Discoveries and Inventions of the Nineteenth Century

Everything yet contrived in the way of recording instruments is eclipsed in wonder and interest by one which is among the latest marvels of the age. It is a recording instrument, and more than a recording instrument, for it can reproduce to the senses the very phenomena it records; and these same phenomena are the most familiar in their effects, and, at the same time, so subtle and delicate, that the impressions they convey are not generally thought of otherwise than in connection with our finest intellectual and emotional perceptions. We are alluding to the phonograph, which can register for us music and song, and articulate human speech in all their tones and modulations, and, like an aërial spirit, address them to the ear again, as often as we wish, and thus Inform the cell of hearing, dark and blind; Intricate labyrinth, more dread for thought To enter than oracular cave, Strict passage through which sighs are brought, And whispers for the heart, their slave; And shrieks, that revel in abuse Of shivering flesh; and warbled air, Whose piercing sweetness can unloose The chains of frenzy, or entice a smile Into the ambush of despair; Hosannas pealing down the long-drawn aisle, And requiems answered by the pulse that beats Devoutly, in life’s last retreats! In order that the reader may understand the action of the phonograph, it is necessary that he should know something of the science of sound. Then we must remember that this word is commonly used to express sometimes those sensations of which the ear is the organ, and at other times the external cause of those sensations. It is with the former meaning that we use such expressions as “a sweet sound”; with the latter, such phrases as “sound travels.” It will not be necessary to speak of the physiology of the organ of hearing; but attention should be directed to the different kinds of audible perceptions we can distinguish, let us suppose, when listening to a song: First, there is the pitch, or the notes in the musical scale, which, by their particular sequence, constitute the air or melody. Second, there is the degree of loudness or lowness of the notes. Third, the enunciation, or those differences by which we distinguish, for 666example, the vowels a, e, i, o, u, one from another. Fourth, the quality of the voice by which we can distinguish between two vocalists singing the same vowel on the same note with equal loudness. Observe that these four kinds of sound perceptions are independent one of another. The last kind of difference may also be well illustrated by the instance of musical instruments of different kinds sounding the same note, in which case the difference of the quality or timbre is readily recognized. We have now to show the nature of the mechanical movements outside of us which act on the ear and reach our perception as sounds, giving the distinguishable impressions that have been enumerated; and for the present we shall consider the case of such musical sounds as those just referred to. That the source of a sustained sound is an elastic body in a state of vibration is a fact of which, in most cases, we are easily made aware by the evidence of sight and touch; as a bell, a violoncello string, a pianoforte wire, or a tuning-fork. On p. 656 is described a simple method by which a tuning-fork may be made to write down its own vibrations, and the more exact plan of recording them on the surface of blackened paper on a revolving and advancing cylinder has also been referred to. By the intervention of appropriate apparatus, a similar record may be obtained from all sounding bodies. From observations of this kind, and others in which totally different methods are used for counting the number of vibrations per vibrations made in a given time, it is known that the pitch of the sound or note depends on the rapidity of the vibrations—the pitch rises with the number of them per second, and the relationship between the notes of a musical scale depends entirely on these numbers. Thus, when the vibrations for the eight notes of an octave are counted, the numbers always have this proportion, beginning from the lowest note-–24, 27, 30, 32, 36, 40, 45, 48. Thus of the two notes— as produced on musical instruments tuned to the concert pitch of the present day, the lower corresponds with 264 complete vibrations per second, the higher with 528. It will be observed, too, that all the harmonies are determined by some simple ratio in the rates of vibration: the interval of the fourth is 3, 4; that of the fifth is 2, 3, etc. Another easily discoverable fact is that the loudness of the sound depends upon the amplitude of the vibrations. This is sufficiently obvious by a few experiments with a tuning fork; and by close examination of such tracings as have been mentioned, we shall soon become aware of another circumstance—namely, that the vibrations not simple, but that the larger or general movement has one or more sets of small vibrations within it. In Fig. 319a, A is the curve that would be traced by the tuning-fork in a state of simple vibration; B and C are tracing such as are given by a fork in two of its modes of vibration. The fork gives out its proper or fundamental note in both cases; but the ear recognizes a difference in the quality of the sound due to the smaller and more numerous vibrations. Differences of the same kind are recognized in the notes produced by different musical instruments; but these are usually more complicated, and their forms are characteristic of the particular quality of the tone, 667which is thus shown to be due to the superposing of several related systems of vibration upon the fundamental one. Thus three out of the four qualities of sound recognized by the ear have had their physical causes assigned. As for the fourth—namely, the distinction we perceive among the different vowels sung on the same note—it has a physical origin identical with the last. For since parts of the vocal organs assume different positions in enunciating the different vowels, they constitute for the time being so many varied musical instruments, and the graphical traces of the sounds (for they can be obtained) show a corresponding modification. Here, in Fig. 319b, for example, are represented the tracings of the vibrations given to the air in various vowel sounds. It is also through the vibrations conveyed by the air to the little membrane called the drum of the ear that the sensations of sound are received, and of the nature of these vibrations a few words must be said presently. In considering the different qualities of sound, we have so far confined ourselves to sustained musical notes, as, for instance, the vowel sounds in singing. This has been done to show the relations of rapidity of vibration to pitch and for simplicity of illustration of the superposition of vibrations, etc. Two other remarks must be added—viz., that the vibrations of musical tones are isochronous—that is, whether the note be loud or soft, the same time is taken up in each vibration corresponding with the same fundamental note. Other vocal sounds than sustained vowel notes are found to be due to still more complicated combinations of vibrations of shorter duration, and noises, as distinguished from musical sound, are formed also by the superposition of a greater or less number of systems of vibrations, the rapidities of which are wanting in harmonic relations such as we have pointed out belong to the musical scale. Even in noises, however, there is often one or more predominant systems of vibrations 668which a musical ear can detect. If the reader will hold a pencil or penholder at one end and tap with it on the edge of the table, passing in quick succession to parts progressively nearer where the pencil is held, he will hardly fail to recognize a rising pitch in the little noises. Fig. 319a.—Traces of Vibrations of a Tuning-Fork. Fig. 319b.—Phonautographic Tracings of Different Vowel Sounds. Fig. 319c.—Diagram. Fig. 319d.—Phases of Sound Waves. A word or two must be said as to the way in which sound is transmitted through the air. This progression is commonly spoken of as a wave motion, but it must not be thought of as taking place in the form familiar to us as waves on water; still less must the reader confound it with the sinuous lines shown in the graphical representations of vibrations given in the figures. It is rather a series of rapid pulsations of the particles of the air taking place in the direction in which the sound is propagated, and resembling waves on water only by presenting periodical phases in uniform succession. The difference may be illustrated from what may be seen in a field of wheat when the wind is blowing over it. The stalks bend down, and rise again when the breeze has passed, and thus the general appearance of the waves of the sea is produced. If we confine our attention, however, to the motion of the several ears of the wheat in a file of stalks, we shall obtain a clearer notion of what takes place in the so-called waves of sound. The positions of the stalks at some one instant of time may be represented by the diagram, Fig. 319c. Each stalk is swinging backwards and forwards like an inverted pendulum, and the successive phases of these vibrations bring the adjacent ears nearest to each other about i, and farthest apart at a and a´. The places of these, and of all the intermediate degrees of approximation and retreat, pass along the file. Instead of the ears of wheat swinging on their elastic stalks, suppose particles of air approaching and receding by virtue of the elasticity by which they resist compression and recover from it, and you will obtain an elementary idea of what takes place in the transmission of sound. Fig. 319d is a picture of a column of air acted upon by a tuning-fork. The swiftly advancing prong is compressing the air in front of it, and in swinging back it will tend to leave a vacuum behind it by which the air is partially rarefied; and these alternate condensations and rarefactions will travel 669along through the air by virtue of its elasticity, and the mechanical action by which they are able to agitate a stretched membrane (or other elastic body), so that its vibrations will correspond with them in period and magnitude, may be easily understood. The vibration produced is a simple one, but any number of other systems may pass at the same time, and each one will be propagated as if the rest did not exist, just as we may see different systems of undulations moving on the surface of water. It should be observed that the velocity of propagation is the same whatever may be the period or the magnitude of the vibrations. The high and the low, the loud and the soft notes of a piece of music played at a distance, all take the same time in reaching the ear. Light as are the particles of air, the mechanical actions which a number of them carrying strong vibratory impulses will produce, may be illustrated by the rattling of window panes by a loud peal of thunder, and may be bodily felt by a person standing close to a very large bell while the hour is striking. We have referred to instruments for registering sound, and even vocal sounds, before anything has been said of the construction of the phonograph, and it is, in fact, many years since the problem was solved of recording the vibrations produced by speech. Mr. Leo Scott, in 1856, invented an instrument, called the Phonautograph, which did this. It consisted of a cone of sheet zinc like a large ear-trumpet, across the smaller end of which was stretched