The Science Notebook
Gilbert Sound - Chapter III  

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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  In addition, some of the information contained in this book is either outdated or inaccurate.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 25 -34

Chapter III

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 the 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-



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.  



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 clock.  


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 manner. 

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


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


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.


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


substances are better conductors of sound than others.  Both water and iron or steel are very much better conductors than air. 

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? 


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 


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.


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. 


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 actually accomplishes. 

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. 


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


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 differences? 

Perhaps the answer to this question had better be left until we have finished our next series of experiments and learned how sound travels.
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