Del etc. Del wrote; >Instead of removing the bell you might try leaving it in place and adding a >wood brace--i.e., another belly-brace--between the inside treble curve and >the bellyrail. If you don't like how much the bolt vibrates you might try >threading the hole in the plate and installing a larger (fully-threaded--in >fact, you may have to thread the bolt yourself) bolt in place of the >original. Then you can also put a nut on top of the bell and another on the >bottom making the connection somewhat more solid. But it is vibrating excessively because the hitch plate is light and the added coupling of the bell is inadequate. Fixing a nut to the under side of the bolt head or on either side of the bell attachment hole will do little to prevent the vibration. It is a systemic lack of rigidity which is allowing the vibration to assume the magnitude it does. >The bell does really serve a somewhat useful function, though it's not the >one S&S has been telling us about. And, no. It has nothing to do with the >Magic Circle of Sound. The bell, along with the coupling bolt, mass-couples >the plate to the inner rim. Yes I agree, but my point is that it's not rigidly coupled enough and the plate's too light. Interestingly, the latest Hamburg S&S plates are a little heavier in section thickness (these are only ones we see in Aus'). >Taking the bolt out--you can do this even with >the piano tuned to pitch, it's not structural--allows the plate to move in >response to the energy in the string backscale and in most pianos will >result in a reduction in sustain time. Indeed it will, but a belly brace and set bolt would increase the sustain time further than that which is achieved by the bell, since it would hold the hitch plate more securely to prevent it waving about like a flag in a breeze. >How much energy loss--hence, how much >of a loss of sustain time--there is in the plate at this point depends >apparently on the precise characteristics of the individual plate--it's >thickness, and certain mechanical characteristics dependent on the pouring >and cooling rate of the casting, etc.--but there is a measurable amount of >vibration in that area that is damped by the bell and coupling bolt. Agreed, but systemic damping is what we are trying to avoid if we are looking for sustain. Holding a light plate with a coupling such as the bell with its poor mechanical advantage is hardly an effective way to build sustain into an instrument. Your mention of the casting freeze rate may also be of significance, since modern castings tend to have longer freeze times. This results in softer castings with higher hysteresis loss potential. We might therefore consider the possibility that plate vibration in more recently poured plates might result in higher energy losses. This is the very thing which got me into winging about the bell in relation to the killer zone, as a possible contributing factor to short tone. Everybody's talking about the short sustain in the killer zone. Energy losses will occur in any mechanical system where there is energy transmission with any degree of hysteresis loss. If the plate vibrates it will lose string energy. The greater the amplitude of plate vibration the greater the loss. If the sound board flops and flexes all over the place as it vibrates (as a consequence of low rib and/or bridge heights), instead of moving more or less uniformly, it will lose relatively more energy in the form of heat also. The string won't care what causes the energy losses, it will still cause the sustain to be shorter. Ron O -- Overs Pianos Sydney Australia ________________________ Web site: http://www.overspianos.com.au Email: mailto:ron@overspianos.com.au ________________________
This PTG archive page provided courtesy of Moy Piano Service, LLC