Soundboard crown

[Overs Pianos]

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Hi all,
At 8:36 PM +0300 10/8/03, Calin Tantareanu wrote:
>
>I have followed the numeorus discussions on this list about rib-crowned vs.
>compression crowned soundboards with great interest.
>However, I am asking myself if a soundboard always needs  crown in order to
>work properly?

Probably not, but if you build a flat board and you dry the panel 
prior to gluing the ribs, you'll end up with compression crown by 
default, possibly the worst type of sound board you could construct.

>I heard about some old pianos with no measurable crown that sound very good.

Maybe, but they likely sounded better at some earlier time.

>I read that harpsichords do not have crowned boards (except the
>Italian ones).

In general, harpsichords have thin board which are dried down before 
installation, the panel is glued in and encouraged to expand upwards 
as it takes up moisture.

>So, why is a crown necessary?

As Dale mentioned in his post (hi Dale its good to read that you're 
back), a down bearing load on a flat panel will cause the panel to 
experience tension as it is forced down into a reverse crown. You can 
imagine how long it might take for the fist hint of summer to take 
out such a board. When considering a crowned panel, as the board 
sinks under the down bearing load at the time of set up, and over 
time as compression/tension set takes its toll, the sound board panel 
will tend to be further compressed as the crown recedes. This will 
tend to encourage an extension of the life of the panel rather than a 
shortening, as would be the case if the panel were to be manufactured 
as a flat panel.

>Would a flat board, but with enough downbearing for the strings, work or
>not?

It would if it was designed appropriately, but why would you build a 
design which would fail earlier than an alternative? No serious piano 
manufacture should continue to build CC boards. They are doomed to 
premature failure, for numerous reasons which have been discussed at 
length on this list.

I am doing a lot of sound board research at present. This is why I 
haven't been posting to the list lately.

There was some list discussion about sound board presses recently. I 
got the first image of my vacuum sound board laminating press 
prepared for the web site yesterday. The image below shows the press, 
with a test strip of sound board panel and one rib being pressed.

http://overspianos.com.au/vacpress.jpg

I haven't written a text page for the image yet. The press is 6' x 8' 
(1800mm x 2400 mm). The vacuum pump is a Becker 3ph 2 hp unit.

A cutaway of the pump can be seen at;

http://www.becker-international.com/en_z010.html

It can pump 28 cfm when unrestricted and the vacuum can be adjusted 
from 1.5 psi to 12 psi. (12 psi will yield a hold-down pressure of 
around 0.75 tons/square foot - or 0.8 atmospheres). We use the 
minimum 1.5 psi vacuum for gluing the panel to the ribs. The vacuum 
diaphragm is a fibreglass reinforced vinyl, fitted to a lightweight 
RHS frame which can be lifted on and off the table with ease. The 
vacuum is delivered from the pump to the the table using the RHS 
frame as a vacuum plenum. A series of holes on the inner edge of the 
RHS allow the air to be evacuated from around the perimeter of the 
table. Apart from its use as a sound board assembly table, the press 
is also very useful for gluing veneer to panels. We will also use it 
for the manufacture of the laminated sound board panels which we are 
using for our own piano. By the way, the poser in the image is my 
Rottweiler 'Harry'. He likes to fossick around the workshop chewing 
the odd sound board off-cut.

Best,
Ron O.
-- 

OVERS PIANOS - SYDNEY
      Grand Piano Manufacturers
      _______________________

Web http://overspianos.com.au
mailto:ron@overspianos.com.au
      _______________________
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