The Science Notebook
Gilbert Sound - Chapter VII

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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  In addition, some of the information contained in this book is either outdated or inaccurate.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

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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. 

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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

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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

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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.  

Go to Chapter VIII or To the A.C. Gilbert Collection

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