ATMOSPHERIC ACOUSTICS
Though air is but one of an unlimited number of elastic substances that
transmit sound, it is the one through which sounds ordinarily reach
our ears. Hence the acoustic properties of the atmosphere are of great
interest to mankind.
Science deals with several kinds of “waves,” and those of the
atmosphere that produce the sensation of sound are quite different
from the waves of the sea. In quiet unconfined air sound travels in
concentric spherical waves, consisting of successive condensations
and rarefactions of the medium. Sound is not transmitted through a
vacuum. A familiar laboratory experiment is to install an electric bell
inside the receiver of an air pump and notice the dying away of the
sound as the air is exhausted. The colossal eruptions that astronomers
witness on the surface of the sun would probably be audible on earth if
interplanetary space were filled with air. In the rarefied air of high
mountains the intensity of sounds is much reduced. Thus we are told
that on the top of Mont Blanc the report of a pistol sounds no louder
than that of a firecracker at sea level.
The speed with which sound travels through air depends upon the
temperature. At 32° F. (the freezing point) it is 1,087 feet per
second, and at 68° F. it is 1,126 feet per second. The increase of
speed with increase of air temperature is very close to 2 feet per
degree Centigrade, or a little more than 1 foot per degree Fahrenheit.
The sounds of violent explosions travel considerably faster than
ordinary sounds near the place of explosion, but slow down to the
normal speed at greater distances. This is true of heavy claps of
thunder. The effect is not important enough, however, to invalidate
the well-known rule that, if you count seconds between the flash and
the detonation and divide the result by 5, you get approximately the
distance of the source of sound in miles.
Since the speed of sound varies with the temperature of the air,
differences in the latter cause deviations of the paths of sound waves
similar to the deviations which rays of light undergo on account of
differences in the density of the air. Sound is also reflected by
obstacles, in the same manner as light. Moreover, whereas light travels
too swiftly to be affected by the wind, this is not true of sound. The
latter travels faster with the wind than against it, and sound waves
are more or less broken up by the gusts and irregularities that are a
feature of most winds near the earth’s surface. For all these reasons
the acoustic qualities of the air are subject to marked variations, as
everybody has observed.
Unusual audibility of distant sounds is a popular prognostic of
rain. The fact underlying this belief is that when the air is full
of moisture it is likely to be of uniform temperature, and therefore
favorable for transmitting sound.
It is impossible to assign any limit to the distance at which loud
sounds may occasionally be heard. No fact of nature has yet, so far
as we know, matched Emerson’s metaphor of the “shot heard round the
world,” but it is literally true that the sounds of eruption of
Krakatoa, in August, 1883, were heard, like the roar of distant heavy
guns, in the island of Rodriguez, in the Indian Ocean, 3,000 miles from
the volcano. Moreover, the atmospheric waves set up by this outburst
actually made the circuit of the globe, not only once, but at least
three times, and the successive journeys were registered by barometers,
if not detected by human ears. During the World War gun firing in
Flanders was very commonly heard at places in England 140 to 150 miles
distant. Several observers also reported that pheasants appeared, from
their disturbed behavior, to hear cannonading over the North Sea that
was beyond the range of the human ear.
Writers have often commented on the fact that thunder cannot be heard
so far as the sounds of artillery. It has been affirmed that 10 miles
or thereabouts is its maximum range of audibility. As a matter of fact,
however, thunder has occasionally been heard at much greater distances,
up to 20 or 30 miles; but it remains true that the distance is always
much less than that at which loud terrestrial sounds are audible. The
reasons why this should be so are not far to seek. In the first place,
the intensity of a sound depends upon the density of the air in which
it is generated, and not upon that of the air in which it is heard. The
air, as we know, diminishes in density upward. Balloonists thousands
of feet above the earth hear with remarkable clearness sounds from
the ground below, but people on the ground cannot hear similar sounds
from the balloon. As thunder is mainly produced at the level of the
clouds, it is subject to this peculiarity. Again, cannonading is heard
at great distances only when the air is comparatively calm, and perhaps
only when it is arranged in well-defined horizontal layers of such a
character as to keep the sound from spreading far aloft. Very different
conditions prevail during a thunderstorm; in fact the conditions are
then just such as would scatter and dissipate the sound waves. Lastly,
the noise of a cannon or the like comes from a single place and the
energy of the disturbance is concentrated to produce a single system of
sound waves; while the disturbance due to lightning is spread over the
long path of the discharge.
The audibility of sounds at abnormally great distances is not usually
a matter of practical importance, but the converse phenomenon--the
failure of sounds to carry to normal distances--has been responsible
for a great number of marine disasters on such fog-ridden coasts as
those of the British Isles, eastern Canada and California. Hence some
of the ablest physicists of both the Old World and the New have tried
to ascertain the conditions under which this phenomenon occurs.
The scientific study of fog signals, dating especially from Tyndall’s
well-known investigations at the South Foreland, in England, in 1873,
and those of General Duane and Professor Joseph Henry in America,
begun somewhat earlier but continued contemporaneously with Tyndall’s,
has probably raised more questions than it has answered. The caprices
of these signals take the shape of variations in the range of
audibility--a signal may at one time carry 10 miles and at another
only 2--and the formation of “zones of silence,” comparatively near
the signal, within which the sound is not heard though audible at
much greater distances. The silent zones are sometimes more or less
permanent and are then generally due to peculiarities of topography;
but in many cases they are transient and opinions differ as to their
cause or causes. Since it is only when fog prevails that fog signals
are sounded (except for experimental purposes), and that vessels meet
with accidents on account of the failure to hear such signals, the idea
has become rooted in the public mind that these acoustic eccentricities
are entirely due to fog. When, however, experiments are made in clear
weather, similar phenomena are observed.
In foggy weather audibility is often better than the average, because
fog prevails chiefly when the air is still and of uniform temperature,
and such conditions favor the transmission of sound. Tyndall strongly
denied that either fog or falling rain, snow, and hail have, as has
been commonly believed, a muffling effect on sound, and he attributed
the peculiar behavior of fog signals to the presence in the atmosphere
of invisible “acoustic clouds,” consisting of patches of air containing
irregularities of temperature and humidity. To the same cause he
ascribed the occurrence of mysterious “aerial echoes,” not due to
any visible object. Several recent investigators have disputed these
conclusions. Thus it is asserted that when the fog signal is in fog
and the observer in a clear atmosphere, or _vice versa_, or when the
signal and the observer are in different fog banks, the fog reflects
the sound very strongly. Apart from the possible effects of fog itself,
the very extensive investigations made by Prof. L. V. King, of McGill
University, at Father Point, Quebec, led him to conclude that the
effects are chiefly due to eddies in the atmosphere. Prof. King used in
his observations the latest devices for obtaining exact measurements
of sound and such up-to-date meteorological apparatus as pilot balloons
for measuring the wind at various levels. He discovered, among other
things, that existing types of fog-signal machinery are very wasteful
of energy, and he has pointed out how their “acoustic efficiency” may
be much improved. Before we dismiss this subject it should be noted
that submarine bells and the radio compass have made mariners much less
dependent than they formerly were upon the types of signals that are
affected by meteorological conditions.
“Zones of silence” on a much more extensive scale than those that
disturb the operation of fog signals have been frequently observed, in
recent years, in connection with great explosions, cannonading, and
volcanic eruptions. The first case of this kind to attract scientific
notice was that of a dynamite explosion at Förde, Westphalia, on
December 14, 1903, the acoustic phenomena of which were investigated
by Dr. G. von dem Borne; and among the many cases that have since
been studied was that of the bombardment of Antwerp in October, 1914.
Without describing these various cases separately, we may state that
when reports were collected from the surrounding country to determine
the places at which the sounds were audible and these reports were
entered on a map, it was found that there was a large and usually
very irregular area of audibility surrounding the source of sound,
beyond which lay a broad, more or less circular zone of inaudibility,
and finally, beginning about 100 miles from the source, there was a
second large region of audibility, extending perhaps 150 miles from
the source. In some cases a single sound at the source gave multiple
reports (double, triple, or quadruple), chiefly in the outer zone of
audibility.
In his attempt to explain these curious silent zones, Von dem Borne
pointed out that the atmosphere at very high levels is supposed to
consist mainly of hydrogen, in which sound travels nearly four times
as fast as in the common gases of the lower air, and that sound
waves ascending to such heights along a slanting course would be
bent strongly toward the earth. Another student of this phenomenon,
Dr. A. Wegener, who is the champion of the idea that the atmosphere
contains an unknown gas lighter than hydrogen (called “geocoronium”
or “zodiacon”), sees in the prevalence of this gas at high levels
the cause of a similar quasi-reflection of sound waves. Probably the
majority of investigators, however, believe that the effect is due
chiefly or entirely to the refraction of sound by wind.
Of acoustic phenomena that belong especially to the domain of
meteorology, probably thunder is the one that excites most general
interest. The sudden expansion of the air along the path of a lightning
discharge, due partly but probably not entirely to the heat generated,
appears to be an adequate explanation of the explosive sound of
thunder, though somewhat different explanations have been suggested. If
the discharge is near at hand, we generally hear a single loud crash.
More distant lightning is usually attended by rumbling. The common and
obvious explanation of rumbling is that it is due to the arrival of the
sound progressively from different points along the path of discharge,
which may be a mile or more in length. A crooked path would account
for reenforcements and diminutions of the sound. Another cause of
irregularities in the sound is probably “interference” (combinations
of waves that tend either to strengthen or to neutralize each other),
especially in the case of multiple lightning discharges, such as we
have described elsewhere. Lastly, thunder is further complicated by
echoes from the ground and probably also from the air (not exclusively
from clouds), though much uncertainty prevails concerning these aerial
echoes. The sounds of thunder have been the subject of some interesting
investigations on the part of an Austrian meteorologist, Dr. Wilhelm
Schmidt, who has devised apparatus for making an automatic registration
of the sound waves that constitute a thunderclap. He finds that there
is a great preponderance of waves of very long period, including many
of too low a pitch to be audible, though perceptible through the
rattling of windowpanes, etc. In fact, the greater part of the energy
involved is represented by these long, inaudible waves, so that one
really _hears_ only a small part of a clap of thunder.
The statement has often been made, on the authority of Humboldt, that
thunder is never heard at sea, at any point far from land. This matter
was investigated by the magnetic survey yacht _Carnegie_ during a long
cruise in the Pacific in 1915. Of twenty-two displays of lightning, six
were accompanied by thunder.
The late war gave prominence to certain acoustic phenomena which,
though hardly mysterious, were novel to the world at large. One of
those was the double report (triple in the case of an exploding shell)
heard near the line of fire of large guns. This effect is due to the
fact that modern projectiles travel much faster than sound. The moving
projectile sets up its own waves in the air, like those at the bow
of a steamer, which may reach the ear of the observer and produce the
sensation of a sharp sound before he hears the sound coming from the
mouth of the gun. Another phenomenon frequently observed when heavy
firing was in progress was the appearance in the sky of rapidly moving
parallel arcs of light and shade. These were generally seen against
clouds, but sometimes they swept across blue sky. They probably
occurred only in calm weather. These arcs were the result of the
successive condensations and rarefactions of the air constituting waves
of sound--visible sound waves. Their visibility was due to contrasts in
the refraction of light passing through air of different densities; the
same sort of refraction contrasts that cause the tremulous appearance
of the air over a hot stove, for example. The same “flashing arcs” of
light have been described by Prof. F. A. Perret as attending explosive
volcanic outbursts at the craters of Vesuvius and Ætna.
The humming of telegraph wires has been the subject of a certain
amount of discussion in meteorological circles, but without altogether
satisfactory results. This sound is not, of course, caused or affected
by the electric currents passing along the wire, and it is almost
certainly due solely to the wind, though the suggestion has been
made that it might be caused by the microseisms, or small and rapid
earthquake tremors, that are so commonly registered by seismographs
while imperceptible to the human senses. The humming is best heard
when one’s ear is placed against a telegraph pole. Several persons
have made systematic observations of these sounds from day to day,
and it has often been alleged that they vary with the temperature,
the movements of storms, etc., and even constitute a safe basis for
weather predictions. They are sometimes heard when the air appears to
be perfectly calm, but in such cases there might be some movement of
the air at the level of the wires, though there was none at the lower
level of the observer. From what is known about “æolian tones” (such
as those of the æolian harp), it would seem that the humming requires
a wind more or less at right angles to the wire, and that the pitch of
the sound depends upon the force of the wind and the diameter (but not
the length or tension) of the wire. For a given wire, the stronger the
wind the higher the pitch of its sound.
Of all the sounds that haunt the air, probably the most mysterious are
those which are best called by the generic name “brontides” (coined,
in the Italian form _brontidi_, by Prof. Tito Alippi from two Greek
words meaning “like thunder”), though they rejoice in scores of other
names in various parts of the world. Brontides take the form of
muffled detonations, resembling the sound of distant cannon or peals
of thunder, and are heard chiefly in warm, clear weather. The first
systematic investigations of these phenomena were made in India. The
fact that they were frequently reported from the neighborhood of
Barisal, a town in the Ganges delta, led to their being called “Barisal
guns,” under which name they were first made known to European science
in 1890. A few years later they were discussed in an extensive memoir
by E. van den Broeck, who had collected numerous reports of their
occurrence in Belgium, especially on the seacoast, where they are
known as “mistpoeffers” (i. e., “fog belchings” or “fog hiccups”).
The majority of descriptions, however, have come from Italy, where
the sounds appear to be extremely common, though peculiar to certain
localities, and where they bear a great variety of names. In Australia
the noises are called “desert sounds,” in Haiti, “gouffre,” etc.
They have been reported from parts of the United States, including
California and, above all, from the vicinity of Moodus, Conn., which
owes its original Indian name, Morehemoodus (“place of noises”), to the
brontides which appear to have formerly been much more common there
than they are to-day. There is a reference in one of Lord Bacon’s
works to “an extraordinary noise in the sky when there is no thunder”;
apparently a description of brontides.
The source of these sounds is undoubtedly subterranean in a great
many cases, though perhaps not in all. Prof. W. H. Hobbs, who has
made a painstaking study of the seismic geology of Italy, concludes
that the brontides of that country are due to the slow settling of
the blocks of the earth’s crust; a process which, in its more abrupt
and violent phases, causes definite earthquakes. Alippi believes that
in order that the sounds may be heard they must be reenforced by a
peculiar configuration of the ground, above or below the surface, and
he attaches special importance to the effects of caverns, which he
suggests act as resonance boxes in the production of audible brontides.
Occasionally an apparent brontide may be due to the explosion of an
unseen meteor. Lastly, a certain proportion of these thunderlike
sounds, if not merely distant thunder, may be such noises as
cannonading, blasting, or the like, made audible at unusual distances
by the refraction of sound waves.