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THE MUSIC TRADE
HORSE POWER OF MUSIC.
Professor Webster of Worcester Tells Ameri-
can Academy How to Measure the Intensity
of Tcnes and to Preserve Their Description
for the Use of Future Scientists.
A very interesting corner of physical science
was considered at the last meeting of the Ameri-
can Academy of Arts and Sciences by Prof. Ar-
thur Gordon Webster, of Clark University, that
of the "Pressure of Sound." "Sound," said the
speaker, "consists in a rapidly varying pressure
of air. If the ton^ be a musical one, the varia-
tions are uniform, as for example in the middle
C they are at the rate of 256 per second." It has
been a work of investigation by Professor Web-
ster during the past ten years to try to deter-
mine certain of the peculiarities of sound, and
in this work he has invented a number of pieces
of apparatus. One of the most important of these
is a device which for want of a better name he
has called the "phone," which will produce always
the same tone, and of a desired intensity. This
in theory and in practice is a standard whereby
Ihe strength of other tones may be measured, and
if one should desire in the future to know
whether a. tone is as loud then as it was to-day,
it will be perfectly practicable to determine the
fact. Such an instrument has many uses. In
his various experiments trom year to year in
which the ear was concerned, Lord Kelvin was
always questioning: "Can I hear as well to-day
as yesterday?" The deaf are always desirous of
getting a precise statement as to whether their
hearing is improving or becoming poorer. Such
an instrument will refer them to a standard, and
give them a scientific statement for comparison.
To-day they depend upon the guess of the aurist.
It is evident at the outset in the matter of
pressure, which is the one considered first by
Dr. Webster, that if a measure could be taken of
amount that the pressure is varied by the sound,
facts with reference to the conditions existing
would be made known. Accordingly physicists
have been at work more or less upon the prob-
lem. But even with an intense sound the varia-
tion in pressure is exceedingly minute, and much
more so in any sound that we are able to bear.
One millionth of an atmosphere variation in the
air pressure would be a large amount to be due
to sound, yet scientific men have set themselves
to measure this infinitesimal variation.
It will occur to any student that if the air is
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REVIEW
varying in pressure it might be measured by
optical means, because denser air will have a
different index of refraction from rarer air, and
rxperiments along this line have been made by
a German. There lies in the method, however,
this difficulty, that the air is affected to so slight
a degree that a very loud noise must be made
before the variations are visible. The efforts
to solve the question have been taken, therefore,
to the mechanical ground and experiments have
been made with diaphragms. The phonograph is
a machine that uses a diaphragm, but this while
it produces a sound that the ear decides to be
like the original, the result is from the scientific
point of view very different, but the principle
of the phonograph is a good one. So Professor
Webster has been experimenting with diaphragms
of different kinds. Membranes are sensitive to
light pressures, but they are also affected by
moisture, so that for the purposes of a standard
they are useless. Metals have been tried, but
they are affected by heat, and in these minute
measurements they become also unreliable, so re-
course has been had a t last to glass. It has
been found that thin glass, the cover glass for
microscope slides, will be affected by the varying
pressure of the air produced by sounds, and so
the later experiments have been made with this.
Even then the movement is so slight that the
microscope cannot read the displacements.
Modern science affords much more delicate
means that the microscope for the viewing of
minute displacements, when these can be made
in light, Michaelson's interferometer being an
instrument of this character, so Professor Web-
ster's next move was to cement a little mirror to
this bit of cover-glass and by means of the in-
terference of light determine the motion of the
glass. By this means a movement of less than
a millionth of an inch is visible and measurable.
Meanwhile the strength of the sound was in-
creased by the use of a resonator, which might
make it even as much as forty times more in-
tense. One of the devices employed by this in-
genious scientist was to measure the vibrations
of the plate by a telescope that was itself set
on a tuning-fork which gave it an opposing mo-
tion so that it nullified in part the rapid vibra-
tions of the glass plate. Meanwhile a German
investigator, Max Wien, of Dantzig, a college
mate of Webster's, in Europe, had been at work
on the same problem. Wien mounted on the
glass plate a little mirror set on a bit of watch
spring, and put the whole within a second resona-
tor, thus by the magnification of two resonators
and the spring of the steel the motion was
brought within the reach of the microscope.
In the statement of his results, Professor Web-
ster made use of many of those graphic curves
with which mathematicians delight to set forth
their work. Some of the ideas, however, may be
expressed popularly, and some of the results
show the enormous waste of energy that our ordi-
nary mechanical methods entail. The "phone"
with which he is experimenting can be heard
for half a mile in the open air, yet it takes no
power to run it, an infinitesimal current merely.
The output of the ordinary cornet (at the bell)
is not more than a millionth of a horse-power,
so that all but the merest fraction of power that
the player puts into his mouthpiece is thrown
away. The organ pipe is a very wasteful device,
for most of the motive power is chrown away and
but a fraction of the wind from the bellows is
really utilized in making music. The power
expended in noise is very great in places, on the
coast of Maine there is a fog whistle that takes
GO horse-power, while in England there is one
consuming about 600 horse-power.
In these not more than a tenth of one per cent,
is really used to make the signal. The outcome
of the investigations is that scientists can now
measure sound without using the human ear, and
that they can express the energy and activity in
units. One sound may be compared with an-
other, sounds may be compared at different times
with other sounds even in different places, and it
is now possible to, so to speak, pack away the
sounds of to-day so that the scientists of the
future can compare them.
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