Wireless Telegraphy and Telephony Simply Explained / A Practical Treatise Embracing Complete and Detailed Explanations of the Theory and Practice of Modern Radio Apparatus and Its Present Day Applications, Together With a Chapter on the Possibilities of Its Future Development

by Alfred Powell Morgan

CHAPTER VII. THE EAR. HOW WE HEAR. SOUND AND SOUND WAVES. THE VOCAL CHORDS. THE STRUCTURE OF SPEECH.

On either side of the head, lodged in a cavity which they do not completely fill, and situated in the midst of a dense and solid mass of bone, entering into the base of the skull and forming the temporal bone, are two membraneous bags called the membraneous labyrinth and the scala media of the cochlea. Each bag is filled with a liquid, and is also surrounded and supported by a fluid which fills the cavity in which they are lodged. Certain small, hard bodies, free to move around, lie in the fluid of the bag. The ends of the auditory nerve of hearing are distributed around the wall of the sac, so that they are subjected to the blows of the little particles of calcareous sand, or otoconia, as they are called, whenever the fluid in the bags is disturbed.

FIG. 120.—Diagram of the ear.

The membraneous lining on which the ultimate ends of the nerves are spread is virtually a sensitive beach, and the little otoconia, showers of pebbles and sand, which are raised and let fall by each succeeding wavelet of sound. This wonderful mechanism constitutes the inner ear.

The ear, as a whole, consists of three parts: the outer ear, which is a trumpet-shaped passageway called the pinna serving to collect the sound waves and pass them on through the auditory canal to a small membrane called the eardrum; the ossicles, a series of three little bones, the hammer, the anvil, and the stirrup, they are called; and the inner ear just described.

FIG. 121.—The ossicles.

The foot of the stirrup is connected with an oval membrane, which closes a hole in the inner ear.

Sounds passing through the auditory canal cause the drum to vibrate and send tremors through the bones to the liquid in the little sacs. The tumbling of the "pebbles" against the filaments of the auditory nerve sends the intelligence to the brain.

The impression which the mind receives through the organ of hearing is called sound.

All bodies which produce sounds are in a state of vibration, and they communicate their vibrations to the surrounding air and thus set it into waves, just as a stick waved back and forth in a pool of water creates ripples.

Sound implies vibration, and whenever a sound is heard some substance, a solid, a liquid, or a gas is in vibration and the surrounding air is in unison with it.

FIG. 122.—Bon jour ("good day" in French) as represented by a wave picture. The picture was made by a mirror arranged to move under the influence of the voice and to cast a beam of light upon a strip of sensitized paper.

Sound has been likened to a picture painted not in the space and color of substance but in time and motion. What really passes out from the source is merely a rhythmical motion of the air particles, manifesting themselves as changes in pressure, spreading out in ever-widening spheres through the atmosphere. The order of these compressions is different for every sound. The musical sounds of an orchestra embody a different set of vibrations for each note of each particular instrument. If the fluctuations in pressure of a sound wave are irregular and non-periodic, the sound is called a noise; if they are cyclic, and follow a regular and sufficiently rapid periodic lag, the sound is musical.

We may easily satisfy ourselves that in every instance in which the sensation of sound is produced the body from whence the sound comes must have been thrown into a state of rapid tremor, implying the existence of a motion to and fro of the particles of which it consists.

FIG. 123.—Experiment showing sounding bodies are in vibration.

If the face of a tuning fork prong be touched with a small ball of cork suspended from a fine silk fiber, after the fork has been struck and caused to emit its note, the cork will be violently repelled from the latter. Why? Because the prong of the fork is in vibration.

If a small wire or bristle is fastened to the prong of the fork and a piece of smoked glass drawn across it while the fork is giving forth a sound, the trace of the point will appear as a wavy line, showing that while the glass was drawn along the prong went to and fro many times.

The vibrations or disturbances set up in the air by a sound emitting body are known as sound waves. These waves consist of a series of condensations and rarefactions succeeding each other at regular intervals, each air particle swinging to and fro in a very short path.

FIG. 124.—Method of registering vibrations of a tuning fork.

Air waves cannot be seen by the naked eye, but their nature may be easily represented or illustrated. Fig. 126 gives a pictorial representation of the crowding together of the air particles during the passage of a wave. The loudness of the sound depends upon the amount and suddenness of the change in pressure, and the note or pitch on the number of complete to and fro motions of the particles per second.

FIG. 125.—Wavy line made by a bristle attached to a tuning fork prong in vibration when passed over smoked glass.

The timbre of a sound or the quality that distinguishes the note of a violin from that of a piano depends upon the smoothness or abruptness of the changes in pressure. Therein lies the difficulty of the production of sound by means of a phonograph or telephone, for the sound waves must resemble each other in every detail in order that the result may be like the original.

FIG. 126.—Illustrating the action of air waves.

The mechanism with which we speak or sing is composed of two flexible membranes, stretched side by side across a small cylindrical box located at the top of the windpipe. The membranes are called the vocal chords, and the box the larynx. The chords are so arranged and controlled by muscles that their tension may be changed at will. In breathing, the air to and from the lungs passes freely between the chords. When the controlling muscles are tightened, so as to stretch the chords, the edges are also brought parallel and quite close to each other. If the breath from the lungs is then forced through the narrow slit between them, they vibrate like the reed of a musical instrument, and produce the sounds of the voice. The multitude of sounds which it is possible for a human being to produce are the result of various degrees of stretching of the vocal chords, together with the movements of the mouth, lips and tongue.

FIG. 127.—The vocal cords in position for making a sound.

FIG. 128.—The vocal when relaxed.

Speech is the sound produced by the vocal chords of a human being, modified by the movements of the lips, tongue, and cavity of the mouth. The consonants are made by movements of the tongue and lips obstructing the sounds at their beginning or end, while the vowels are formed by a steady voice modified by the resonance of the different shapes or sizes given the parts of the mouth. The waves produced in this manner are transmitted to the ear, and the sensation of sound is caused by the impact of the otoconia against the auditory nerve, giving a series of impressions, musical or unmusical, pleasing or displeasing, as the case may be. Many interesting experiments showing the nature of the sounds of the human voice may be performed by means of a simple apparatus invented by Koenig of Paris. A box is separated into two compartments by a rubber membrane. Gas is led into one of these compartments by a rubber tube, and then allowed to issue to a burner. The other compartment is connected to a megaphone.

FIG. 129.—Koenig's manometric flame apparatus.

FIG. 130.—Appearance of manometric flames in a revolving mirror.

Two pieces of mirror are arranged so as to revolve in front of the lighted jet or burner. When the human voice is produced in front of the megaphone, the air waves strike the membrane and cause changes of pressure in the gas. The height of the flames varies with each change in the pressure, and when viewed in the mirror resemble a band of light having an edge like a saw. The teeth are faithful representations of the changes in the voice, and immediately take on a new appearance when a new sound is emitted. The shape of the teeth changes with the tone, and the number of teeth with the pitch. Fig. 130 shows the flames produced by singing the sound oo, as in tool. The same sound an octave lower in pitch will show as in B, where there are just one-half as many teeth or vibrations. The sound of oo is a simple sound. If o on the note is sung into the megaphone, the image in the mirror will appear like that shown by C, being made up of alternating large and small teeth, the former corresponding to every alternate vibration of the octave of the higher sound coinciding with a vibration of the octave below.

FIG. 130 a.

The sound causing the flame to appear, as in D, is made up of two simple vibrations combined.