upper part, it will
travel faster and cause the wavefront to change its direction and
may even cause it to curve of upward, as here illustrated (Figure
48).
WIND
WILL FACILITATE THE HEARING OF SOUND WAVES
If you live back of a hill, there will be times when you cannot hear
sounds at all from the other side of the hill and at other times
they will be quite audible. The reason for this is that the
wind will carry the sound waves over the hill when it blows, and
when the air is still the sound waves will not be carried in this
way.
Experiment No. 37. Take
a tumbler or glass jar and fill it
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with water and into this insert a glass tube. Over the glass
tube put a tuning fork, as in Figure 49. Now set the tuning
fork in vibration and moved the glass tube up and down until it
reaches a position where the air in the tube responds to the tune of
the fork and you will find that the sound will be increased and
stimulated. In physics they call this glass tube a resonator.
The prongs, in vibrating, push the particles of air down, forming a
condensation, which goes to the bottom of the tube or to the surface
of the water. Here it is reflected back to the prongs just in
the nick of time to join the next condensation produced by the fork
vibrating upward, and this strengthens the sound.
A straight-sided tube is a poor resonator in that it responds to a
limited number of tones. Resonators have been made in various
shapes, usually with curved bottoms, the object being to obtain
resonance with as many different tones as possible.
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SOUND EXPERIMENTS 73
WHAT
MAKES THE SOUND LIKE WAVES WE HEAR IN THE SEA SHELL?
You are probably familiar with the sounds that are emitted from a
sea shell. They are generally known as sea waves, because some
people imagine that because the shell is found in the sea the noises
is in them are waves. These sounds are really air waves, and
the reason we can hear them so well in a sea shell is because the
shell is a wonderful sounding box. There is nothing better
constructed than the shell as a resonator. The sounding box of
the violin or guitar can be likened to the sea shell, but it is not
nearly as perfect a resonator as the sea shell. A perfect
resonator such as the sea shell will pick up sounds which are not
possible to detect with the naked ear. Although to you
everything about you may be absolutely quiet, the little seashell
will pick up sound waves and magnify them. It has been
demonstrated that the sea shell will not emit sounds in places that
are absolutely sound proof and where there are no sounds.
TABLE
RAPPING
Through the Science of Sound you will be able to understand the
various manifestations which have been given the name of "rapping"
and which are practiced by so-called Spiritualists.
There are many mechanical inventions for producing this phenomenon,
electrically and otherwise, but the most interesting method and the
one that was the origin of all Spiritualism was produced by a
physical action of certain elements in the body. The sounds
were produced in a way that could not be reasonably understood by
people who attended the Spiritualistic seances.
In March, 1848, there began in little rustic cottage in Mydesville,
Arcadia, near Newark, Wayne County, N.Y., manifestations by two
children, known as the Fox sisters in the form of
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GILBERT BOY ENGINEERING
noises that were attributed to the spirits. This was the
beginning of Spiritualism and the news of the phenomenon produced by
these two girls traveled around the world. We will not attempt
to describe the interesting child play that took place and led up to
this discovery, but the facts in the matter are that these two girls
discovered that by disjointing certain bones in their feet they were
able to produce noises and when their feet were placed in contact
with a resonant body such as a vibrating floor or wooden table,
mysterious noises or rappings were produced, the origin of which
could not be detected by anyone. These girls became very
expert and absolutely mystified the entire world with these
phenomenal manifestations. They were nothing more than sound
waves produced by articulations of joints and ligaments which
produced sympathetic vibrations when in contact with resonant bodies
that magnified the sound.
THE
HUMAN VOICE
There is no musical instrument - not even the violin - which is
capable of producing sounds that are so marvelously rich in
overtones as those produced by the vocal cords. This fact is
yet more wonderful when you consider that the vocal chords of a bass
singer, for example, are only about an inch long. Of course,
however, we must remember that they act under forced vibration,
which always results in a greater variety of overtones than free or
natural vibration. It must also be remembered that the length
and tension of vocal cords may be changed at will by the
singer.
Perhaps the most important feature in vocal tones, however, is the
resonating system which Nature has provided. The chest and the
cavities of the mouth and nose serve as resonators and the beauty of
them is that they can be changed appropriately from moment to
moment. The overtones of the lower notes
GILBERT
SOUND EXPERIMENTS 75
are reinforced by the chest as a resonator, which does best when
well expanded. This is why a singer has plenty of air in his
lungs, especially when singing the low notes.
It is not only in singing that the effect of our natural resonators
is felt, but in talking as well. The qualities of sound which
enter into the sounding of vowels and which determine the kind of
voice a person has are controlled by the upper resonators - the
mouth and nose. These can be quickly varied within a wide
range by members of the higher races of mankind. The lower
races, however, such as the South Sea Islanders, do not have this
control over the upper resonator and hence their language consists
almost entirely of consonants, giving one the impression of nothing
but a series of clicks, coughs and sneezes, or even grunts.
INTERFERENCE
IN WAVE MOTIONS
I will ask you to repeat the first experiment with the coil springs,
by which we demonstrated a wave set up by a blow struck on a
wire. Now, just as this wave, which was started by striking
the spring, reaches the other end of the spring[,] strike another blow and produce a
second wave. These two waves will meet in the middle of the
spring and, being of the same amplitude, will counteract each other;
this is called interference. You will notice, though, that
this does not stop vibration of the spring except at the point where
the two waves meet. On the contrary, each wave continues
on in its original direction.
Isn't this just what happens when two sets of swells meet in the
water? Watch the swells from two steamers as they pass one
another. Where the two sets of swells meet, the water is
choppy and you cannot distinguish either set of waves. After a
few seconds, however, you can see one set traveling off in one
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GILBERT BOY ENGINEERING
Direction and the other in another direction, neither of them any
the worse for their encounter.
Experiment No. 38. Take
your
tuning fork and set it in vibration, preferably on a sounding box,
and move it toward a smooth wall. (See Figure 50.) Here you
will observe the phenomenon of the interference of wave motion the
same as in the spring. You will find a position of the fork in
which the sound
GILBERT
SOUND EXPERIMENTS 77
will be faint. In this position, the condensations as
reflected back from the wall just match up with the rarefactions at
the fork, and interference takes place at that point. You can
move the fork and find a position in which the sound is
loudest. This you should recognize as the same principle which
was at work in the case of the resonating tube. The
condensations are reflected back from the wall just in time to join
other condensations produced by the vibration of the fork.
Experiment No. 39. Use
the apparatus described in Experiment No. 37. When you have
adjusted the length of the air
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column so that it is in resonance with your tuning fork, hold the
fork horizontally over the glass tube and revolve it slowly about
its axis - that is, with a rolling motion. You will find a
point where the sound is practically inaudible, it is so
faint. With the fork in this position, slip a cardboard tube
over one prong of the fork, being careful not to touch the
fork. (See Figure 51.) This sound will now be heard very
plainly again.
This experiment shows interference of air waves. Each prong
sends a set of waves down the jar; but when one prong is higher than
the other, the two sets of waves do not reach the bottom of the jar
in the same manner. In other words, a condensation and a
rarefaction reach the bottom at the same time, neutralizing each
other. When the cardboard tube is put over one prong, one set
of waves is cut out and you are able to hear the sound from the
other prong of the fork.
BEATS
The hands of a clock move at different speeds. There are times
when both hands are together, but most of the time they are more or
less separated. In the same manner, two tuning forks may be
vibrating at different rates, yet there will be times when the
prongs of both forks will be in the same relative position.
The result of such a condition may be nicely be shown in the
following manner:
Experiment No. 40. If
a little wax is placed upon one of the prongs of one tuning fork, it
will not vibrate as fast as the other fork. (Refer to Figure
34.) When both forks are vibrating at the same time, you will
hear a sound whose intensity increases and diminishes at regular
intervals. These are called beats and their cause should be
clear to you now. When the prongs of both forks are vibrating
together, the sound waves from the two forks reinforce each other;
but when the prongs
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SOUND EXPERIMENTS 79
are going in opposite directions, interference takes place and the
sound becomes faint.
You may now press a piece of shot into the wax and thereby increase
the load that fork will have to carry. This will still further
cut down the rate of vibration. When the two forks are now set
in vibration, the beats will be more frequent and less pleasing to
the ear. If you reason it out, you will see that the greater
the difference in the rates of vibration the more frequently the two
forks will be in and out of unison.
This feature of musical tones is the cause of harmony and discord in
music which are so often spoken of. It has been found that
when the beats are about thirty-two per second the greatest discord
results. When the beats are fewer than ten or more
than seventy per second, they are somewhat unpleasant, but do
not produce discord.
Take two tuning forks, a long one and a short one. Set one in
vibration and place it firmly on a box cover. Do the same
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With the other fork, placing it near the first one, as shown in
Figure 52. The tone you will now hear will be a pleasing sound
instead of series of beats. The phenomenon of beats takes
place, but the ear does not detect them as such, as in the case of
notes that are nearly the same.
Experiment No. 41. Set
up the apparatus described in Musical Flames in duplicate. In
other words set up two metal
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SOUND EXPERIMENTS 81
tripods covered with wire gauze and place a gas jet under
each. After lighting the gas above the wire gauze, hold a
glass tube over each flame. If these two tubes are of the same
size, you should hear a uniform loud tone. By slipping a
cardboard cylinder over one of these tubes (see Figure 53) and thus
varying its length, you can cause it to give a sound of a different
pitch. If the pitch of the sound made by this tube is only
slightly higher or lower than the other tube, you should hear
distinct beats. By sliding the cardboard cylinder up or down
and thus causing the pitch of the tone from that tube to be changed,
the frequency of the beats will also be changed. You will find
that a point is found where the beats are so rapid that you cannot
detect them, and the result will be a sound similar to that produced
when you vibrated two tuning forks of decidedly different
lengths.
