and the middle point of a side is touched while a bow is drawn across
the edge near a corner, the sand is seen to gather in the form of a
cross dividing the square into four equal squares—like a cross of St
George. If the finger touches a corner, and the bow is drawn across the
middle of a side, the sand forms a cross dividing the square along its
diagonals—like a cross of St Andrew. Touching two points equidistant
from two corners, and drawing the bow along the middle of the opposite
edge, we get the diagonal cross and also certain curved lines of
sand systematically placed in each of the four quarters into which
the diagonals divide the square. We also have, in this case, a far
shriller note from the vibrating plate. And so, by various changes in
the position of the points clamped by the finger and of the part of the
edge along which the bow is drawn, we can obtain innumerable varieties
of nodal lines and curves along which the sand gathers upon the surface
of the vibrating plate.
When we take a circular plate of glass, clamped at the middle, and
touching one part of its edge with the finger, draw the bow across a
point of the edge half a quadrant from the finger, we see the sand
arrange itself along two diameters intersecting at right angles. If the
bow is drawn at a point one-third a quadrant from the finger-clamped
point, we get a six-pointed star. If the bow is drawn at a point
a fourth of a quadrant from the finger-clamped point, we get an
eight-pointed star. And so we can get the sand to arrange itself into a
star of any even number of points; that is, we can get a star of four,
six, eight, ten, twelve, etc., points, but not of three, five, seven,
etc.
In these cases the centre of the plate or disc has been fixed. If,
instead, the plate or disc be fixed by a clip at the edge, or clamped
elsewhere than at the centre, we find the sand arranging itself into
other forms, in which the centre may or may not appear; that is, the
centre may or may not be nodal, according to circumstances.
A curious effect is produced if very fine powder be strewn along
with the sand over the plate. For it is found that the dust gathers,
not where the nodes or places of no vibration lie, but where the
motion is greatest. Faraday assigns as the cause of this peculiarity
the circumstance that “the light powder is entangled by the little
whirlwinds of air produced by the vibrations of the plate; it cannot
escape from the little cyclones, though the heavier sand particles are
readily driven through them; when, therefore, the motion ceases, the
light powder settles down in heaps at the places where the vibration
was a maximum.” In proof of this theory we have the fact that “in vacuo
no such effect is produced; all powders light and heavy move to the
nodal lines.” (Tyndall on “Sound.”)
Now if we consider the meaning of such results as these, we shall begin
to recognize the perplexing but also instructive character of the
evidence derived from the telephone, and applied to the construction
of the phonograph. It appears that when a disc is vibrating under such
special conditions as to give forth a particular series of tones (the
so-called fundamental tone of the disc and other tones combined with it
which belong to its series of overtones), the various parts of the disc
are vibrating to and fro in a direction square to the face of the disc,
except certain points at which there is no vibration, these points
together forming curves of special forms along the substance of the
disc.
When, on the other hand, tones of various kinds are sounded in the
neighbourhood of a disc or of a stretched circular membrane, we may
assume that the different parts of the disc are set in vibration after
a manner at least equally complicated. If the tones belong to the
series which could be emitted by the diaphragm when struck, we can
understand that the vibrations of the diaphragm would resemble those
which would result from a blow struck under special conditions. When
other tones are sounded, it may be assumed that the sound-waves which
reach the diaphragm cause it to vibrate as though not the circumference
(only) but a circle in the substance of the diaphragm—concentric,
of course, with the circumference, and corresponding in dimensions
with the tone of the sounds—were fixed. If a drum of given size is
struck, we hear a note of particular tone. If we heard, as the result
of a blow on the same drum, a much higher tone, we should know that
in some way or other the effective dimensions of the drum-skin had
been reduced—as for instance, by a ring firmly pressed against the
inside of the skin. So when a diaphragm is responding to tones other
than those corresponding to its size, tension, etc., we infer that the
sound-waves reaching it cause it to behave, so far as its effective
vibrating portion is concerned, as though its conformation had altered.
When several tones are responded to by such a diaphragm, we may infer
that the vibrations of the diaphragm are remarkably complicated.
Now the varieties of vibratory motion to which the diaphragm of the
telephone has been made to respond have been multitudinous. Not only
have all orders of sound singly and together been responded to, but
vocal sounds which in many respects differ widely from ordinary tones
are repeated, and the peculiarities of intonation which distinguish one
voice from another have been faithfully reproduced.
Let us consider in what respects vocal sounds, and especially the
sounds employed in speech, differ from mere combinations of ordinary
tones.
It has been said, and with some justice, that the organ of voice
is of the nature of a reed instrument. A reed instrument, as most
persons know, is one in which musical sounds are produced by the
action of a vibrating reed in breaking up a current of air into a
series of short puffs. The harmonium, accordion, concertina, etc.,
are reed instruments, the reed for each note being a fine strip of
metal vibrating in a slit. The vocal organ of man is at the top of
the windpipe, along which a continuous current of air can be forced
by the lungs. Certain elastic bands are attached to the head of the
windpipe, almost closing the aperture. These vocal chords are thrown
into vibration by the current of air from the lungs; and as the rate
of their vibration is made to vary by varying their tension, the sound
changes in tone. So far, we have what corresponds to a reed instrument
admitting of being altered in pitch so as to emit different notes.
The mouth, however, affects the character of the sound uttered from
the throat. The character of a _tone_ emitted by the throat cannot be
altered by any change in the configuration of the mouth; so that if a
single tone were in reality produced by the vocal chords, the resonance
of the mouth would only strengthen that tone more or less according to
the figure given to the cavity of the mouth at the will of the singer
or speaker. But in reality, besides the fundamental tone uttered by
the vocal chords, a series of overtones are produced. Overtones are
tones corresponding to vibration at twice, three times, four times,
etc., the rate of the vibration producing the fundamental tone. Now the
cavity of the mouth can be so modified in shape as to strengthen either
the fundamental tone or any one of these overtones. And according as
special tones are strengthened in this way various vocal sounds are
produced, without changing the pitch or intensity of the sound actually
uttered. Calling the fundamental tone the first tone, the overtones
just mentioned the second, third, fourth, etc., tones respectively
(after Tyndall), we find that the following relations exist between the
combinations of these tones and the various vowel sounds:—
If the lips are pushed forward so as to make the cavity of the mouth
deep and the orifice of the mouth small, we get the deepest resonance
of which the mouth is capable, the fundamental tone is reinforced,
while the higher tones are as far as possible thrown into the shade.
The resulting vowel sound is that of deep U (“oo” in “hoop”).
If the mouth is so far opened that the fundamental tone is accompanied
by a strong second tone (the next higher octave to the fundamental
tone), we get the vowel sound O (as in “hole”). The third and fourth
tones feebly accompanying the first and second make the sound more
perfect, but are not necessary.
If the orifice of the mouth is so widened, and the volume of the cavity
so reduced, that the fundamental tone is lost, the second somewhat
weakened, and the third given as the chief tone, with very weak fourth
and fifth tones, we have the vowel sound A.
To produce the vowel sound E, the resonant cavity of the mouth must be
considerably reduced. The fourth tone is the characteristic of this
vowel. Yet the second tone also must be given with moderate strength.
The first and third tones must be weak, and the fifth tone should be
added with moderate strength.
To produce the vowel sound A, as in “far,” the higher overtones are
chiefly used, the second is wanting altogether, the third feeble, the
higher tones—especially the fifth and seventh—strong.
The vowel sound I, as in “fine,” it should be added, is not a simple
sound, but diphthongal. The two sounds whose succession gives the sound
we represent (erroneously) by a single letter I (long), are not very
different from “a” as in “far,” and “ee” (or “i” as in “ravine”);
they, lie, however, in reality, respectively between “a” in “far” and
“fat,” and “i” in “ravine” and “pin.” Thus the tones and overtones
necessary for sounding “I” long, do not require a separate description,
any more than those necessary for sounding other diphthongs, as “oi,”
“oe,” and so forth.
We see, then, that the sound-waves necessary to reproduce accurately
the various vowel sounds, are more complicated than those which would
correspond to the fundamental tones simply in which any sound may be
uttered. There must not only be in each case certain overtones, but
each overtone must be sounded with its due degree of strength.
But this is not all, even as regards the vowel sounds, the most
readily reproducible peculiarities of ordinary speech. Spoken sounds
differ from musical sounds properly so called, in varying in pitch
throughout their continuance. So far as tone is concerned, apart from
vowel quality, the speech note may be imitated by sliding a finger up
the finger-board of a violin while the bow is being drawn. A familiar
illustration of the varying pitch of a speech note is found in the
utterance of Hamlet’s question, “Pale, or red?” with intense anxiety of
inquiry, if one may so speak. “The speech note on the word ‘pale’ will
consist of an upward movement of the voice, while that on ‘red’ will
be a downward movement, and in both words the voice will traverse an
interval of pitch so wide as to be conspicuous to ordinary ears; while
the cultivated perception of the musician will detect the voice moving
through a less interval of pitch while he is uttering the word ‘or’
of the same sentence. And he who can record in musical notation the
sounds which he hears, will perceive the musical interval traversed in
these vocal movements, and the place also of these speech notes on the
musical staff.” Variations of this kind, only not so great in amount,
occur in ordinary speech; and no telephonic or phonographic instrument
could be regarded as perfect, or even satisfactory, which did not
reproduce them.
But the vowel sounds are, after all, combinations and modifications
of musical tones. It is otherwise with consonantal sounds, which, in
reality, result from various ways in which vowel sounds are commenced,
interrupted (wholly or partially), and resumed. In one respect this
statement requires, perhaps, some modification—a point which has not
been much noticed by writers on vocal sounds. In the case of liquids,
vowel sounds are not partially interrupted only, as is commonly stated.
They cease entirely as vowel sounds, though the utterance of a vocal
sound is continued when a liquid consonant is uttered. Let the reader
utter any word in which a liquid occurs, and he will find that while
the liquid itself is sounded the vowel sounds preceding or following
the liquid cease entirely. Repeating slowly, for example, the word
“remain,” dwelling on all the liquids, we find that while the “r” is
being sounded the “ē” sound cannot be given, and this sound ceases so
soon as the “m” is sounded; similarly the long “a” sound can only be
uttered when the “m” sound ceases, and cannot be carried on into the
sound of the final liquid “n.” The liquids are, in fact, improperly
called semi-vowels, since no vowel sound can accompany their utterance.
The tone, however, with which they are sounded can be modified during
their utterance. In sounding labials the emission of air is not stopped
completely at any moment. The same is true of the sibilants s, z, sh,
zh, and of the consonants g, j, f, v, th (hard and soft). These are
called, on this account, _continuous_ consonants. The only consonants
in pronouncing which the emission of air is for a moment entirely
stopped, are the true mutes, sometimes called the six _explosive_
consonants, b, p, t, d, k, and g.
To reproduce artificially sounds resembling those of the consonants
in speech, we must for a moment interrupt, wholly for explosive and
partially for continuous consonant sounds, the passage of air through a
reed pipe. Tyndall thus describes an experiment of this kind in which
an imperfect imitation of the sound of the letter “m” was obtained—an
imitation only requiring, to render it perfect, as I have myself
experimentally verified, attention to the consideration respecting
liquids pointed out in the preceding paragraph. “Here,” says Tyndall,
describing the experiment as conducted during a lecture, “is a free
reed fixed in a frame, but without any pipe associated with it, mounted
on the acoustic bellows. When air is urged through the orifice, it
speaks in this forcible manner. I now fix upon the frame of the reed
a pyramidal pipe; you notice a change in the clang, and, by pushing
my flat hand over the open end of the pipe, the similarity between
the sounds produced and those of the human voice is unmistakable.
Holding the palm of my hand over the end of the pipe, so as to close
it altogether, and then raising my hand twice in quick succession, the
word ‘mamma’ is heard as plainly as if it were uttered by an infant.
For this pyramidal tube I now substitute a shorter one, and with it
make the same experiment. The ‘mamma’ now heard is exactly such as
would be uttered by a child with a stopped nose. Thus, by associating
with a vibrating reed a suitable pipe, we can impart to the sound of
the reed the qualities of the human voice.” The “m” obtained in these
experiments was, however, imperfect. To produce an “m” sound such as an
adult would utter without a “stopped nose,” all that is necessary is
to make a small opening (experiment readily determines the proper size
and position) in the side of the pyramidal pipe, so that, as in the
natural utterance of this liquid, the emission of air is not altogether
interrupted.
I witnessed in 1874 some curious illustrations of the artificial
production of vocal sounds, at the Stevens Institute, Hoboken,
N.J., where the ingenious Professor Mayer (who will have, I trust,
a good deal to say about the scientific significance of telephonic
and phonographic experiments before long) has acoustic apparatus,
including several talking-pipes. By suitably moving his hand on the top
of some of these pipes, he could make them speak certain words with
tolerable distinctness, and even utter short sentences. I remember
the performance closed with the remarkably distinct utterance, by one
profane pipe, of the words euphemistically rendered by Mark Twain (in
his story of the Seven Sleepers, I think), “Go thou to Hades!”
Now, the speaking diaphragm in the telephone, as in the phonograph,
presently to be described, must reproduce not only all the varieties
of sound-wave corresponding to vowel sounds, with their intermixtures
of the fundamental tone and its overtones and their inflexions or
sliding changes of pitch, but also all the effects produced on the
receiving diaphragm by those interruptions, complete or partial, of
aerial emission which correspond to the pronunciation of the various
consonant sounds. It might certainly have seemed hopeless, from all
that had been before known or surmised respecting the effects of aerial
vibrations on flexible diaphragms, to attempt to make a diaphragm speak
artificially—in other words, to make the movements of all parts of
it correspond with those of a diaphragm set in vibration by spoken
words—by movements affecting only its central part. It is in the
recognition of the possibility of this, or rather in the discovery
of the fact that the movements of a minute portion of the middle of
a diaphragm regulate the vibratory and other movements of the entire
diaphragm, that the great scientific interest of Professor Graham
Bell’s researches appears to me to reside.
It may be well, in illustration of the difficulties with which formerly
the subject appeared to be surrounded, to describe the results of
experiments which preceded, though they can scarcely be said to have
led up to, the invention of artificial ways of reproducing speech.
I do not now refer to experiments like those of Kratzenstein of
St. Petersburg, and Von Kempelen of Vienna, in 1779, and the more
successful experiments by Willis in later years, but to attempts
which have been made to obtain material records of the aerial motions
accompanying the utterance of spoken words. The most successful of
these attempts was that made by Mr. W. H. Barlow. His purpose was “to
construct an instrument which should record the pneumatic actions”
accompanying the utterance of articulated sounds “by diagrams, in
a manner analogous to that in which the indicator-diagram of a
steam-engine records the action of the engine.” He perceived that the
actual aerial pressures involved being very small and very variable,
and the succession of impulses and changes of pressure being very
rapid, it was necessary that the moving parts should be very light,
and that the movement and marking should be accomplished with as
little friction as possible. The instrument he constructed consisted
of a small speaking-trumpet about four inches long, having an ordinary
mouthpiece connected to a tube half an inch in diameter, the thin end
of which widened out so as to form an aperture of 2¼ inches diameter.
This aperture was covered with a membrane of goldbeater’s skin, or thin
gutta-percha. A spring carrying a marker was made to press against the
membrane with a slight initial pressure, to prevent as far as possible
the effects of jarring and consequent vibratory action. A light arm
of aluminium was connected with the spring, and held the marker; and
a continuous strip of paper was made to pass under the marker in the
manner employed in telegraphy. The marker consisted of a small, fine
sable brush, placed in a light tube of glass one-tenth of an inch in
diameter, the tube being rounded at the lower end, and pierced with a
hole about one-twentieth of an inch in diameter. Through this hole the
tip of the brush projected, and was fed by colour put into the glass
tube by which it was held. It should be added that, to provide for the
escape of air passing through the speaking-trumpet, a small opening
was made in the side, so that the pressure exerted upon the membrane
was that due to the excess of air forced into the trumpet over that
expelled through the orifice. The strength of the spring which carried
the marker was so adjusted to the size of the orifice that, while the
lightest pressures arising under articulation could be recorded, the
greatest pressures should not produce a movement exceeding the width of
the paper.
“It will be seen,” says Mr. Barlow, “that in this construction of the
instrument the sudden application of pressure is as suddenly recorded,
subject only to the modifications occasioned by the inertia, momentum,
and friction of the parts moved. But the record of the sudden cessation
of pressure is further affected by the time required to discharge the
air through the escape-orifice. Inasmuch, however, as these several
effects are similar under similar circumstances, the same diagram
should always be obtained from the same pneumatic action when the
instrument is in proper adjustment; and this result is fairly borne out
by the experiments.”
The defect of the instrument consisted in the fact that it recorded
changes of pressure only; and in point of fact it seems to result,
from the experiments made with it, that it could only indicate the
order in which explosive, continuant, and liquid consonants succeeded
each other in spoken words, the vowels being all expressed in the
same way, and only one letter—the rough R, or R with a burr—being
always unmistakably indicated. The explosives were represented by a
sudden sharp rise and fall in the recorded curve; the height of the
rise depending on the strength with which the explosive is uttered,
not on the nature of the consonant itself. Thus the word “tick” is
represented by a higher elevation for the “t” than for the “k,” but the
word “kite” by a higher elevation for the “k” than for the “t.” It is
noteworthy that there is always a second smaller rise and fall after
the first chief one, in the case of each of the explosives. This shows
that the membrane, having first been forcibly distended by the small
aerial explosion accompanying the utterance of such a consonant, sways
back beyond the position where the pressure and the elasticity of the
membrane would (for the moment) exactly balance, and then oscillates
back again over that position before returning to its undistended
condition. Sometimes a third small elevation can be recognized,
and when an explosive is followed by a rolling “r” several small
elevations are seen. The continuous consonants produce elevations less
steep and less high; aspirates and sibilants give rounded hills. But
the results vary greatly according to the position of a consonant; and,
so far as I can make out from a careful study of the very interesting
diagrams accompanying Mr. Barlow’s paper, it would be quite impossible
to define precisely the characteristic records even of each order of
consonantal sounds, far less of each separate sound.
We could readily understand that the movement of the central part of
the diaphragm in the telephone should give much more characteristic
differences for the various sounds than Barlow’s logograph. For if
we imagine a small pointer attached to the centre of the face of the
receiving diaphragm while words are uttered in its neighbourhood, the
end of that pointer would not only move to and fro in a direction
square to the face of the diaphragm, as was the case with Barlow’s
marker, but it would also sway round its mean position in various small
circles or ovals, varying in size, shape, and position, according to
the various sounds uttered. We might expect, then, that if in any way
a record of the actual motions of the extremity of that small pointer
could be obtained, in such sort that its displacement in directions
square to the face of the diaphragm, as well as its swayings around
its mean position, would be indicated in some pictorial manner, the
study of such records would indicate the exact words spoken near the
diaphragm, and even, perhaps, the precise tones in which they were
uttered. For Barlow’s logograph, dealing with one only of the orders
of motion (really triple in character), gives diagrams in which the
general character of the sounds uttered is clearly indicated, and the
supposed records would show much more.
But although this might, from _à priori_ considerations, have been
reasonably looked for, it by no means follows that the actual results
of Bell’s telephonic experiments could have been anticipated. That the
movement of the central part of the diaphragm should suffice to show
that such and such words had been uttered, is one thing; but that these
movements should of themselves suffice, if artificially reproduced, to
cause the diaphragm to reproduce these words, is another and a very
different one. I venture to express my conviction that at the beginning
of his researches Professor Bell can have had very little hope that
any such result would be obtained, notwithstanding some remarkable
experiments respecting the transmission of sound which we can _now_
very clearly perceive to point in that direction.