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 seven 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 taking pictures at 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.”
Kinetoscope Film of Explosion.
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 focussed 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 up 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.
