Re: Force Versus Distance of a Pneumatic
By John Grant
On Fri, 22 Mar 96 16:24:12 PST Robbie Rhodes wrote:
[Snip]
> Now consider how lifting the hammer rest rail alters things. The
> Soft Pedal moves the rest rail so that the hammer travel distance is
> reduced to one-half. We assume that the travel of the key and action
> pneumatic is also halved, and therefore the pneumatic is working
> between 50% open (note at rest) and 20%. The integrated force in this
> operating range is considerably less than half of normal, and the
> result is that the hammer velocity is much less than it would be if
> the force had remained constant.
>
> You questioned, John, if halving the hammer blow distance predicts a
> 3-decibel reduction in sound intensity. You bet it does, and the
> equations are simple, so let's look at it.
> [Physics discussion snipped.]
> But the above assumes that the pneumatic force is constant, yielding
> constant acceleration. In reality, because the pneumatic is operating
> in the "mostly closed" region, the acceleration term "a" is also
> reduced, and the result is that the Soft Pedal produces much more than
> 3 db intensity reduction.
>
> I'd like very much to read the articles in AMICA Bulletin that you
> mention. Can someone help me obtain them?
>
> -- Robbie Rhodes
>
OK, I think I see part of my problem here: My confusion arose from your first mentioning of the hammer rail pneumatic in your original post, but then the rest of your discussion shifted to note striker pneumatics, a distinction I failed to make. I think you will agree that the speed/force of the hammer rail pneumatic, per se, has no effect on note intensity, other than from the changed blow distance it manifests, and I readily agree that a striker pneumatic, starting from a partially closed position will impart a smaller force/acceleration to the hammer, resulting in a softer note. Beyond that, generalities can be a little dangerous as we need to discuss the design differences among the various manufacturers. (Grand rather than upright actions are assumed unless noted otherwise, although much of this analysis applies to both.)
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.) On the one hand there is the Duo-Art mechanism, none of which, in my experience, have any means to DIRECTLY compensate for the "lost motion" resulting from the lifting of the hammer rail. 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 than pushing 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. Also, in those pianos with side-shift pneumatics (some Steinways and larger Webers), IF tubed according to the official charts, the softening effect is achieved by striking only two (of the tri-chord) strings with a normally uncompacted portion of the hammer felt. 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.)
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.)
So, while in the three mechanisms, properly adjusted hammer rails will all raise the hammers halfway to the strings in response to a soft pedal perforation, the response of each mechanism is less consistent and predictable. Happily, the roll editors seemed to be aware of these considerations and developed coding changes which produce pleasing and believable, if not 100% faithful, playback. As Robbie originally noted, some empirical/experimental data would be useful here. I really don't have time right now to research the tech articles for what I seem to remember reading. Terry Smythe, can you help out here? And Robbie: on what mechanism(s) are you making your observations?
-John Grant
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(Message sent Sat 23 Mar 1996, 09:10:39 GMT, from time zone GMT-0800.) |
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