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.”
[Illustration: 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.
[Illustration: FIG. 1. SOUND WAVE REFLECTED FROM A PLANE SURFACE.]
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.
[Illustration: FIG. 2. SPHERICAL SOUND WAVE.]
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.[D] Intricate as
these wave surfaces are, they have all been verified by geometrical
constructions, as I shall presently show.
[D] 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.
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.)
[Illustration: FIG. 3. A WAVE REFLECTED FROM A PORTION OF A SPHERE.]
[Illustration: FIG. 4. A WAVE FROM A CYLINDRICAL MIRROR.]
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.
[Illustration: FIG. 5. A WAVE FROM AN ELLIPTICAL MIRROR.]
[Illustration: FIG. 6. A WAVE STARTING OUTSIDE THE FIELD OF THE LENS.]
[Illustration: FIG. 7. A CASE OF REFRACTION.]
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.
[Illustration: FIG. 8. A MUSICAL TONE.]
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.
[Illustration: FIG 9. THE REFLECTION INSIDE THE HOLLOW SPHERE.]
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.