Jim: I don't think the RC&S board is what dictates the need for cut-offs/fish and radial patterns. The compression board will also benefit from cut-offs and other features. The benefit here is the shortening of the ribs through the tenor and into the lower treble and making it easier to achieve the required stiffness where it's necessary and creating a speaker, as it were, that graduates more uniformly from tweeter (treble) to woofer (bass). It creates a panel with a more balanced distribution of frequencies and helps eliminate distortions associated with the weird acoustic properties of the shape of the bass corner. Bosendorfer, among others, uses a very aggressive cutoff bar connecting to the belly rail around note 50 on some models. Of course there are other differences including rim construction and grain angle (somewhat less than 45 degrees) as well as panel thickness but just thought I would mention it. And while the use of the fish also makes it easier to use lighter weight ribs through the higher treble, it's not required on an RC&S board to make the treble stiff enough and the greater need will depend somewhat on the shape of the rim at that end. Additionally, if you are not careful with the use of the fish you will need to add sufficient mass to compensate for getting the panel too stiff without adequate mass with poor results. Even when counterbalanced by adequate mass, it creates a somewhat different high treble than even a successful upper end without it and that difference one may or may not like as well (though both are fine to my ear--just a bit different). As far as the radial rib pattern goes, in my experiments with changing various features, I've done several RC&S boards with original rib positions that when calculated correctly are without problems of achieving the necessary stiffness through the panel all the way to the treble. The main benefit that I've found from adding more ribs and putting them in a radial pattern is that you reduce the incidence of soundboard resonances. More and smaller ribs means that the difference in the soundboard response at the point where the rib is glued to the panel versus the space between the ribs will be less differentiated (it can be that localized). That's part (if not all) of the reason that Darrell Fandrich uses the riblets between the full length ribs. It not only helps to stiffen the panel, which means that you need less beefy ribs overall, but it makes a more uniform transition from rib to rib. All soundboards have resonances (even the ones with smaller and radial rib patters) but the more and smaller ribs that you use in addition to going to a radial pattern the more they are reduced. One other thing about radial rib patterns, when employing a bass float, it's better to have the first rib more parallel to the belly rail setting it back away from the low end of the tenor bridge. The radial rib pattern allows you to rotate the rib array as it descends from the treble in order to achieve that. At least that's my current take on it. David Love www.davidlovepianos.com -----Original Message----- From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of jimialeggio Sent: Thursday, February 11, 2010 7:57 AM To: pianotech Subject: Re: [pianotech] Hammer strike line. Was-----whatever This is really interesting. Could you back up a couple of steps and help me understand what the reasoning for increasing the angle from 45deg in rc&s boards is? I'm thinking out loud here... On a sample old compression board I have (non-radial rib pattern), there is a 42 deg grain orientation relative to the belly rail. This grain orientation puts the cross grain consistently 12 deg off of perpendicular to the ribs through the entire scale. Since cross grain dead perpendicular to the ribs would be the weakest structural orientation of the grain, I assume they were looking for some consistent degree of stiffening from the grain orientation, but not much. Read...entire assembly somewhat flexible...stiffness mainly defined by compression.. On an rc&s board, the fact that the panel itself is not trying to be a significant structural member demands that the ribs be oriented in a fashion which allows them (the ribs) to create the assembly's structure. This dictates the need for the cutoffs/fish and radial rib pattern. Given the radial rib pattern, if you were to leave the grain orientation at 45 deg, the panel's grain orientation would be well off perpendicular to the tenor/low tenor/bass ribs, and pretty close to perpendicular in the high treble. Read; @ 45 deg the panel would add too much stiffness in the tenor/low tenor/bass, and hardly any in the high treble. Seems backwards of what you would like to happen, ie you'd want the bottom end appropriately stiff (from the ribs) but not constrained by a too stiff a panel, and in the high treble you'd want whatever stiffness you could muster, rib and panel (to a point). So shifting of the grain angle up adjusts this, putting tenor/low tenor/bass ribs closer to perpendicular to the grain, and high treble closer to parallel to long grain. I think I answered my own question...does this make sense? (...Let me down easy boys...) Jim Ialeggio grandpianosolutions.com 978- 425-9026 Shirley, MA
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