Hi Sarah I of course have to bow to your grasp of physics, its not always easy to use correct terms and descriptions when relating to so many different folks... especially when I havent gone through that mill myself to begin with. That said I think the basic idea is more or less correct. I stated that the magnets would impart more stiffness to the assembly. I mean that the combined spring rate of the magnet componet and whatever is in the soundboard will be greater then by the soundboard alone. Secondly, the strings downward pressure potential is not changed, so if the soundboard suddenly has the ability to hold an extra 2 mm of crown against whatever downbearing is applied, then it has in the sense you seem to be describing below with your leaf spring expample an increase in stiffness. Not the panel in itself mind you.. but the whole (now including the magnets) assembly. I havent gotten so far as to finding out about the specifics of the spring constant curve for these particular magnets. I do know that there are several classes of so called super magnets, and I imagine each have their own characteristics thus. My experiements have gone more along the lines of straight forward empiri... ie... place the magnets such and such, observe the results. Mostly to demonstrate the basic principle, and to fashion a basic working example. I could measure a clear increase in sustain on this beater, and a raise in pitch. That indicates the soundboard increases its deflection of the string plane, ie it essentially takes more N's downward to maintain the start point crown. On the side.... you can get the magnets very close indeed to the soundboard... it has quite a bit less of an amplitude then I had imagined. One other concern would be perhaps the footprint of the magnets on the underside of the panel... need to disperse that a bit I think. Someone asked about the magnets mass. They are very light weight and these I am useing measure 32 mm Ø and 5.5 mm thick with a 4 mm Ø hole in the middle for attachment with screws. You are sure right about the pros / cons of adding stiffness bit. Too much is no better then not enough. But that seems to be one of the neat points with this. Its quite adjustable to begin with. And one probably would not see a need to install such a system unless one needed a bit more stiffness anyways. Thanks as always for your informative post ! Cheers RicB Hi Ric, Sixty pounds?! Whoa! ;-) You suggest that upwards force on the SB increases stiffness, but that's not necessarily so. Stiffness would be the same thing as spring constant, which would be defined as the amount of force needed to deflect the SB by a given amount. So if it takes 100 N of pressure to deflect the SB by 1 mm (just pulling numbers out of the air, without any clue as to whether they would be close to anything "real"), we could say the SB has a stiffness of 100 N/mm. If we make the SB twice as stiff, it would take 200N to deflect the SB by 1 mm. Now set a 100 N weight atop the SB, and the SB will sink by 1 mm. Apply 100 N of force from the bottom, and the original position will be restored. Apply 100 N of force instead to the top of the SB, and the SB will sink another mm (all assuming ideal spring properties). Either way, the relative movement from 100 N of force will be the same. Thus, the addition of the 100 N of weight in no way affects the stiffness of the assembly. (Mass loading is another issue, but please ignore it for now. I'm just talking about stiffness.) Now let's apply 100 N of pressure to the underside of the SB with an enormous leaf spring with a stiffness of 100 N/mm (same stiffness as the SB). Now, when we apply a 100 N force the SB, the deflection of the board will be only half as much (0.5mm). To deflect the SB 1mm, we now have to apply 200N of force, and thus the combined stiffness is 200 N/mm. That's because we're deflecting both the SB and the spring, both of which require force for deflection. If we repeat the above experiment by simply attaching the spring to the SB but not applying any force against the SB from the spring, we'll get the same results -- a combined stiffness of 200 N/mm. In other words, force doesn't matter. What *does* matter? What matters is the stiffness of the "spring" that is attached to the SB. In other words, what matters is the amount of force needed to change the position of the "spring" by a given amount. In the case of gravity, that force is zero. (Zero?! OK, consider two equal weights attached to a rope, hanging over a frictionless pulley. Now move one of the weights up or down. Yup, zero.) In the case of a leaf spring, that force is greater than zero and is equal to the spring constant. If you want to do the equivalent of the pulley/weight demonstration, tie two springs to a rope, and fasten the other ends of the springs to two walls, such that the rope is under tension. Now move the rope back and forth (longitudinally). It takes force, right? This leads to the final and most important point. If you want to add pressure to the underside of a SB to support downbearing, but you DON'T want to increase stiffness very much, then you need to use a support "spring" that has a relatively low spring constant (i.e. a spring that's not too stiff). In other words, the force needed to compress the spring by a given increment must be small. You can still apply the same force. It would simply require greater compression of the spring to achieve that force. But if the force is more uniform with a given increment of deflection, then the spring constant is lower, and the stiffness added to the SB is less. But what about magnets? Well, they're a bit like springs too, albeit more nonlinear ones. It sounds like the ones you're using are potentially adding LOTS of stiffness to the SB assembly. That's not necessarily bad, if part of the objective is to add stiffness. My only point is that your choice of device to apply force to the bottom of the SB will determine how much stiffness you add to the entire assembly. You can use a weaker spring to achieve the same force with less added stiffness, or you can use your magnets to apply the same force with much more added stiffness. Sometimes added stiffness is needed. Sometimes it is not. If stiffness were universally good, we'd be casting SBs out of reinforced concrete! Having said all this, I like your magnets idea. It achieves the application of force while at the same time eliminating the potential for resonances. For instance, if coil springs were used, they might "ring," unless they were somehow damped. Of course damping the springs would be a bit like damping the entire soundboard. The best application of your magnets idea may well be with a larger number of magnets spaced at larger distances from the SB. It is the larger distance that would yield an overall lower added stiffness, while still supporting the downbearing with the same force. Peace, Sarah
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