THE TUNING-FORK—THE SYREN—SOUND FIGURES—SINGING FLAMES.
We cannot close the subject of Sound without some mention of the
Musical Pitch, and various instruments and experiments which have from
time to time been made to discover the pitch, sound, and vibrations,
and even to _see_ Sound. To understand the vibrations or “pitch”
of a musical note we may study the illustration, which shows us a
tuning-fork in vibration.
You will perceive that each prong of the tuning-fork beats the air
in an opposite direction at the same time, say from _a_ to _b_ (fig.
196). The prong strikes the air, and the wave thus created strikes
again outward, and the condensation thus created travels along the
back beat, rarefying the air, and both these, the rarefaction and the
condensation, move with the same rapidity one behind the other.
[Illustration: Fig. 196.]
[Illustration: Fig. 197.]
The tuning-fork of course vibrates a very great many times in a second,
every vibration generating a wave. “Pitch,” in a general sense, is the
number of vibrations per second which constitute a note. For instance,
the note A, the standard pitch consists of four hundred and thirty-five
complete vibrations per second. Concert pitch is slightly higher,
for there are a few more vibrations in the second. The lowest sound
pitch is forty vibrations, the highest forty thousand. “Pitch” may be
determined by an instrument termed the “Syren,” or by a tooth-wheeled
apparatus.
The SYREN was invented by Cagniard de Latour. It consists of
a metal cylinder, a tube passes through the bottom, and through the
tube air is blown into the cylinder. On the top a number of holes are
drilled, while just over the cylinder top, almost in contact with it,
is a metallic disc, which rotates upon a vertical axis. The disc is
perforated with holes equal in number to those in the cylinder top, but
the holes are not perpendicular, they slope in opposite directions. So
when the air is forced through the holes in the top of the cylinder it
impinges upon one side of the holes in the rotating disc, and blows it
round.
The disc in one revolution will therefore open and shut as many holes
as there are in the disc and cylinder, and the air blown in will escape
in so many puffs—the number of puffs in a given time depending upon the
rapidity of rotation. There is an arrangement to show the number of
turns. By these rotations a sound is produced which rises in pitch as
the revolutions are increased in number.
[Illustration:
Fig. 198. Sound Figures. Fig. 199.]
To determine the pitch of a certain sound we must find the number of
times the plate revolves in that time, then we shall have the number
of vibrations per second required to produce the note we desire.
The arrangement working in a notched wheel tells us the number of
rotations of the disc. Successive, and rapidly-successive puffs or
beats are heard as the rotation increases, and at length the two
sounds will disappear, and merge into one, which is perhaps that of
the tuning-fork, whose note you require to find the “pitch” of. By
maintaining this rate for a minute or less, and setting the gear to
tell the revolutions, the number will be found marked on the dial of
the apparatus. So by multiplying the number of revolutions of the disc
by the number of the holes, and dividing the product by the number of
seconds during which the disc was in connection with the recording
gear, we shall have the number of vibrations per second necessary to
produce the pitch corresponding to the given sound. The above is the
description of one form of Syren; there are others, which, however, we
need not detail.
We have seen that there are certain _nodal points_, or resting-places,
in vibrations, and this can easily be shown upon a fiddle-string, from
which paper discs will fall off except on the nodal point, showing
that there is no vibration there. The same experiment may be made by
means of plates, which will give us what are termed Chladni’s figures.
Suppose we strew a glass-plate with fine sand, and stroke the edge with
a fiddle-bow. The vibrations of the plates will make certain patterns,
and cast the sand upon those points of repose to form nodal lines in
various directions. The plates must, of course, be held or fastened,
and a variety of _sound figures_ may be produced. (_See_ figs. 198 and
199.)
The relation between the number of segments on the plate and the pitch
of the note, can be ascertained by using a circular plate clamped
in the centre. “If the finger on the plate and the fiddle-bow are
one-eighth of the circumference apart, the fundamental note will be
produced. If one-sixteenth apart, the higher octave will be heard.”
Sensitive flames will detect air vibrations, and flames can also be
made to sing. Sensitive flames were discovered by Mr. Barrett, who
noticed the effect a shrill note had upon a gas flame from a tapering
jet. The flame was a very long one (fourteen inches), and when the
sound was produced it shortened at once, while the upper part expanded
like a fan; the same effects, in a less marked degree, were observable
when the shrill sound was prolonged from a distance of forty feet.
Professor Tyndall was immediately interested in this discovery, and in
January 1867 he lectured upon it at the Royal Institution.
If any one wish to try the experiment, a piece of glass tubing should
be obtained, and let the mouth be tapered down to a small orifice
one-sixteenth of an inch in diameter. Then when the highest pressure
is on for the evening, light the gas and sound a shrill whistle. The
flame will sink down and spread out. The illuminating power may thus be
increased, and many experiments may be made. For instance, if a person
be in the room and try to read, he will probably not be able to do so
at a little distance; but if his friend whistle to the gas it will so
expand itself as to enable him to read, so long as the whistle lasts.
A very ingenious burglar-detector was made upon the principle of the
sensitive flame, which expands at a noise and heats a welded plate of
gold, silver, and platinum. The plate swerves aside, the metals being
unequally affected by heat, and as it is connected with a battery,
rings a bell by electricity. A small high flame has been made sensitive
to the chinking of coin, or even to the ticking of a watch. We will now
give some explanation, derived partly from Professor Tyndall, of the
cause of sensitive flames.
A sensitive flame is one just on the point of “roaring,” and about
to change its aspect. “It stands,” says Tyndall, “on the edge of
a precipice. The proper sound pushes it over.... We bring it to
the verge of falling, and the sonorous pulses precipitate what was
already imminent.” The flame is in a state of vibration, so sounds
being vibrations, practically increase the pressure; and the flame
acknowledges the pressure thus invisibly applied by air waves.
SINGING FLAMES are produced by burning hydrogen in a tube;
a musical note is thus produced in the same way as the air causes a
note in an organ pipe. Faraday attributed the sound to rapid vibration
caused by successive explosions of the burning gas. The Gas Harmonicon
has been made on this principle. The air, being heated in the glass
tube, ascends, and the flame is thus permitted to come up more forcibly
in the tube; so violent agitation results when the air tries to get
into the opening above. The size of the flame and its position in the
tube will give a certain note which will be the same note as the air
would emit if in a pipe, for the vibrations give the sound.
Sir Charles Wheatstone has shown by experiment how sound can be
transmitted by placing a rod on a musical-box, and carrying the rod
through the ceiling. When a guitar or violin was placed upon the rod,
the sounds of the musical-box were distinctly heard in the upper room.
A _Phantom Band_ can be made by connecting certain instruments with
others being played on under the stage. Every one will then appear to
play by itself.