Popular Science Monthly/Volume 57/August 1900/The Photography of Sound Waves
|THE PHOTOGRAPHY OF SOUND WAVES.|
UNIVERSITY OF WISCONSIN.
ANY one who has stood near a large naval gun during its discharge, will, I think, be prepared to admit that the sound of the explosion affects not only the ears, but the whole body as well, which experiences something not unlike a sudden blow. This blow, or concussion, as it is generally termed, is merely the impact of the wave of compressed air, spreading out in all directions around the gun. In the case of ordinary sounds, the compression of the air in the wave is so slight that only the delicate auditory nerves respond to the impact, hence we naturally conclude that sounds are perceived only by the ear. When dealing with sounds of very great intensity, this notion must be somewhat modified, for they certainly can be felt as well as heard. In some extreme cases, in fact, the sensation of feeling may be stronger than that of hearing, as in the case of which I shall speak presently. Is it also possible that we can perceive sound through the medium of any other sense organ, say the eye? 'To see a noise' certainly sounds like an absurdity; yet under certain conditions, sound waves in air can be made as distinctly visible as the ripples on a pond surrounding the splash of a stone. That they are not seen under ordinary conditions does not justify us in assuming them to be invisible. We all know that the currents of hot air rising from a stove, while not usually conspicuous, can be made visible by properly regulating the illumination, as by looking along the surface of the stove towards a window. The hot air is visible because in its optical properties it is different from the cold air surrounding it. The rays of light, passing through the unequally heated portions of the air, are bent in different directions, causing a distortion of objects seen through the heated currents. What we see, strictly speaking, is not the hot air itself, but a wavering and swimming of the objects seen through it. Yet I think we are justified in saying that the eye perceives the hot air.
Now sound waves in air, which are merely regions where the air is somewhat compressed, differ in their optical properties from the uncompressed portions, just as the hot air differs from the cold. As the pictures illustrating this article testify, they may be seen and photographed under proper conditions of illumination as readily as solid objects. We must remember, in the first place, that a sound wave travels with a velocity something greater than a thousand feet a second, rather less than the speed of a modern rifle ball, yet ten times faster than the fastest express train. The wave, even if it were stationary, could be seen only by adjusting the illumination with far greater care than was necessary in the case of the hot air, and we consequently can easily understand why we never see the waves under ordinary conditions.
While it is true that laboratory appliances are generally required to render them visible, I should like at the outset to cite an example to show that in the case of very loud sounds occurring in the open air the wave can be perceived by the eye, without the aid of any apparatus whatever. I will quote from an article by Prof. C. V. Boys, which appeared in 'Nature,' June 24, 1897. Mr. Boys first cites the following letter from Mr. E. J. Ryves: "On Tuesday, April 6th, I had occasion, while carrying out some experiments with explosives, to detonate one hundred pounds of a nitro-compound. The explosive was placed on the ground in the center of a slight depression, and in order to view the effect, I stationed myself, at a distance of about three hundred yards, on the side of a neighboring hill. The detonation was complete, and a hole was made in the ground five feet deep and -even feet in diameter. A most interesting observation was made during the experiment. The sun was shining brightly, and at the moment of detonation the shadow of the sound wave was most distinctly seen leaving the area of disturbance. I heard the explosion as the shadow passed me, and I could follow it distinctly in its course down the valley for at least half a mile; it was so plainly visible that I believe it would photograph well with a suitable shutter."
Professor Boys at once made preparations for photographing the phenomenon at the first opportunity. On May 19th the experiment was made. One hundred and twenty pounds of a nitro-compound were exploded, and an attempt made to photograph the sound shadow, both with the camera and the kinematograph, the latter instrument designed and operated by Mr. Paul. Writing of the experiment, Professor Boys says: "On the day on which I was present, about one hundred and twenty pounds of a nitro-compound were detonated, and ten pounds of black powder were added to make sufficient smoke to show on the plate. As the growth of the smoke cloud is far less rapid than the expansion of the sound shadow, no confusion could result from this. At the time of the explosion my whole attention was concentrated upon the camera, and for the moment I had forgotten to look for the 'Ryves ring' as I think it might be called; but it was so conspicuous that it forced itself upon my attention. I felt, rather than heard, the explosion at the moment that it passed. We stationed ourselves as near as prudence would allow, at a distance of one hundred and twenty yards, so that only about one third of a second elapsed between the detonation and the passage of the shadow. The actual appearance of the ring was that of a strong, black, circular line, opening out with terrific speed from the point of explosion as a center. It was impossible to judge of the thickness of the shadow; it may have been three feet, or it may have been more at first, and have gradually become less in thickness, or possibly in depth of shade."
Unfortunately, Professor Boys's apparatus did not work satisfactorily, but a most interesting series of pictures was secured by the kinematograph. This instrument had been constructed especially for takingat a very high rate of speed, viz. . eighty exposures a second, or four times the usual number. The sound wave appears in the first dozen pictures as a hazy ring of light, opening out from the center of explosion. The ring, though not very conspicuous when the pictures are viewed singly, becomes a striking object when they are projected in rapid succession on the screen. We see the rush of smoke along the ground to the box in which the explosion is confined (the smoke of the quick fuse); then comes the burst of the explosion with such startling reality that we involuntarily jump. The image of the sound wave flies out in the form of a white ring, and is gone in a moment; and there remain only the rolling clouds of smoke. It is interesting to observe the development of the explosion by running the machine quite slowly, and by thus magnifying time to follow the changes which ordinarily occur in such rapid succession that the eye is unable to perceive them.
Of this series of pictures, Professor Boys says: The "kinematograph fails to show any black ring; and this is not surprising, as with the exposure of about one one hundredth of a second the shadow would have to be at least eleven feet thick in order that some part should remain obscured during the whole exposure. As a fact, there is clearly seen a circular light shading, which does—so far as one can judge from the supposed rate of working and the known distances—expand at about the same rate as the observed shadow, but it is lighter than the ground and shaded, instead of being dark and sharp, as seen by the eye."
So much for the visibility of sound under ordinary conditions. In the laboratory, by means of an optical contrivance due to the German physicist Toepler, we can secure a means of illumination so sensitive that the warm air rising from a person's hand appears like dense black smoke. Moreover, since we are working on a small scale, we can use the electric spark as the source of light, and dispense with the photographic shutter. This is a great advantage, for the time of the exposure is, under these conditions, only about one fifty-thousandth of a second, during which time the sound wave will move scarcely a quarter of an inch. During the past year I have made a very complete series of photographs of sound waves, which illustrate in a most beautiful manner the fundamental principles of wave motion. It is not practicable to give here a full description of the apparatus used, but a brief outline may make the method intelligible. The sound photographed in each case is the crack of an electric spark, which is illuminated and photographed by the light of a second spark, occurring a brief instant later. In front of a large lens (a telescope objective, for example) two brass balls are mounted, between which the 'sound spark.' as I shall call it, passes. The instant the spark jumps across the gap, a spherical wave of condensed air starts out, which, when it reaches our ear, gives the sensation of a snap. The object is to photograph this wave before it gets beyond the limits of the lens. The camera is mounted in front of the lens and on the
brass balls, which appear in line in the picture so that the sound spark is always hidden by the front one. The spark, on jumping between the balls, charges a Leyden jar, which instantly discharges itself between two wires placed behind the lens, producing the illuminating spark. This second spark can be made to lag behind the first just long enough to catch the sound wave when it is but a few inches in diameter, notwithstanding the fact that the spherical wave is expanding at the rate of eleven hundred feet a second. The photographs show in every case the circle of the lens filled tip with the light of the illuminating spark, the brass balls (in line) and the rods that support them, and the sound wave, which appears in the simplest case as a circle of light and shade surrounding the balls. By placing an obstacle in the way of the wave we get the reflected wave or echo, and we shall see that the form of this echo may be very complicated.
It will be well at the outset to remind the reader of the close analogy between sound and light. A burning candle gives out spherical light waves, just as the snapping sparks give out sound waves. The ~form of the reflected light wave will be identical with that of a sound wave reflected under similar conditions. As we can not see the light waves themselves, we can only determine their form by calculation, and it is interesting to see that the forms photographed are identical in every case with the calculated ones. The object in view was to secure acoustical illustrations of as many of the phenomena connected with light as possible. We will begin with the very simplest case of all: the reflection of a spherical sound wave from a flat surface, corresponding to the reflection of light from a plane mirror. It can be shown by geometry that the reflected wave or echo will be a portion of a
sphere, the center of which lies as far below the reflecting surface as the point at which the sound originates is above it. In the case of light, this point constitutes the image in the mirror. Referring to the photograph, we see the reflected wave in three successive positions, the interval between the sound spark and the illuminating spark having been progressively increased. The brass balls are shown at A, and beneath them the flat plate B, which acts as a reflector. In the first picture the sound wave C appears as a circle of light and shade, and has just intersected the plate. The echo appears at D. In the next two pictures the original wave has passed out of the field, and there remains only the echo.
It may, perhaps, be not out of place to remind the reader of the relation between rays of light and the wave surface. What we term light rays have no real existence, the ray being merely the path traversed by a small portion of consecutive wave surfaces. Since the wave surface always moves in a direction perpendicular to itself, the rays are always normal to it. For instance, in the above case of a spherical wave diverging from a point, the rays radiate in all directions from the point; the same is true in the case of the echo, the rays radiating from the image point below the reflecting surface. In all subsequent cases the reader can, if interested in tracing the analogy between sound and light, draw lines perpendicular to the reflected wave surfaces representing the system of reflected waves.
We will now consider a second case of reflection. We know that if a lamp is placed in the focus of a concave mirror, the rays, instead of diverging in all directions, issue from the mirror in a narrow beam. The headlight of a locomotive and the naval searchlight are examples of the practical use made of this property. If the curvature of the mirror is parabolical, the rays leaving it are parallel; consequently mirrors of this form are employed rather than spherical ones. But what has the mirror done to the wave surface which is obviously spherical when it leaves the lamp, and what is its form after reflection? The wave surface, I have said, is always perpendicular to the rays: consequently in cases where we have parallel rays we should expect the wave to be flat or plane.
Examine the second photograph, which shows a spherical sound wave
starting at the focus of a parabolic mirror. The echo appears as a straight line, instead of a circle as in the previous case, which shows us that the wave surface is flat.
If now our mirror is a portion of a sphere instead of a paraboloid, our reflected wave is not flat, and the reflected rays are not all parallel, the departure from parallelism increasing as we consider rays reflected from points farther and farther away from the center of the mirror. A photograph illustrating the reflection of sound under these conditions is next shown, the echo wave being shaped like a flat-bottomed saucer. As the saucer moves upward the curved sides converge to a focus at the edge of the flat bottom, disappearing for the moment (as is shown in the fourth picture of the series), and then reappearing on the under side after passing through the focus, the saucer turning inside out.
If, instead of having a hemisphere, as in the last case, we have a complete spherical mirror, shutting the wave up inside a hollow ball, we get exceedingly curious forms; for the wave can not get out, and is bounced back and forth, becoming more and more complicated at each reflection. This is illustrated in our next photograph, the mirror being a broad strip of metal bent into a circle. Intricate as these wave surfaces are, they have all been verified by geometrical constructions, as I shall presently show.
Another very interesting case of reflection is that occurring inside an elliptical mirror. When light diverges from one of the two foci of such a mirror, all the rays are brought accurately to the other focus. If rays of light come to a focus from all directions, it is evident that the wave surface must be a sphere, which, instead of expanding, is collapsing. This is very beautifully shown in the photographs. The sound wave starts in one focus and the reflected wave, of spherical form also, shrinks to a point at the other focus. (See fig. 5.)
In the next series the wave starts outside of the field of the lens,
and enters a hemispherical mirror. We know that a concave mirror has the power of bringing light to a focus at a point situated half-way between the surface of the mirror and its center of curvature. If the light comes from a very distant point, and the mirror is parabolic in form, the rays are brought accurately to a focus; which means that the reflected wave is a converging sphere,—a condition the opposite of that in which spherical waves start in the focus of such a mirror. If, however, the mirror is spherical, only a portion of the light comes to a focus. On examining the pictures we see that the reflected wave has a form resembling a volcanic cone with a bowl-shaped crater. See the third and fourth pictures of the series. The bowl of the crater shrinks to a point half-way between the surface of the mirror and its center of curvature, and represents that portion of the light which comes to a focus, while the sides of the cone run in under the collapsing bowl, and eventually cross. (No. 6 of the series.) From now on the portion which has come to a focus diverges, uniting with the sides of the cone, the whole passing out of the mirror in the form of a horseshoe.
We will now consider a case of refraction, and show the slower
velocity of the sound wave in carbonic acid. A narrow glass tank, covered with an exceedingly thin film of collodion, was filled with the heavy gas and placed under the brass balls. When the sound wave strikes the collodion surface, it breaks up into two components, one reflected back into the air, the other transmitted down through the carbonic acid. An examination of the series shows that the reflected wave in air has moved farther from the collodion film than the transmitted wave, which, as a matter of fact, has been flattened out into a hyperboloid. Exactly the same thing happens when light strikes a block of glass. We have rays reflected from the surface, and rays transmitted through the block, the waves which give rise to the latter moving slower than the ones in air.
A complete discussion of all of the cases that have been studied in this way would probably prove wearisome to the general reader. Prisms and lenses of collodion filled with carbonic acid and hydrogen gas have been made, and their action on the wave surface photographed. Diffraction, or the bending of the waves around obstacles, and the very complicated effects when the waves are reflected from corrugated surfaces, are also well shown. I shall, however, omit further mention of them and speak of but one other case, possibly the most beautiful of all.
In all the cases that we have considered, it must be remembered that we have been dealing with a single wave—a pulse, as it is called. Musical tones are caused by trains of waves, the pitch of the note corresponding to the distance between the waves, or to the rate at which the separate pulses beat upon the drum of the ear. For studying the changes produced by reflection, wave trains would have been useless,
owing to the confusion which would have resulted from the superposition of the different waves. Moreover, it is doubtful whether an ordinary musical tone could be photographed in this way; for the distance between the waves, even in the shrillest tones, is four or five inches, and the abrupt change in density, necessary for the perception of the wave, is not present. It is possible, however, to create a wave train or musical tone which can be photographed. The reader may perhaps have noticed that on a very still night, when walking beside a picket fence or in front of a high flight of steps, the sounds of his footsteps are echoed from the palings as metallic squeaks. Each picket, as the single wave caused by the footfall sweeps along the fence, reflects a little wave; consequently a train of waves falls on the ear, the distance between the waves corresponding to the distance between the pickets. The closer together the pickets, the shriller the squeak. In point of fact, the distance between the waves in such a train is twice the distance between the palings, since they are not struck simultaneously by the footstep wave, but in succession.
This phenomenon, of the creation of a musical tone by the reflection of a noise, was reproduced by reflecting the crack of the spark from a little flight of steps. In the first picture the wave is seen half way between its origin and the reflecting surface. In the second it has struck the top stair, which is giving off its echo, the first wave of our artificially constructed musical tone. In the third we find the original wave at the sixth step, with a well-developed train of five waves rising from the flight. The following three pictures show the further development of the wave train. The height of each step was about a quarter of an inch; consequently the distance between the waves was half an inch. This would correspond to a note about three octaves above the highest ever used in music.
While experimenting with the complete circular mirror, which, it will be remembered, gave the most complicated forms, it occurred to me that a very vivid idea of how these curious wave surfaces are produced could be obtained by preparing a complete series in proper order on a kinetoscope film, and then projecting them in succession on the screen. The experimental difficulties were, however, too great to make it seem worth while to attempt to obtain a series of pictures of the actual waves, it being very difficult to accurately regulate the time interval between the two sparks. The easier method of making a large number of geometrical constructions, and then photographing them in succession on the film, was accordingly adopted. Three complete sets of drawings, to the number of about one hundred each, were prepared for three separate cases of reflection;—viz.: the entrance of a plane wave into a hemispherical mirror, the passage of a spherical wave out from the focus of a hemispherical mirror, and the multiple reflection of a spherical wave inside of a complete spherical mirror. Special methods were devised for simplifying the constructions, and much less labor was required in the preparation of the diagrams than one would suppose. The results fully justified the labor, the evolutions of the waves being shown in a most striking manner. These films I exhibited before the Royal Society in February last, and a more complete description of the manner of preparing them may be found in the Proceedings of the Society.
A portion of one of these series is reproduced, about one in four or five of the separate diagrams being given. The series runs from left to right in horizontal rows. When projected on the screen, the spherical wave is seen gradually to expand from the focus point, like a swelling soap bubble; it strikes the surface, and the bowl-shaped echo bounces off and follows the unreflected portion across the field; these two portions are then reflected in turn, and the curiously looped wave flies back and forth across the mirror, changing continuously all the time, and becoming more complicated at each reflection. These diagrams should be compared with the photographs shown in the fourth series.
One must not suppose that these beautiful forms exist only in the laboratory. Every time we speak, spherical waves bounce off the floor, ceiling and walls of the room, while in any ordinary bowl or basin the curious crater-shaped echoes are formed. Glance once more at the wave surfaces produced within a hollow sphere, and try to imagine the complexity of the aerial vibrations caused by a fly buzzing around in an empty water-caraffe! The photographs enable us to realize what is going on around us all the time—this our perceptions are fortunately too dull to perceive. Life would be a nightmare if we were obliged to see the myriads of flying sound waves bounding and rebounding about us in every direction, and combining into grotesque and ever-changing forms. It is just as well, on the whole, that the light of the electric spark and the delicate optical device of Toepler are necessary to bring them into view.
- Cylindrical mirrors have been used instead of spherical, for obvious reasons. A sectional view of the reflected wave is the same in this case as when produced by a spherical surface.