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TECHNICAL^SUPPLY DEPARTMENT
William Braid White, Technical Editor
Vibratory Powers of the Piano Sound
Board to Be Subject of Wide Research
Vibration Forms of Strings Alone and Vibration Forms of Strings and Sound Board
to Be Studied Intensively, With Differences Graphically Shown
HE other day I was talking about sound
boards and made the usual remarks.
When I say the "usual remarks" I mean
that I said at some length what so far all of
us must say on such occasions; namely, that
very little actual fact is available concerning
the physical behavior of the sound board. So
long as very little is thus known, it follows
that very little that amounts to anything can
be definitely said about the relations between
the accepted construction of the board and its
functions. Of course, piano makers have been
making sound boards for a hundred years in
this country and for two hundred years, very
nearly, in Europe, while behind all this practice
has been the earlier practice of the harpsichord
makers. Naturally the sound board construc-
tion, which the unbroken tradition of many
hundred years has gradually developed, is not
to be brushed aside with a shrug of the shoul-
ders. If it were not satisfactory it would have
been rejected ere this. Because it has been
on the whole satisfactory, and because there
has come to be a considerable fiel
provement and development within its well-
known limits, we are obliged to admit that
fundamentally it must be more or less sound.
Rut we need not admit that there is, therefore,
nothing more to be said on the subject, for
in fact evidence is constantly accumulating to
show that there is everything yet to be dis-
covered and to be said; if by that we refer to
scientific research which will yield definite data
accurate and capable of being practically used.
Evidence
One such piece of evidence came to my hands
the other day. It happens unhappily that I am
obliged to write these words away from my
field of correspondence and therefore must
speak from memory. Nevertheless the facts
are as I state them. A certain tuner corre-
spondent, whose name I shall look up and
gladly acknowledge later, writes me to say that
some time ago he had occasion to work upon
an upright piano of Western make which had
at some time previously been badly damaged.
So badly was it damaged that the sound board
was rendered completely useless, and the
owner, himself a mechanic, decided to replace
it. This he did, using such lumber as he
thought would do, and placing the bridges the
best he could by measurement. I presume he
took off the bridges from the old board and
placed them on the new one by measurement
as accurately as he could. This need not have
been a very difficult matter. Well, anyhow,
the piano thus repaired was restrung and set
to playing again. My tuner correspondent as-
sures me that it stands in tune very well and
sounds at least as good as the average piano
of the same make and age. The new sound
board was made, so far as I recollect, from
some kind of table-top lumber and was much
thicker than the usual sheet of spruce.
T
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Doubtless, if one were to look up the records
one would find that other readers of this de-
partment have seen or heard of similar occur-
rences. There have been tried at various times
many varying constructions of the sound board.
Such boards have always worked well enough
apparently and the principal cause for their
having been abandoned, as most of them have
been, seems to lie in the fact that the new
construction gave no results conspicuously dif-
ferent from the results given by the traditional
methods. Much the same might be said about
the metal sound boards which have been tried.
The Swedish invention shown in this country
a few years ago was very ingeniously thought
out. It was a sheet of corrugated steel with
wooden bridges fastened upon it. It was quite
good; but it was not conspicuously different in
its results.
It is also well known that Steinway & Sons
(and possibly other piano manufacturers, too)
did at one time experiment with the use of
various metals, including aluminum; but that
they found the results obtained hardly worth
the trouble of the experimentation. Most
probably, too, the supply of good suitable New
England and Adirondack spruce was in those
days abundant and obtainable at a moderate
price. That fact would, of course, be very nearly
decisive. Had the metal boards come out with
astonishing superiority, the story would, of
course, have been different.
Now all this may be somewhat vague, but
one cannot deny that it does suggest something.
It does suggest that the material of which the
sound board is made is of lesser importance
than we have been accustomed to think. Or
perhaps, rather, we should say that it suggests
that we have been paying too much attention
to species of wood -or other raw material and
too little to what the material is expected to
do. It is absolutely certain that the principal
duty of the board is to amplify the vibratory
forms impressed upon it by the strings. In
my book, "Modern Piano Tuning," published
ten years ago, I elaborated the idea that the
sound board, being in a state of tension over
its upper surface, being furthermore pressed
upon by a battery of high-tension stretched
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TUNERS' TRADE SOLICITED
(Continued on page 35)
William Braid White
Associate, American Society of Mechanical
Engineers; Chairman, Wood Industries
Division. A. S. M. E.; Member, American
Physical Society; Member, National Piano
Technicians' Association.
Consulting Engineer to
the Piano Industry
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34
steel strings, was in the condition of a very
sensitive nervous system which would have the
faculty of greatly elaborating any impression
made upon it from an outside source. Such an
impression must always come, of course, from
the strings, and it is evident that the sound
board must always find its principal function
in thus elaborating, that is, in amplifying, the
string motions. That it to an extent modifies
them is also clear, because we find that the
sound of the piano string alone is different in
quality as well as in quantity, though not as
much. Mainly the board is an amplifier, which
means that its main function is to produce,
at its surfaces, the string motions in increased
amplitude. And if that be true, as it undoubt-
edly is true, then certainly the ideal board is
one which can execute the surface motions with
the greatest ease, accuracy and amplificatory
power.
But how are we to relate a fact of this
nature to the other fact of the favor shown
for so long to certain species of wood in sound
board construction? It is a remarkable fact
that European makers have also found that a
fir of species closely allied to our picea alba
is the best for their purposes. The answer to
the question can only be found by (1) making
a close study of the structure of the spruce
pine used for sound board work and (2) an
equally close study of the vibratory forms of
the board in action.
Structure
During the piano technicians' conference held
under the auspices of the American Steel &
Wire Co. and the chairmanship of Frank E.
Morton, Dr. Laufer, the company's agricultural
commissioner, on two occasions furnished close-
ly reasoned and illustrated analyses of the
structure of the spruce pine. Among other
things, he showed that the cells of which the
wood is built up are themselves disc-like In
form and capable of executing periodic vibra-
tions. The period of each disc separately taken
will doubtless be very high, but the effect of
the presence of these cells throughout a sheet
of wood will be to make that sheet into some-
thing rather like a large, thick diaphragm,
which, of course, will have its own period,
but now a period of low frequency.
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