_Sound._
Besides the function which air discharges as the great agent in the
changes of meteorology and vegetation, it has another office, also
of great and extensive importance, as the vehicle of sound.
1. The communication of sound through the air takes place by means
of a process altogether different from anything of which we have
yet spoken: namely, by the propagation of minute _vibrations_ of
the particles from one part of the fluid mass to another, without
any local motion of the fluid itself.
Perhaps we may most distinctly conceive the kind of effect here
spoken of, by comparing it to the motion produced by the wind in a
field of standing corn; grassy waves travel visibly over the field,
in the direction in which the wind blows, but this appearance of
an object moving is delusive. The only real motion is that of the
ears of grain, of which each goes and returns, as the stalk stoops
and recovers itself. This motion affects _successively_ a line of
ears in the direction of the wind, and affects _simultaneously_ all
those ears of which the elevation or depression forms one visible
wave. The elevations and depressions are propagated in a constant
direction, while the parts with which the space is filled only
vibrate to and fro. Of exactly such a nature is the propagation
of sound through the air. The particles of air go and return
through very minute spaces, and this vibratory motion runs through
the atmosphere from the sounding body to the ear. Waves, not of
elevation and depression, but of condensation and rarefaction, are
transmitted; and the sound thus becomes an object of sense to the
organ.
Another familiar instance of the propagation of vibrations we have
in the circles on the surface of smooth water, which diverge from
the point where it is touched by a small object, as a drop of
rain. In the beginning of a shower, for instance, when the drops
come distinct, though frequent, we may see each drop giving rise
to a ring, formed of two or three close concentric circles, which
grow and spread, leaving the interior of the circles smooth, and
gradually reaching parts of the surface more and more distant from
their origin. In this instance, it is clearly not a portion of the
water which flows onwards; but the disturbance, the rise and fall
of the surface which makes the ring-formed waves, passes into wider
and wider circles, and thus the undulation is transmitted from its
starting-place, to points in all directions on the surface of the
fluid.
The diffusion of these ring-formed undulations from their centre
resembles the diffusion of a sound from the place where it is
produced to the points where it is heard. The disturbance, or
vibration, by which it is conveyed, travels at the same rate in
all directions, and the waves which are propagated are hence of a
circular form. They differ, however, from those on the surface of
water; for sound is communicated upwards and downwards, and in all
intermediate directions, as well as horizontally; hence the waves
of sound are spherical, the point where the sound is produced being
the centre of the sphere.
This diffusion of vibrations in spherical shells of successive
condensation and rarefaction, will easily be seen to be different
from any local motion of the air, as wind, and to be independent of
that. The circles on the surface of water will spread on a river
which is flowing, provided it be smooth, as well as on a standing
canal.
Not only are such undulations propagated almost undisturbed by
any local motion of the fluid in which they take place, but also,
many may be propagated in the same fluid at the same time, without
disturbing each other. We may see this effect on water. When
several drops fall near each other, the circles which they produce
cross each other, without either of them being lost, and the
separate courses of the rings may still be traced.
All these consequences, both in water, in air, and in any other
fluid, can be very exactly investigated upon mechanical principles,
and the greater part of the phenomena can thus be shown to result
from the properties of the fluids.
There are several remarkable circumstances in the way in which air
answers its purpose as the vehicle of sound, of which we will now
point out a few.
2. The _loudness_ of sound is such as is convenient for common
purposes. The organs of speech can, in the present constitution of
the air, produce, without fatigue, such a tone of voice as can be
heard with distinctness and with comfort. That any great alteration
in this element might be incommodious, we may judge from the
difficulties to which persons are subject who are dull of hearing,
and from the disagreeable effects of a voice much louder than
usual, or so low as to be indistinct. Sounds produced by the human
organs, with other kinds of air, are very different from those in
our common air. If a man inhale a quantity of hydrogen gas, and
then speak, his voice is scarcely audible.
The loudness of sounds become smaller in proportion as they come
from a greater distance. This enables us to judge of the distance
of objects, in some degree at least, by the sounds which proceed
from them. Moreover, it is found that we can judge of the position
of objects by the ear: and this judgment seems to be formed by
comparing the loudness of the impression of the same sound on the
two ears and two sides of the head.[14]
The loudness of sounds appears to depend on the _extent_ of
vibration of the particles of air, and this is determined by the
vibrations of the sounding body.
3. The _pitch_, or the _differences of acute and grave_, in sounds,
form another important property, and one which fits them for a
great part of their purposes. By the succession of different
_notes_, we have all the results of melody and harmony in musical
sound; and of intonation and modulation of the voice, of accent,
cadence, emphasis, expression, passion, in speech. The song of
birds, which is one of their principal modes of communication,
depends chiefly for its distinctions and its significance upon the
combinations of acute and grave.
These differences are produced by the different _rapidity_ of
vibration of the particles of air. The gravest sound has about
eighty vibrations in a second, the most acute about one thousand.
Between these limits each sound has a musical character, and from
the different relations of the number of vibrations in a second
arise all the differences of musical intervals, concords and
discords.
4. The _quality_ of sounds is another of their differences. This is
the name given to the difference of notes of the same pitch, that
is the same note as to acute and grave, when produced by different
instruments. If a flute and a violin be in unison, the notes are
still quite different sounds. It is this kind of difference which
distinguishes the voice of one man from that of another: and it is
manifestly therefore one of great consequence; since it connects
the voice with the particular person, and is almost necessary in
order that language may be a medium of intercourse between men.
5. The _articulate_ character of sounds is for us one of the most
important arrangements which exist in the world; for it is by this
that they become the interpreters of thought, will and feeling, the
means by which a person can convey his wants, his instructions,
his promises, his kindness, to others; by which one man can
regulate the actions and influence the convictions and judgments
of another. It is in virtue of the possibility of shaping air into
words, that the imperceptible vibrations which a man produces in
the atmosphere, become some of his most important actions; the
foundations of the highest moral and social relations; and the
condition and instrument of all the advancement and improvement of
which he is susceptible.
It appears that the differences of articulate sound arise from
the different form of the cavity through which the sound is made
to proceed immediately _after_ being produced. In the human voice
the sound is produced in the larynx, and modified by the cavity of
the mouth, and the various organs which surround this cavity. The
laws by which articulate sounds are thus produced have not yet been
fully developed, but appear to be in the progress of being so.
The properties of sounds which have been mentioned, differences
of loudness, of pitch, of quality, and articulation, appear to be
all requisite in order that sound shall answer its purposes in
the economy of animal and of human life. And how was the air made
capable of conveying these four differences, at the same time that
the organs were made capable of producing them? Surely by a most
refined and skilful adaptation, applied with a most comprehensive
design.
6. Again; is it by chance that the air and the _ear_ exist
together? Did the air produce the organization of the ear? or the
ear, independently organized, anticipate the constitution of the
atmosphere? Or is not the only intelligible account of the matter,
this, that one was made for the other: that there is a mutual
adaptation produced by an Intelligence which was acquainted with
the properties of both; which adjusted them to each other as we
find them adjusted, in order that birds might communicate by song,
that men might speak and hear, and that language might play its
extraordinary part in its operation upon men’s thoughts, actions,
institutions, and fortunes?
The vibrations of an elastic fluid like the air, and their
properties, follow from the laws of motion; and whether or not
these laws of the motion of fluids might in reality have been other
than they are, they appear to us inseparably connected with the
existence of matter, and as much a thing of necessity as we can
conceive any thing in the universe to be. The propagation of such
vibrations, therefore, and their properties, we may at present
allow to be a necessary part of the constitution of the atmosphere.
But what is it that makes these vibrations become sound? How is it
that they produce such an effect on our senses, and, through those,
on our minds? The vibrations of the air seem to be of themselves
no more fitted to produce sound, than to produce smell. We know
that such vibrations do not universally produce sound, but only
between certain limits. When the vibrations are fewer than eighty
in a second, they are perceived as separate throbs, and not as a
continued sound; and there is a certain limit of rapidity, beyond
which the vibrations become inaudible. This limit is different to
different ears, and we are thus assured by one person’s ear that
there are vibrations, though to that of another they do not produce
sound. How was the human ear adapted so that its perception of
vibrations as sounds should fall within these limits?--the very
limits within which the vibrations fall, which it most concerns
us to perceive: those of the human voice for instance? How nicely
are the organs adjusted with regard to the most minute mechanical
motions of the elements?