----- Original Message -----
From: "Tony Caught" <caute@optusnet.com.au>
To: <pianotech@ptg.org>
Sent: April 20, 2001 7:08 PM
Subject: Re: Steinway Sustain
-----------------------------------
About the soundboard spring idea . . .
The idea is pretty much as others have described, but I'll give you what I
remember of the work I did and extrapolate a bit on what I'd do now given
the need and an appropriate situation.
First, the idea is not to 'restore crown' in a collapsed soundboard, that
isn't going to happen. What can happen, however, is that some stiffness, or
springiness, can be added to a soundboard system that has too little because
whatever mechanism that originally formed its crown, and the resultant
stiffness, is no longer doing its job.
Typically, with a compression-crowned soundboard assembly, this is because
of compression set within the soundboard panel itself. As the wood cells
collapse due to compression set the stress interface between the soundboard
panel and the across-grain rib dissipates and the soundboard begins to
return to its original flat condition. Since what we consider to be adequate
sustain depends on some proportion of both mass and stiffness within the
soundboard assembly, when one of these decreases, so does sustain.
What you're going to do by installing a spring, or springs, under the
soundboard is substituting some of the stiffness of the spring(s) for the
lost stiffness of the soundboard assembly.
The exact mechanism you devise will depend on the materials you have
available. When I first tried this I used the progressive coil valve springs
from a 1957 Corvette engine -- they were the used ones I had left over from
a valve job I did on my car back then. Later I graduated to various springs
chosen just for the purpose. You want them to be of reasonably large
diameter, say 1" (25 mm) or more, so they don't squirrel around on you. And
you want them to be reasonably long so you have some range of adjustment. As
I recall, 3" to 4" (75 to 100 mm) was about right. You'd also like them to
be closed and ground on both ends so they will apply even pressure without
distorting. You want the compression rate to be somewhere between 50 and 100
lbs/inch (say between 25 and 50 kgf).
A quick look at my McMaster-Carr catalog has a couple of promising
candidates. The one I'd start with is:
#9657K136 4" (100 mm) o/a length, 1 1/4" (32 mm) diameter.
The load is given as 58 lbs (26 kgf) at 1" (25 mm) deflection.
You could pick up another 30 lbs (14 kgf) or so by compressing it
another
0.5" (12.5 mm) without stressing it overly much.
A package of 6 runs about US$7.50 plus shipping.
Given the typical dead killer octave I'd locate the center of the worst part
and then locate that point along the bridge underneath the soundboard. Then
I'd find the spot along the bridge line between the two ribs that comes
closest to this point and mark it. Hopefully this will be near a couple of
belly braces. Fabricate some type of mounting board between the belly
braces, along side one of the belly braces or along the belly rail itself on
which to mount the spring adjustment bolt. This should be made of maple or
some other such wood and should be of a size that will be able to support a
several hundred pound (at least a hundred kgf of so) load for the duration.
As I recall it worked best to glue a maple strip to the back side of the
soundboard in line with the bridge to seat the spring against. This strip
should extend nearly to the ribs and should be about 8 to 10 mm thick and
about 10 mm wider than the spring diameter. You want to spread the spring
force around a bit. Put a piece of heavy leather on the maple strip to seat
the spring against.
Locate the appropriate point on your fabricated mounting board or bracket
and drill through for a T-nut of appropriate size. Your adjustment screw can
be most anything of a suitable size. In the States we would use something
around a 3/8" to 1/2" course thread machine screw or bolt. Run this through
the T-nut and put on a nut, a large fender washer with a hard leather washer
contact cemented in place and another nut to hold it all in place with.
Fill the spring cavity with soft felt packing (old, rolled up muffler strips
will do nicely) and place it between the wood strip on the soundboard and
the fender washer. The one problem I recall had to do with the springs
resonating and ringing. Filling the cavity with loose felt dampened this
down nicely. Run the adjustment screw in until you have compressed the
spring by the desired amount (I'd start with about 25 mm). Then go up and
listen.
If you hear some improvement, but not enough you can always put in a couple
more springs. With experience you will learn to plan ahead for this when you
fabricate your mounting board and/or brackets. It was my experience that
three, four or five medium pressure springs worked better than one or two
very heavy springs. Also, you don't want the springs close to fully
compressed. The spring described above has a compressed length of 1.8" (46
mm). You probably shouldn't compress it to less than 2.5" (64 mm). A typical
installation might have two springs located between the ribs closest to the
worst sounding area and one more between the ribs on either side (or on both
sides) adjacent to that area.
If you're careful you can dry fit all this stuff until you determine whether
or not it is giving you improved results. If it does you can then mount the
stuff more permanently and dress things up to make them look pretty. If not,
you can remove the whole mess and no one will be the wiser except for you
and your lost time.
Now to respond to a couple of other comments I've read about the procedure:
-- No, I do not consider this something I would only do on an old
clunker. If the procedure works on an old clunker why shouldn't it work just
as effectively on a new clunker? If the situation arose today I would
consider doing it to any piano that exhibited these symptoms -- namely a
localized loss of sustain. No, I agree that the results I remember were not
as good as a new soundboard installation. And I'd surely recommend a new
soundboard first, but I've not found all that many owners of five to ten
year old $50,000 pianos who wanted to pay another few thousand to install a
new soundboard no matter how bad the killer octave sounded.
-- You may use any method available to mount the springs. Someone
mentioned angle iron and this will work as well. I generally stuck with wood
because I could work with it more easily in the field and it has better
damping qualities. Use whatever you are familiar with. Whatever you use it
will sometimes require considerable ingenuity to get the job done.
-- This process will generally improve sustain (if it is going to be of
any help at all), but not 'power.' By adding stiffness to the soundboard
assembly it will make the assembly more resistant to motion in response to
relatively lower frequencies. This changes the energy spectrum within the
acoustic wave envelope by reducing the percussive attack tone characteristic
in the killer octave and improving the sustain time of the fundamental.
-- It is not necessary to reduce string tension before installing these
springs. I've not done this for a number of years and I don't remember what
effect it had on the long-term tuning stability of the piano. Initially,
though, it will raise the pitch through the affected area somewhat. It won't
be all that much but I still install them before you do your pitch raise.
-- As to the life of the repair, again my experience goes back a few
years, but I see no reason why the acoustic affect shouldn't be relatively
permanent. The damage is already done in the soundboard panel and probably
won't get much worse over time. Springs do age over time, but not as fast as
we do.
-- Is it worth the time and effort? Probably yes. If it improves the
sustain time and makes a relatively ugly sound better for a few hundred
bucks -- why not?
-- No, the writer is correct; the soundboard is not vibrating as the
manufacturer intended after the installation of these springs. But, of
course it also was not vibrating as the manufacturer intended before their
installation -- at least I hope the percussive, dead sound through the
killer octave was not intended -- so I fail to understand the reluctance to
use them. Assuming that they work, of course. Remember, my experience with
them goes back probably 25 years. I quit experimenting with them because the
various 'experts' of the time said they should never be used on 'good'
pianos or by 'good' piano technicians. I didn't know enough about the
function of the piano soundboard back then to challenge those experts. Now I
do. Nowadays the naysayers will now have to make a good theoretical case as
to why they shouldn't be used if they do the job. I want chapter and verse,
folks!
Now, having stuck my neck out thus, let me repeat: my experience with these
things goes back a few years. Experimental work with the idea was just
getting nicely started when the traditionalists and experts managed to put a
stop to it. What I've outlined above is a good starting place, not a
well-tested end product. It's another of those ideas that needs to be worked
with some to test its limits and its potential. Only with a full backlog of
thoughtful trial and error over some period of time will the various
strengths and pitfalls be discovered. But let us not, either out of
ignorance or prejudice, discard yet another idea that may well have merit
just because we've never done it that way before.
Good luck to those adventurous enough to go where few have gone before.
Regards,
Del
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