Re: Force Versus Distance of a Pneumatic
By Craig Brougher
|Great discussion! John Grant is answering Robbie:
> I accept for the moment that a 50% blow distance DOES predict a
> nominal 3 dB reduction in intensity (I see no reason to quarrel with your
> analysis, PROVIDED ALL other factors are constant.)
This can be shown to be so very easily, by "moment of inertia, ie ~momentum. A moment is proportional to a mass X distance. Forget the vectors. Given the same "force" component, this cancels out, being the same on both sides of the equation. We are left with mv or md-- whichever you would rather measure. I prefer mxd. Halve the d, you halve the kinetic energy as long as acceleration of gravity remains the same. On the db scale, that directly relates to 3. No question, and no argument. It's just a fact. It is also a fact that the same equation does not at all hold true for an upright, whose hammer operates "overcenter." Continuing the quotes:
> When the hammer rail lifts the hammer heads,
> thereby removing their weight from the rest of the action chain, the key
> fronts, by gravity, fall a short distance, having no longer to support
> the hammer weight. This of course raises the back end of the key,
> resulting in a gap ("lost motion") between the underside of the key and
> the top of the striker poppet. BUT, the open span of the striker
> pneumatics does not change.
> While it might be argued that it they will
> continue to develop the same force(s) as when the hammer rail is down,
> there are some subtle considerations. First, the initial movement of the
> pneumatic is against no resistance so that by the time the poppet
> reaches the underside of the key, it is traveling faster than otherwise.
> This and the complex dynamic of having the poppet "impact" the key
> (rather thankpushing it smoothly) may affect final hammer velocity in
> non-intuitive ways. Complicating this (as if we needed more!) are the
> purely pneumatic changes which also typically take place when the
> hammer rail rises, i.e., opening the soft pedal port also cues accompaniment
> level #2. This would seem to be working at cross purposes (raising
> accompaniment vacuum while engaging the soft pedal) but it simply
> re-emphasizes the complex factors at work here.
The #2 level only comes on with the soft pedal when the Duo-Art is placed in "Soft" mode. The reason is different. When in "Soft," a Duo-Art's vacuum is clamped to 20", or whatever you want to set it to, at the modulator. Since the lost motion will cause drop out of the light staccatto notes, the soft pedal on "Soft" play is actually not in the expression circuit any longer. Instead, it is used to widen the lattitude of tolerance to weak, quickly played notes, yet being counteracted by the #2 int. which is exactly the same amount of loudening that the soft pedal is in softening. Forget the hammer rail expression in "Soft." It doesn't express in that mode.
> The hammer lift rail only comes into play
> when the modify switch is placed in the "Soft" position. (In a later
> discussion I will express my reasoning why I believe this is backwards.)
Aeolian, in my opinion, is correct, John. I believe you are confusing the action shift soft pedal with the hammer rail. Very understandable, and I do this same kind of thing myself, and will continue to do so. Doesn't mean you don't know better, so have patience with me too when I toss off a wrong idea now and then--and I promise you, I do it all the time! But All Duo-Arts, regardless of the shift feature of a few Webers & Steinways, exclusively used the hammer rail lift for Duo-Art soft pedal. If the shifter were being used for the soft pedal normally, you would be bending pitman arm strikers and actually trapping a few of them with the side motion during the playing of a piece.
> On the other hand is the Ampico B mechanism (and most late A's
> as well.) Here, the keys will still visually "dip" when the hammer rail
> rises, BUT the span opening also is reduced, violating the "all other
> factors remaining constant" premise. Here the effect will likely be MORE
> than a 3 dB reduction for the reasons Robbie previously stated. (In the
> B, an extremely ingenious mechanism facilitates the span adjustment
> procedure using the "Note Compensation Roll" which allows the
> extremely quiet sub-intensity capability of the "B", below levels that
> would likely drop notes in other mechanisms.)
John, the basic premise of impulse and momentum is, "Any action creates an equal and opposite reaction (conservation of momentum). It doesn't matter if the pneumatic starts up ON the key, or rises first before it contacts the key. The total energy is either going to be the same, or my old physics teacher gets a "Dear John" letter. The difference may be in timing. You may notice that quick, light notes don't strike when the pneumatic has to travel very far up to reach the key, but that is another matter. BUT... If you don't believe me, build the impulse gauge I mentioned to Robbie and prove me wrong. (no impulse--force-- can be developed until a resistance is encountered.)
> As Robbie originally noted, some empirical/experimental data would be
> useful here.
Oh yeah! I can just see it all now. Somebody is going to chalk up a pitman striker and start shooting pool with it.
(Message sent Sun 24 Mar 1996, 00:58:33 GMT, from time zone GMT.)