Soundboard re-engineering priorities

[Delwin D. Fandrich]

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  ----- Original Message -----=20
  From: BobDavis88@aol.com=20
  To: pianotech@ptg.org=20
  Sent: Thursday, April 24, 2003 8:51 AM
  Subject: Soundboard re-engineering priorities


  One really needs to START with 50 installations under his belt, to try =
out parameter changes one at a time and throw them all away. However, =
since this is impossible, it would be nice to have guidance on the order =
of precedence. Rather than throwing fifty experimental boards at one =
piano, I'm going to have to make small changes in the next fifty pianos. =
I am willing to accept that they may not be their ultimate.
This is a generally good and safe way to proceed. It also has the =
advantage of allowing you to quantify the improvements (or lack thereof) =
made by one specific change.=20

  I have previously used crowned ribs, glued on in a bellied press, in a =
moderate EMC environment, and they sound good and should presumably last =
longer. I have (for now!) four specific questions:=20

  1) Is there some small change to try next, to improve the treble =
sustain and projection, without changing the rib layout, or making a new =
bridge root?
You might try a soundboard cutoff bar. It won't help the high treble any =
but will help through the upper tenor and lower treble. Don't forget to =
adjust the rib specification to compensate for their shorter length. =
Sooner or later, however, you're going to have to tackle those bridges. =
You might start by recapping the top two treble sections to accommodate =
something resembling semi-log scaling.


  2) I thought the general deal was that stiffer=3Dsofter tone/longer =
sustain and less stiff=3Dlouder, more abrupt. We recently strung a =
Chickering 109C, the treble of which had both weak output AND a very =
short sustain in the top section. The scale lengths and sizes looked =
normal. Whassup? Too much mass?
What do you mean, "The scale lengths and sizes looked normal....?" What =
is normal? Where is the hammer striking point? What are the hammers =
like?=20


  3) A third bridge obviously goes on a different spot on the =
soundboard, with different impedance, and loudness, characteristics from =
the hockey-stick area of the tenor bridge it replaces. How does one =
figure that difference into the string scale the FIRST time? And would =
you leave the hockey stick area or cut it away?
Are you going to be in Dallas come July?


  4) What difference do you think the grain angle to the belly rail =
makes?
In general, as the grain angle becomes closer to perpendicular to the =
bellyrail the stiffness felt by the bridge increases (i.e., the =
stiffness component of impedance increases) wherever the bridge is close =
enough to the belly rail or the rim for this stiffness to be felt. This =
by virtue of the physical characteristics of the spruce panel. It is =
stiffer along the grain line than it is across the grain line. Changing =
the grain angle (relative to the belly rail) from the normal 45=BA to =
something like 60=BA or 70=BA (depending on the length and design of the =
piano) can be particularly helpful through the killer octave region. In =
practical terms, and all other factors being equal, this change works to =
increase sustain of the treble. In theory it may also reduce slightly =
the available potential power, though this may not be noticeable. Of =
course, this change has the same effect on the bass to a greater or =
lesser degree depending on the design of the bass bridge and its =
location on the soundboard. Around the bass bridge this added stiffness =
may not be desirable since it will work to increase the sustain of the =
higher partials, something rarely desirable in the bass section. =
Compensation can be made by thinning the board out through the bass =
region or, better yet, partially or fully floating the bass region.

It is a good idea to re-orient the ribs to take advantage of any changes =
made to the soundboard grain angle. Though we don't always.

Del

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