Sound wave

“ Although each atom of air moves a very little way forwards and then back, yet, as a long row of atoms may be crowded together before they begin to part, a wave is often very long. When a man talks in an ordinary bass voice, he makes sound-waves from 8 to 12 feet long; a woman's voice makes shorter waves, from 2 to 4 feet long, and consequently the tone is higher, as we shall presently explain. ”

“ A sound wave, set up by the vibrations of a bell, or other sound producing device, travels through the air at the rate of 1,090 feet per second, while light and heat waves set up by the vibrations of a flame, the Sun or other hot body, travel through the ether at the rate of 186,500 miles per second. ”

“ What in physics is called a sound-wave is constructed as follows: We have a line A X, which represents the normal condition of the air through which the sound is traveling, and curves which represent to the eye—first, the relative amounts of compression (+) and rarefaction (—) brought about by the sound in the case of each pulse, and secondly the relationship of this to the actual length of the wave, or, what is the same thing, the time taken for the pulse to travel. ”

“ To account for sound originating in one place being heard in another, it is observed, that sound is propagated by undulation, wave after wave, radiating in every direction, from a centre, like the blossoms of a Nauclea. ”

“ The wave theory of sound requires, if we expect to hear sound by means of a C3 fork of 256 vibrations, that the molecules of the air composing the sound wave must not be interfered with in such a way as to prevent them from traveling a distance of at least 1/50 to 1/1000000 of an inch forward and back in the 1/256 of a second. ”

“ It will be necessary now to prove experimentally that the same things can be done with sound, in order that a body of proof may be built up in your minds convincing you that the external cause of sound-sensation must be a wave-motion in the air. ”

“ 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. ”

“ When the air is set in motion by the vibration of certain bodies of wood, metal and other material, we know that sound waves, striking upon the tympanum of the ear, penetrate to the brain and imagination. ”

“ A simple noise being a sound-wave, has its wave-form or make-mark. The blending of many such would produce a tone in that likeness; hence, a musical note is a blending of like noises, and every noise is really the first wave, the key-note of a musical tone. Take the ringing of a door-bell. Here the ear hears not only a musical tone, a sound-blend, in the ring of the bell, but also the noise of the clapper's clang, clang, clang. The vibrations of the bell throw the air into musical waves, shown in Fig. 5, while a huge clang-wave will sweep
Fig. ”

“ 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. ”

“ Each reflection being thus again and again divided and reflected, we have, following close upon the primary wave, a multitude of secondary waves, which, falling later and later upon the ear, greatly mask the distinctness of the original sound, and give rise to indistinctness and confusion. ”

“ Nature of Sound Waves.—If some sonorous body, as a bell, be struck, it is given a quivering, or vibratory, motion. This is not confined to the bell, but is imparted to the air and other substances with which the bell comes in contact. These take up the movements and pass them to objects more remote, and they in turn give them to others, until a very considerable distance is reached. Such progressive vibrations are known as waves, and, since they act as stimuli to the organs of hearing, they are called sound waves. ”

“ Highest and Deepest Tones—Rapidity of Vibration Determined by the Siren—Determination of the Lengths of Sonorous Waves—Wave-Lengths of the Voice in Man and Woman—Transmission of Musical Sounds through Liquids and Solids
IN OUR last chapter we considered the propagation through air of a sound of momentary duration. We have to-day to consider continuous sounds, and to make ourselves in the first place acquainted with the physical distinction between noise and music. As far as sensation goes, everybody knows the difference between these two things. ”

“ Hearing, feeling (as regards warmth) and seeing are all produced by vibrations. It is quite in accord with the doctrine of modern science to believe that the morning stars did “sing together,” for light is essentially rhythmic, and to senses adapted to the perception of their harmonies, the sunbeams would make music. The various colors of the spectrum differ solely in the wave-lengths of their vibrations. The red corresponds to low pitch in music and the violet to high pitch. As the vibrations of air striking upon the ear increase in rapidity, the sound [324] rises in the scale. ”

“ Not only must the object, either solid, liquid, or gaseous, be in motion to produce sound, but the air surrounding the vibrating body must also be moving in unison with it. And lastly there must be some medium of receiving the sound waves—the ear or some part of the body. ”

“ In all departments of natural science we find ourselves confronted by the phenomena of wave-motion. In the study of earthquakes and tides, telegraphs and telephones, as well as terrestrial temperature, no less than in the examination of water waves and ripples, sound, music, or light and heat, we are bound to consider waves of some particular kind. ”

“ Whether the sound waves are long or short, whether they can be detected by the ear or not, we still say that all are sound waves, and that all are due to the vibrations of the sounding body, which vibrations are transmitted through the air, in waves, that fall upon the tympanum or drum of the ear, and set that vibrating, which vibrations are transmitted to the auditory nerve and so give rise to the sensation of hearing. ”

“ This sets in motion a series of waves, which, traveling with the enormous velocity of which I have spoken, produce such rapid vibrations that the ear, unaided, cannot perceive the sound, but by means of the other half of the seismaphone these sound waves are collected and so transformed by the corresponding wires and diaphragm [57] that the voice is reproduced by one instrument in exactly the tone spoken. ”

“ As sound moves with different velocities through media of different densities, the wave which produces the sound will be partly reflected in passing from one medium to the other, and the direction of the transmitted wave changed ”

“ Nature of sound—The ear—Musical instruments—The vibration of strings—The sounding-board and the frame of a piano—The strings—The striking mechanism—The quality of a note.
Sound differs from light, heat, and electricity in that it can be propagated through matter only. Sound-waves are matter-waves, not ether-waves. This can be proved by placing an electric bell under the bell-glass of an air-pump and exhausting all the air. Ether still remains inside the glass, but if the bell be set in motion no sound is audible. ”

“ There is also a vast difference between the velocity of vibration of the air waves and those of the ether. The human ear is sensitive to sound waves of somewhere between twenty-nine per second to thirty-eight thousand per second; while the eye responds to light [52] waves of from five hundred million to one billion per second. Some ears are better adjusted to the low vibrations and some to the high, and the ears of no one hear any but a small part of the melody of a great symphony. ”

“ Value of Sound Waves to the Body.—From a physiological standpoint, the value of sound waves is not easily overestimated. In addition to the use made of them in the communication of ideas, they serve the purpose of protecting the body, and in the sphere of music provide one of the most elevating forms of entertainment. ”

“ We see, then, that the sound-waves necessary to reproduce accurately the various vowel sounds, are more complicated than those which would correspond to the fundamental tones simply in which any sound may be uttered. ”

“ First, its intensity, or loudness, which is governed by the height, depth, amplitude—for these amount to the same thing—of the waves produced in the medium. Second, the timbre, or quality, which is regulated by the shape, or outline, of these waves. Third the pitch, high or low, which is controlled by the distance from crest to crest of the sound-waves—or, as we say, from node to node of the vibrations. ”

“ They learned that sound travels through some medium, like the air, by means of waves—waves of alternate condensation and rarefaction of the air, for example. Then these men studied the human ear, and they learned that as these sound waves struck the ear, the drum of the ear was made to vibrate back and forth as a condensed part of the air or a rarefied part of the air struck it. ”

“ The tuning fork is musical. Here we have rapid, regular motion; vibrations follow each other at perfectly definite intervals, and the air disturbance produced by one vibration is exactly like the disturbance produced by a later vibration. The sound waves which reach the ear are regular in time and kind and strength, and we call the sensation music.
To produce noise a body must vibrate in such a way as to give short, quick shocks to the air; to produce music a body [279] must not only impart short, quick shocks to the air, but must impart these shocks with unerring regularity and strength. ”

“ When a sounding body is moving rapidly forward, the sound-waves are crowded together a little, and therefore shortened; when it is moving backward, the sound-waves are drawn out, or lengthened a little. ”

“ The slippery big trout dropped from his fingers. He sat motionless, and stared about him.
Nothing stirred; the ripple on the lake had died away; there was no wind; the forest lay a single purple mass of shadow; the yellow sky, fast fading, threw reflections that troubled the eye and made distances uncertain. But there was no sound, no movement; he saw no figure anywhere. Yet he knew that someone watched him, and a wave of quite unreasoning terror gripped him. The nose of the canoe was against the bank. In a moment, and instinctively, he shoved it off and paddled into deeper water. ”

“ Sufficient has already been said to make it clear to you that one essential difference between sound or noise and music, as far as regards the events taking place outside of our own organism, is that, in the first case, we have a more or less irregular motion in the air, and, in the second, a rhythmical movement, constituting a train of air waves. ”

“ As the waves [Pg 743] of light touch the optic nerve, so do the grosser waves of air touch the auditory nerve. But sound is only a recognized sensation when the waves of air are within a certain measurement, a maximum and minimum of length. The rush of a whirlwind has no sound, except when arrested by some object, and smaller waves of the vast billows of rolling air are created. We say that the wind roars. ”

“ The waves of air carried the sounds along until they reached a pine wood which shut in the field. They struck the tall trees, and were reflected, or sent back again, almost as clearly as when first spoken.
Just in this way echoes of sound are, like birds, ever on the wing: the whole air is alive with them. The walls of our rooms give back the tones of our voices, but we hear no echo, because they are so near that the repeating of the sound comes almost at the same moment as the sound itself. There are echoes on all sides of us, and no sound is ever lost. ”

“ Light comes in undulations to the eye, as tones of sound to the ear. Must not light also sing? The lowest tone we can hear is made by 16.5 vibrations of air per second; the highest, so shrill and "fine that nothing lives 'twixt it and silence," is made by 38,000 vibrations per second. ”

“ The Various Qualities of Sound.—Physically, sounds consist of vibrations, and these are, generally speaking, aërial waves. When the waves are non-periodic the result is a{54} noise; when periodic it is what is nowadays called a tone, or note. The loudness of a sound depends on the force of the waves. When they recur periodically a peculiar quality called pitch is the effect of their frequency. ”

“ By various contrivances we can compel the vibrations of the air to declare themselves; we know the length and frequency of sonorous waves, and we have also obtained great mastery over the various methods by which the air is thrown into vibration. We know the phenomena and laws of vibrating rods, of organ-pipes, strings, membranes, plates, and bells. We can abolish one sound by another. We know the physical meaning of music and noise, of harmony and discord. In short, as regards sound we have a very clear notion of the external physical processes which correspond to our sensations. ”

“ Thus the air must include a vast variety of molecules of various degrees of elasticity, and capable of vibrating in as many different periods as there are tones from all kinds of sonorous bodies; and these molecules, set in motion by the musician and falling on our ear, answer to our ideas, according to each man's temperament. I myself believe that sound is identical in its nature with light. Sound is light, perceived under another form; each acts through vibrations to which man is sensitive and which he transforms, in the nervous centres, into ideas. ”