Audion

“ Although the audion is familiar to all amateur radio-operators, we shall have to give a brief outline of its construction and operation for the benefit of those who have not had the opportunity to dabble in wireless telegraphy.
The audion is a small glass bulb from which the air is exhausted to a high degree of vacuum. The bulb contains three elements. One is a tiny filament which is heated to incandescence by a battery, so that it emits negatively charged electrons. The filament is at one side of the bulb and at the opposite side there is a metal plate. ”

“ Of all instruments and devices used in radio work, the audion bulb is by far the most difficult to learn how to handle. Probably 90% of the amateurs of the country do not know how to get the best results from an audion bulb nor how to keep the bulb in best working condition. The information given in Mr. Cole's article is the first time that all of the latest details known about the audion have been put together in one article in such form that the amateur can understand it. ”

“ If you will think, you will realize that if you pass through an Audion Bulb or any other apparatus, two or three times as much energy as it is designed to carry, the result will be a short life! This appears in “amplifying” circuits in operation of the filament at excessive brilliancy and in rapid deterioration of the filament from carrying abnormal power. The vacuum increases to an extremely high value in most cases. ”

“ I’ll say this much, however, it is 64 the vacuum or audion tube detector that gives results, and the application of a loud speaker is only possible with a vacuum or audion tube. It is as easy to build a vacuum tube set as a crystal set and only a very little more expensive. So, whether you are building or buying a set, make it a good set, something that you can hear with a good many hundred miles.
“Now, you can buy the parts and build a receiving set that will generally give more satisfaction than a bought set.” (Bill stepped over to the blackboard and took up a pointer.) ”

“ If there were another type of generator, for undamped waves, as efficient as the audion, and if the beat effect produced by the generator were utilized on an ordinary detector circuit as sensitive as the ordinary audion receiver circuit. would this circuit be as sensitive and satisfactory as an Armstrong circuit?
What is meant by “increase in audibility”—an amplifier concern claims to have an instrument that increases the audibility 1500 times. ”

“ The operator will find “A” and “G” never need a large capacity. The whole secret of getting effective results from this connection is to tune the condenser combination properly. That is, to vary the capacity of the various condensers so that the Audion will oscillate. Once the Audion is generating undamped waves, the set is tuned with the loose coupler and the signals heterodyned. This merely means “beats” are produced. The operator can vary the frequency of the signals at will, as the “beat” frequency depends upon the capacity of the condensers. ”

“ Regular Audion Detector Bulbs are not adapted for the reception of continuous waves, because the vacuum is not correct for the purpose and because the filaments must be operated at such a high intensity that they give very short service, making them unnecessarily expensive. ”

“ I’ll tell you about wave lengths later. First let’s see how to control the frequency of an audion oscillator like that of Fig. 38.
It takes time to get a full-sized stream going through a coil because of the inductance of the coil. That you have learned. And also it takes time for such a current to stop completely. Therefore, if we make the inductance of the coil small, keeping the condenser the same, we shall make the time required for the current to start and stop smaller. That will mean a higher frequency for there will be more oscillations each second. ”

“ For transmitting a 1 kw. transformer is used with an aerial 75 feet long, and 70 feet, high. This station has worked 1ZL, 8NB, 8ZO, 8YL, and has been heard by many stations in the East. The Club has at its disposal an audion detector and for receiving uses an aerial 200 feet long 150 feet high at one end, and 60 feet at the other. ”

“ Not only is the vacuum tube a detector of electric wave signals and speech and music but it can also be used to amplify them, that is, to make them stronger and, hence, louder in the telephone receiver and further its powers of amplification are so great that it will reproduce them by means of a loud speaker, just as a horn amplifies the sounds of a phonograph reproducer, until they can be heard by a room or an auditorium full of people. ”

“ In using the Audiphone during conversation there is no movement or vibration felt by the teeth; in listening to a piano there is a very faint sensation as if the Audiphone vibrated slightly, while with the handle of the Audiphone resting on the sounding-board of the piano the vibrations are so violent as to be painful to the teeth. By closing the ears a person with even acute hearing can observe the admirable manner in which the instrument conveys spoken words to the ear. ”

“ The tone-waves, as they rush out of the open mouth, communicate their vibrations to the air, which conducts the sound onward until it reaches our ears, provided we are within the reach of these atmospheric vibrations. The difference between a cultivated voice or note is soon detected in the purity and regularity with which its sounds reach us as compared to the harsh, irregular, discordant waves impelled by one not so cultivated. Johannes Müller places the extreme range of the human voice at four octaves, but it is quite seldom that the range exceeds two and a half octaves. ”

“ The action is the same on the voice of the speaker. The sounds he utters are complex and as they reach the walls certain components are reinforced and the quality of the sound is changed. This action of resonance may also be caused by the air in a room. Each room has a definite pitch to which it responds, the smaller the volume of the room the higher being the pitch. A large auditorium would respond to the very low pitch of the bass drum. ”

“ The cure for the trouble is brought about by the introduction of materials such as carpets, tapestries, and the like, which act as absorbers of sound and reduce the time of reverberation.
When music is played in an auditorium with a prolonged reverberation, the tones following one another blend and produce the same effect as that of a piano when played with the loud pedal in use. A reverberation is more advantageous for music than for speech, since the prolongation and blending of the musical tones is desired, but the mixing of the words in a speech is a distinct disadvantage. ”

“ This exemplifies the complication of our subject, and indicates that the heavy diaphragm and other details of the loud speaker must produce serious distortion.
Let us be honest at once. We can only [Pg 23] hear such distant places as America by the grace of heaven. Even Sunlight can tune sweet song into vague cracklings. Until true tuning can be obtained we are largely at the mercy of the reproducing instrument, which too often exaggerates every fault and gives the impression that wireless and music are in no way related. ”

“ We recognize at once a difference between the sound of a vowel, say ah, sung by different persons to the same note of the piano and with the same loudness. There is a personal element, an individuality, about voices which at once arrests our attention, apart altogether from the tone or loudness. This sound-quality is determined by the form of the wave-motion, that is, by the nature of the movement of the air-particle during its little excursion to and fro in which it takes part in producing a zone of compression or rarefaction in the air and so forms a sound wave. ”

“ As a scientific proposition it is easy, but the results are not good enough at present, and, if we can improve, let us first consider the loud speaker.
The talking cinema will come as a matter of course. It is so easy to record sounds upon the film by [Pg 30] photography with reproduction by the selenium cell or the neon tube; it is easy to photograph the wave; it is easy actually to impress the sound wave upon the film with the picture, or to use a gramophone; but reproduction is not like a human voice. Neither as yet is any reproduction ever like the voice itself. ”

“ If thus the reproduction [of tones] in exactly similar height and depth is easily attained, it is however difficult for our ear, amidst the always smaller vibrations, to which the diminished strength of the tone is due, to evaluate exactly the magnitude of the vibrations. But the character of the tone depends upon the number of variations of amplitude (Anschwellungen) , that is to say, depends upon whether, for example, in the tones which have similar pitch and therefore a similar number of waves per second, the fourth, sixth, eighth, tenth, or sixteenth wave is stronger than the others. ”

“ 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. ”

“ By speaking into the phonograph and reproducing the words, we are enabled for the first time to hear ourselves speak as others hear us; for the vibrations of the head are understood to mask the voice a little to our own ears. Moreover, by altering the speed of the barrel the voice can be altered, music can be executed in slow or quick time, however it is played, inaudible notes can be raised or lowered, as the case may be, to audibility. ”

“ The quality or timbre of sound, i.e. that which differentiates a note sounded on one instrument from the same note on another instrument, depends neither on amplitude nor on frequency or wave-length. We can only conclude that it depends on wave form, a conclusion fully borne out by investigation. The displacement curve of the waves from a tuning-fork on its resonance box, or from the human voice sounding oo, are nearly smooth and symmetrical, as in fig. 7a. That for the air waves from a violin are probably nearly as in fig. ”

“ It may be here noted that in a paper on experiments in audition published in 1827 Wheatstone said: “The great intensity with which sound is transmitted by solid rods at the same time that its diffusion is prevented, affords a ready means of augmenting the loudness of external sounds and of constructing an instrument which, from its rendering audible the weakest sounds, may with propriety be named the microphone.” ”

“ No human voice, however, has so great a range of tone; the highest soprano notes of women correspond to [282] but 1000 vibrations a second, and the deepest bass of men falls but to 80 vibrations a second.
While the human voice is limited in its production of sound,—rarely falling below 80 vibrations a second and rarely exceeding 1000 vibrations a second,—the ear is by no means limited to that range in hearing. ”

“ If there is no regular recurrence of waves, it is a noise. The rapidity of these regular beats determines the pitch of tones. That quality or timbre by which one sound is distinguished from another of the same pitch and intensity is due to the different complications of waves in the air; the ability to discriminate the various waves in the vibrating air is, therefore, the condition of our finding music in it; for every wave has its period, and what we call a noise is a complication of notes too complex for our organs or our attention to decipher. ”

“ There are some directions in this play which may be new to you, but these are necessary, for you are now in a radio broadcasting studio, talking in front of a microphone. The word [in] means that the character is standing close to the microphone, while [off] indicates that he is farther away, so that his voice sounds faint. When the directions [off, coming in] are given, the person speaking is away from the microphone at first but gradually comes closer. The words [mob] or [crowd noise] you will understand mean the sound of many people talking in the distance. ”

“ In the phonograph, a diaphragm replaces the tuning fork and a cylinder (or a disk) coated with wax replaces the glass plate. When the speaker talks or the singer sings, his voice [305] strikes against a delicate diaphragm and throws it into vibration, and the metal point attached to it traces on the wax of a moving cylinder a groove of varying shape and appearance called the "record." Every variation in the speaker's voice is repeated in the vibrations of the metal disk and hence in the minute motion of the pointer and in the consequent record on the cylinder. ”

“ The acoustic quality of a room may be improved by breaking up the smooth surfaces by curtains or by arrangement of furniture. The echo is then broken up into small waves, none of which may be sufficiently distinct to interfere with the direct voice. Sometimes a sounding-board over the head of a speaker improves the hearing probably by preventing echo from a smooth wall behind him. A large bare floor is undoubtedly bad for acoustics, for when a room is filled by an audience the hearing is much improved. ”

“ All voice-sounds are made by two elastic bands or cushions, called vocal chords, stretched across the end of the tube or windpipe through which we breathe, and as we send the air through them we tighten or loosen them as we will, and so make them vibrate quickly or slowly and make sound-waves of different lengths. But if you will try some day in the woods you will find that a bird can beat you over and over again in the length of his note ”