Acoustic resonance

“ And if, on the other hand, the resonance-sphere be moved in such a manner that the coincidence of the serrations is not disturbed, it is evident that the motion must be in lines traced upon the curved surface of a body of air—exactly similar in size and form to one, two, three, etc., pulsations sent forth by the organ-pipe. Prof. Mayer was the first to trace the surface of sound-waves by this beautiful and ingenious method. It is highly probable that, by this arrangement, some hitherto unapproachable acoustic problems may meet with a solution. ”

“ And fluid–structure interactions would generate dipole acoustic radiation with its locus situated within the chimney structure. The deployment of a hydrophone array in the near-field may therefore help discriminate between these possible sources by measuring the fall-off rate of the pressure fluctuations and by locating the sound sources.
Potential narrowband sources within black smokers are related to the excitation of the acoustic modes of resonators by unsteady flow. Such resonance typically involves flow past cavities, or flow impinging upon solid bodies. ”

“ At present this is best accomplished by resonance; that is, through the harmonic vibrations created by musical instruments, bringing out the acoustic world as the microscope reveals the hidden visual world. ”

“ We are now prepared to realize the error of the common notion that loudness of tone is due entirely to increase of breath pressure on the vocal cords. Simple experiments with the tuning-fork have shown that while the volume of sound it gives forth is due in part to the amplitude of its vibrations, its loudness is chiefly due to the character of the resonance provided for it.
The larger the resonance chamber the greater is its reinforcing capacity. The largest air chamber in the body is the chest, which serves not only as a wind-chest, but as a resonance chamber. ”

“ Placing or "focusing the voice" I have found to be chiefly [Pg 52] a matter of control and use of the resonator, consisting of chest, pharynx, mouth, and the nasal and head cavities.
A tone lacking in resonance is ineffective,—devoid of carrying power,—is diffuse and unfocused; while a resonant tone, no matter how soft dynamically, has carrying power and is focused in its vibration.
Now "voice placing" depends primarily on correct vowel placing, which in turn depends on proper adjustment of the resonators, which again depends chiefly on the positions and motions of the organs of articulation. ”

“ Increased loudness of the voice will not improve its carrying power if the resonator is improperly used; it will often lead to a rise of pitch and the production of a harsh, shrill tone associated with a sense of strain and effort. Aikin claims that by studying the whispering voice we can find for every vowel sound that position of the resonator which gives us the maximum of resonance. By percussing2 the resonator in the position for the production of the various vowel sounds you will observe a distinct difference in the pitch of the note produced. ”

“ About 25 per cent. of the diameter of the tube must be added to the length of the air column to make it just one-fourth the wave length. The sound heard in seashells and in other hollow bodies is due to resonance. Vibrations in the air too feeble to affect the ear are intensified by sympathetic vibration until they can be heard. A tuning fork is often mounted upon a box called a resonator, which contains an air column of such dimensions that it reinforces the sound of the fork's sympathetic vibration. ”

“ Another action of sound causes the interference of waves. Thus the reflected waves may meet the oncoming ones and set up concentrations of sound in certain positions and a dearth of sound in others.
Summing up, it is seen that the effects of sound which may exist in a room are loudness, reverberation, echoes, resonance, and interference, and that the most common defects are reverberation and echoes. We now turn to the discussion of the methods of cure. ”

“ Every body capable of producing sound has a tone peculiar to itself; a stringed instrument, for instance, and a trombone differ in the tones they give forth, and we may even divide the air into definitely circumscribed portions, or “chambers of resonance,” each of which will have its own peculiar tone. The form assumed by the double vibrations, the ultimate causes of sound, determines these differences in the quality of the tones we hear. Sometimes the vibrations will run in zigzag course through the elastic medium; sometimes their shape will be rounded ”

“ This law of sound secures the appropriate resonance for every pitch much more accurately than it could be secured by an effort to develop chest, middle, and head registers through calculating the differences. Again, we need the higher chambers of resonance to reinforce even the low pitch, because every note has its overtones that enrich it, and if these cannot find their proper resonance the tone is impoverished. ”

“ Each push in and out of the base of the stapes must cause a movement of the fluid, or a pressure, in the scalae as a whole.
There are difficulties in the way of applying the resonance theory to the perception of noises. Noises have pitch, and also each noise has a special character ”

“ It is apparently also necessary for successful binaural listening that the two portions of the incident wave which enter the two receivers should be free from subsequent distortion; in particular, that the sound receivers should be as nearly as possible non-resonant for the vibrations in question. Any amplification of the sound which depends upon resonance, therefore, such as the use of Helmholtz resonators already referred to, is incompatible with efficient direction-finding by observations of phase difference. ”

“ Among the many possible resonators are Helmholtz resonators, half-wave or quarter-wave resonators, and solid structures such as tubes, plates, or cavities within the chimneys [24] – [25] . Considering the typical acoustic properties of black smoker fluids, and the geometry of these structures, the frequencies of the observed resonant signals are reasonable. ”

“ The resonance-chambers influence volume also. A tone is augmented by resonance.
The larynx bears certain resemblances to both stringed and wind instruments, but it is really unique (sui generis) . The vibrations of the vocal bands are caused solely by the expiratory current of air, which is more or less held back by the cords, owing to their approximation, so that the greater the obstruction the stronger must the blast of air be, other things being equal, and the result increase in pitch. The problem Nature had to solve is very complex. ”

“ Of course the more elastic the body which is struck, the louder and clearer will be the by-note, and of all elastic bodies none are better than those chambers of resonance into which we can divide the air. Such chambers of resonance are afforded by wind instruments of all kinds, whose shape determines the peculiar tone they are to emit. If the instrument is so constructed as to change its shape at will, now round, now straight, now broad, now narrow, the number of different chambers of resonance, and consequently the number of different peculiar tones, may be almost indefinitely increased. ”

“ As to Resonance, any elastic body fastened so as to be permitted to vibrate will have its own proper tones, and will respond sympathetically to the influence of other periodic vibrations, as may be commonly observed with violins, pianofortes, harps, and other stringed instruments, where the comparatively faint sound of the strings is materially reinforced by the responsive sound-board. ”

“ 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. ”

“ In respect to resonance, as in other respects, the human voice is far superior to all other instruments, for their resonators are fixed and unchanging, while the human resonator is flexible,—in Helmholtz's words "admits of much variety of form, so that many more qualities of tone can be thus produced than on any instrument of artificial construction." ”

“ It depends on the fact that many waves of sound that reach the ear are compound wave systems, built up of constituent waves, each of which is capable of exciting a sensation of a simple tone if it be singled out and reinforced by a resonator (see Sound) , and which may sometimes be heard without a resonator, after special practice and tuition. Thus it appears that the ear must have some arrangement by which it resolves every wave system, however complex, into simple pendular vibrations. When we listen to a sound of any quality we recognize that it is of a certain pitch. ”

“ This inaudibility to our organ of hearing is no proof whatever of any reduction of the acoustic force evolved on the introductory impulse given to the bell. It is only a proof that the number of the molecules left for the acoustic force to act upon has been so reduced by increasing the vacuum, that the concentration of sound from the diminished number cannot be heard. The ear is not susceptible to the acoustic force emanating from one molecule, nor even from the concentration of one hundred millions of billions of molecules. ”

“ Let R and R' be two resonators accurately tuned to the note given by the vibratory body at S. Suppose both resonators at the same instant on the wave-surface, then they both receive, at the same instant, the same phase of vibration, on the planes of their mouths. If two tubes of equal length lead from the resonators and join into one tube just before they reach the ear, E, then the sound-pulses will act together, being of the same phase, and the ear will receive double the action which it would if only one resonator were connected with the ear. ”

“ Not only so, but the amplitude of vibration in the one will determine the amplitude of vibration in the other, for the intensity of the induced current is determined by the amplitude of the inducing vibration, and the amplitude of the vibration at the receiving end depends upon the intensity of the attractive impulses. When we sing into a piano, certain of the strings of the instrument are set in vibration sympathetically by the action of the voice with different degrees of amplitude, and a sound, which is an approximation to the vowel uttered, is produced from the piano. ”

“ This picture of the complete happenings during an acoustic event enables us to understand how such a process can be the vehicle for conveying certain astral impulses in such a way that, when met by them, we grow aware of them in the form of a direct sensation. Taking as a model the expression 'transparent' for the perviousness of a substance to light, we may say that the air, when in a state of acoustic vibration, becomes 'trans-audient' for astral impulses, and that the nature of these vibrations determines which particular impulses are let through. ”

“ Each vibration of the sound adds to the pressure upon the glass of its own vibration. Instead of the edges drawing closer by the one-thousandth of an inch, let us say, the sound vibration causes the edges to draw closer by two-thousandths of an inch. And that strain, repeated, not normal, will sometimes shatter the glass. But the sound vibrations must be exactly as numerous as the glass vibrations. Otherwise, the sound vibrations, instead of accelerating the glass vibrations, will impede and stop them. ”

“ This remarkable case of interference, which was first noticed by Dr. Thomas Young, and thoroughly investigated by the brothers Weber, may be rendered audible397 by means of resonance. Bringing a vibrating fork over a jar which resounds to it, and causing the fork to rotate slowly, in four positions we have a loud resonance; in Fig. 161. four others absolute silence, alternate risings and fallings of the sound accompanying the fork’s rotation. ”

“ If the aperture be narrowed without providing a proportional increase of wind-pressure, the harmonic overtone may be heard, but either the intonation will suffer or the intensity of the tone will be reduced, because the force required to set the tenser membrane in vibration is insufficient to give the vibrations the requisite amplitude as well as the frequency. ”

“ The conditions may be still further complicated by the effect of earths or open circuits, which sometimes may produce, temporarily, appalling resonance phenomena, through bringing into action the capacity and inductance of the apparatus and introducing surges. In ordinary working the resonance of the harmonics is not very conspicuous, and the fact that it occurs not systematically, but only in special ways and under special conditions, indicates more strongly than anything else that the vital point is not the time-constant of the line alone, but those of the apparatus connected thereto. ”

“ Musical sound, or tone, is produced by regular vibrations, and differs from mere noise whose vibrations are irregular and confused. The pitch of a musical tone rises in proportion with the rapidity of the vibrations that produce it. Tones may be perceived by the human ear ranging from about sixteen vibrations in a second to nearly forty thousand, more than eleven octaves. Only about seven octaves are used in music. The science of acoustics is full of interesting facts of this kind, and is of profound value to any one who would gain an insight into the structure of music. ”