Sound Board Crown

[Horace Greeley]

Richard,

Snips on Ed's notes:

At 04:11 PM 3/19/97 -0500, you wrote:
> Richard Moody asks;
>
>>How do you measure crown on a sound board?  (With strings attached)
>>How much should there be?
>
>      The most accurate, easy way I have found is to use a piece of white,
>thin  1/4 "leader" tape "   . It is used to start reel to reel tapes, it is
>NOT adhesive.
>

This is getting harder to find, but is worth the work to do so.  If not
available, something
sufficiently opaque to throw a shadow - thin white cord (sometimes sold as
"kitchen twine")
will work.

>     Tape one end under the soundboard, at one edge,and then thread it over
>the beams, etc.  to the other side, ( staying between the ribs, of course).
>Make sure there are no twists and then, pull it tight and tape the other end
>to the bottom of the board.

I have seen folks use map tacks to do this.

>     You may be able to see a gap between the tape and the board,  it will
>show up as a shadow between the tape and the board.  This is why you want to
>use this very thin, white tape.  If you want to measure the crown,
>accurately, then use a set of feeler gauges between the tape and the board.
> With a strong light shining on the shiny tape, it is very easy to get the
>bearing  read to several thousandths, though that is not so important.

For me, using a drill index is easier, perhaps because of vision issues.  A
drill
will fit most of the places that a feeler gauge will.  If you use a feeler
gauge, get
what used to be called a "machinist's gauge".  These have longer blades
which are
bent an inch or so from the end of the blade, thus making insertion in
tight places
easier.

>        You may want to measure the bearing at several places under the tape,
>before you move it up or down several ribs.  Doing it this way can give you
>an idea of the board's overall shape.
>

If you're serious about doing this, do it wherever you can.  You can get a
general idea
on site and then do a more complete evaluation in your shop, with the piano
inverted.

>       Amount?  Seems like all pianos have their own optimum about of bearing
>pressure.  It must be enough to tighten the whole structure, but not so much
>that the sound board has been forced to it's elastic limit.
>

This last is a critical issue.  It doesn't much matter if you have 3/8"
crown if, in
achieving that, you create a board which is too "bound-up" to speak.  One
test of this
(on assembled pianos, anyway) is to rap soundly on the board with the end
of a finger or
your clenched fist.  (Do not pound.  Rather, a good solid thump.)  If the
board sounds like a
good bass drum, e.g., a nice, whole boom, without too much (if any)
discernable pitch
center - chances are that you are on the way to ok.

If, however, the board, at one or more points, sounds more like your dining
room
table top - well, you _may_ have a problem.  Particularly if, as a part of
this more dull,
non-resonant sound, you find yourself able to discern a (relatively)
specific pitch(s).

>     I would be interested in how much downbearing others find appropriate
>for old soundboards; who does what?

This is a real judgment call, on all fronts.  What constitutes an "old"
board?  We've all
seen boards that were, arguably, "old" well before "their time".  We've all
also seen 100
year old pianos with perfect boards.

Since for me, everything takes a back seat to a musical judgment, I want to
know "what
do I think this piano is capable of doing?"  Thus, a piano may have
virtually no crown (or bearing, for that matter), and yet still be
musically sound for its use.
This is one of those areas where reductive analysis is secondary to
subjective judgment.

The more important questions include:  "If the basic instrument can do
more, what can be
done, what 'improvements' can be made, in order to take maximum advantage
of the piano
as it stands?  How even is the crown?  How even is the bearing?  What
can/should be done
to make these things more even?"

We should not confuse "can" with "should".  So here, as elsewhere, the
numbers provide a
set of guidelines which are to be followed as far as is reasonable.

How's that for a non-answer?

Best.

Horace


Horace Greeley

Stanford University
email: hgreeley@leland.stanford.edu
voice mail: 415.725.9062
LiNCS help line: 415.725.4627