PTG Chat: Soundboard Loading

[davehugh]

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Hello Folks!

    David Hughes, RPT, signing in here. Dale Erwin, RPT, was kind enough =
to forward some PTG Tech-Chat to me this morning which apparently took =
place yesterday regarding pre-stressing of soundboards during bearing =
setting. My name was mentioned so I thought I'd chime in.
    First of all, I must apologize for not partaking regularly of the =
PTG tech chat room (or whatever I should call it). I used to subscribe =
several years ago, but at the ripe old age of 46 I've decided I've got =
to leave the pianos in the shop from 5:00pm until 8:30 the next morning. =
I'm not sure of etiquette here - I hope my comments will be =
accepted/welcomed. Furthermore I'd like to add that I'm astonished and =
flattered that I was even mentioned in your discussions. The fact that =
you know who I am is noteworthy to me, and that some even listen to my =
ramblings at PTG conventions is even more pleasurable. Okay, enough =
mushy talk, I'll put the Kleenex away. Back to the chat thing, I'm never =
quite sure who is contributing what in this format, so if I'm not =
crediting someone correctly, please forgive me.
    I believe it was Terry Farrell who quite accurately described our =
method of loading the board. As was alluded to, before you decide if/how =
you are going to do this, you've got to know how your soundboard is =
built. Is it a compression crowned panel? Are the ribs pre-crowned? What =
are the ribs made of? How thick is the board? How is it tapered? And on =
and on. All of this is a little more than I care to delve into tonight, =
so let's get back to the loading thing.
    With our pre-crowned eastern white spruce ribbed soundboard (except =
that the first 3 ribs in the high treble are Sitka), we wedge the middle =
of the board down 3mm using the plate strut which passes over the board =
at about note 40 on most Steinways. This is done with the plate =
suspended on the nosebolts with the cap nuts fastened down, and the =
plate is also simultaneously, temporarily screwed down to the new =
pinblock. This hovers the plate, fixed, at its ultimate resting place. =
No acoustic dowels in yet? Don't sweat it - not needed. I use a long =
screwdriver as a pry bar and force the board down using this central =
strut as a fulcrum point and shove a wooden wedge in there. Real =
hi-tech. So, if the unloaded "free air" between the top of the new raw =
bridge and the underside of this strut is, for example, 10mm, I wail on =
that puppy until I get 13mm and shove the wedge in to hold. Rocket =
science accomplished. Our boards are quite stiff. This takes some muscle =
- no broken plate struts yet :-).=20
        Let's talk about a model A-2, just as an example: for the plate =
strut at note 40: 3mm; the plate strut at 51/52 break: 2mm; the plate =
strut at the 68/69 break: 1.5mm, or some visible movement. Let's face =
it, if you push this down 3mm up there, you'll be picking up kindling =
off the floor....... and you'd need a hydraulic jack to do it in the =
first place. Terry states very wisely that pushing the board down using =
the plate strut at note 20 (actually about note 25 on this scale) is =
risky business. Why? The plate is flexy back there and you'll do more =
bending upward of the plate than shoving down of the soundboard. But you =
do have to do something to assure the board is captured. Make sure the =
plate does not rise and deflect the board a gentle 1.5mm, tops. For me, =
this is the area of the board I've striven to keep "free".=20
    Likewise with the bass bridge. The soundboard (correction: our =
soundboard as we build it in our shop, which in no way infers this is =
the way to do things) "sees" the bass strings as big, albeit pretty, =
earthworms. They have more than enough mass to couple adequately to the =
soundboard given the right bridge pinning, etc. So why would we =
(correction: why would I) want/need to establish any bearing there? I =
want the board to be as flaccid as possible back there. Downbearing is =
only going to foul this up. The amount of long-term sag in the =
soundboard we construct, especially in this area, is so minimal, truly =
no "downbearing" is desired. So, when the bass bridge is cut for =
bearing, I look for the string to virtually touch the top of the bridge =
at the same instant it contacts the hitch area. Maybe a beam of light =
could fit in there, maybe.
    Now that we've got the board screaming in a squashed condition =
(how'd you like to live the rest of your life like that poor guy?), I =
set 0 bearing at note 20, a dime at note 40, a nickel at note 51 through =
note 69, and about a nickel up to note 88. The backscale lengths are a =
determining factor here. And forget trying to get it right on a model S =
in the high treble: the plate casting elevations are all wrong up there =
relative to the elevation of the crappo disastro bar. If you get the =
highest handful of notes to actually say something interesting on an S, =
you've done a fine job.
    This much pressure always alarms people. For the third time I offer =
that this system works for our soundboards, which are high impeadance, =
stiff darned things. I've got about 100 belly jobs out there over an =
eleven year span and I haven't eaten a job yet...... and the pianos =
sing...... and the rims haven't pushed out 1/4". Of course, if we were =
the Beatles, it would be our bird that would be doing the singing, =
right?
    And David Love is right: I don't sweat it too much beyond this. And =
as far as I'm concerned, Terry's head is screwed on just fine.

    Dave Hughes, RPT
    ps: Would someone kindly forward this to David Love? I don't have =
his e-mail address. Thanks.

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