This is a multi-part message in MIME format. ---------------------- multipart/alternative attachment Thank you very much for your response Dale. Some good info here. I will = read those articles. Terry Farrell =20 ----- Original Message -----=20 From: Erwinspiano@AOL.COM=20 To: pianotech@ptg.org ; davehugh@msn.com=20 Sent: Sunday, January 20, 2002 8:47 PM Subject: Re: Soundboard Pre-Stressing/Downbearing, was: Baldwin = accu-hitch pins In a message dated 1/20/2002 6:41:51 AM Pacific Standard Time, = mfarrel2@tampabay.rr.com writes: Terry >>>Welcome to the insomnia club. Considering all the variables in = the always in flux soundboard system I've always thought that setting = bearing to the degree you are speaking ofis a bit like making a jello = foundation for a house. Just when you thought you had the right = consistency poof it wiggled and the house fell over. Well it's not quite as bad as all that I have always been curious about the soundboard pre-stressing method = of targeting downbearing, but when I ask how much to push the soundboard = down, I get answers like: "enough", "not too much" and the like. The = idea of getting a feel for how much the soundboard will deflect under = load I should think would help target proper downbearing Now I see here = we are using some numbers. This is a good thing. You say David Hughes = deflects the board 3 mm at the center strut. You state that you drive = wedges under each strut. How much do you deflect the board at each = strut? >>>>The short answer is after driving the maple wedges in, readings = of deflection will be appox.3 to 3.5 at the central strut about 2 ish on = the next treble strut and 1 to.05 on the top. The lowest tenor strut can = be forced lower than is needed and should not be over driven. It's value = will be in the 2 mm range. But hey if you don't want to take my word for = it e- mail Dave hughes at davehugh@msn.com and ask him. Terry, save some time and read John Hartman 'effects of = downbearing on tone' aug,sept oct nov. 1996. Very good reference = material. The last article will give you some numbers that I can see a = person like you is ready to digest. I have posted a description of this = in the last year on this list. Now the other question I have is that I assume the reason one would = pre-stress the soundboard is to incorporate into your work how much the = soundboard will deflect under load. >>>>>>.. Yes an approximation But if you deflect the soundboard 3mm or whatever, wherever, for = any given=20 soundboard, what does that do for you (i.e., you are pushing a stiff = board 3mm down, and you would be pushing a highly flexible board 3mm = down - under strung conditions, obviously the flexible board would = deflect more under final strung downbearing)? .>>>>>> Ist question. Your making it stiffer and changing the = impedance .>>>>>Yes but for the sake of discussion lets stick to newly crowned = boards. The same principles can be modified for other situations. Rib = crowned boards with either sugar pine or spruce ribs and duplicated to = original rib heightswill result i a stiffer board than the old = compression crowned boards and the amount they compress is fairly = predictable.=20 >>>>>>>> Remember jello? It moves. So does a new unstrung = soundboard. Up And down with moisture content. So at what point in the = moisture content cycle are you going to set your bridge = heights/downbearing. More insomnia material. =20 You could simply calculate the needed angles without = pre-stressing the board.=20 Now if you could pre-stress the board with downward pressure = approximately equal to that of the final string downbearing in the = various regions of the scale, then we would be truly simulating the = soundboard's reaction to the strung-up load and accurately calculate our = desired string downbearing angles, etc. Of course to do this we would = need some sort of pressure gauge/device. >> Terry I know Steinway doesn't carry much weight with you but = they've done it this way for a hundred fifty years but so has mason and = others >>>See Bob Hofs article on this last year he uses go bars off the = Ceiling. The down side to that is I can't feel what go bars are doing = in the same way as I can with my fist. >> With the maple wedges I insert them between the struts with the = left hand as I pound with my right fist on the bridge top. As the board = sinks to appox. 3 mm it gets so stiff it feels like your pounding on a = gym floor. Or you pound till it feels real stiff It's an aquired feel = that's not difficult to get the hang of. Repeat at each strut.=20 What about a rig similar to a pneumatic rib clamp. Just use the half = with the fire hose. Clamp it to the rim above the bridge, inflate, and = there you go with measurable pressure applied all along the long bridge = in much the same manner as downbearing would be once the piano is = strung. You could also use four segments of fire hose to apply = appropriate variable pressure to each of the four sections (and one on = the bass if you want to do that). The result would be a board stressed = in very much the same manner as it would be after stringing. Then you = could set your downbearing exactly where you want it, armed with the = knowledge of exactly (or there abouts) how much the soundboard will = deflect! >>>>Dave Love was right, no access to the bridge. So now, readers..........you are thinking one of two things: Either = Terry Farrell needs to get his head screw on properly, i.e. it's a dumb = idea, or it sounds like a great idea, now go try it Terry and write us = back. So what do you think? >>>Your doing some good thinking. no worrys you're on your way Maybe it is the case that with a given soundboard construction, the = boards are consistent enough to yield predictable result when = pre-stressed 3mm or whatever. Is this the case, and thus would yield a = more accurate pre-stressing a waste of time? >>>>This what I'm gettin at I guess I'm just thinking and probing for information. Any input = would be appreciated. I realize the thing to do is put in a couple dozen = boards, try some different methods, and see what happens. >>> This helps a lot. Of course don't forget the fun factor. I hope = this is helpful verses annoying. But boy oh boy, that will be years, and I would fear, some tears. I'm = just trying to=20 arm myself with as much info a possible before I screw up too darn = many pianos. Terry Farrell SNIP ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/db/44/7f/85/attachment.htm ---------------------- multipart/alternative attachment--
This PTG archive page provided courtesy of Moy Piano Service, LLC