H. W. Stephenson's Monograph
By Dave Saul
The monograph by H. W. Stephenson appears to be something that deserves wider distribution. I believe he is discussing a single valve system if his main concern has been the Duo-Art. (Was he looking at cross valves, later types, or both?) To offer any final and objective comments on any of Mr. Stephenson's conclusions, we really need to examine his data in its proper context. In formulating his conclusions about valve travel, for example, did his tests consider recovery time -- the time required for the hammer to come back to a state of readiness for the next note to play? This is important for rapidly repeating notes.
Repetition can and does vary according to the placement of a note in the piano scale; the moment of inertia of a shank with hammer attached, rotating about its pivot point, is greatest at the bass extreme of the scale. The effective rotational mass decreases markedly toward the treble end of the piano. That's why faster repetition is generally achievable in the higher regions of the scale.
Consider also that when a note is struck lightly, it is not caught by the back check as it would be with a more intense strike. Aside from mass considerations, the checked/not checked possibility requires that two distinct conditions be dealt with during the release. This is, of course, influenced by stack pressure. These and other factors (e.g., energy stored and later released by a grand's repetition lever spring) could have a profound effect on ability of a pneumatically activated note to achieve good repetition.
A valve gap on the small side not only constricts air being exhausted from a pneumatic during activation, but also flowing INTO it after the note is released. The latter, of course, is not influenced at all by stack pressure (the pneumatic is open to atmosphere over the top of the valve), but rather by the dynamics of the action parts trying to achieve a rest state after a note has been played. My gut feeling is that a slightly larger valve gap may be needed for release than for the strike that precedes it. I would give it at least .030 in.
Excessively large valve gaps need to be avoided because of noise, if for no other reason! Valves with large gaps tend to add annoying clatter to any music produced. Excessively large gaps may also tend to cause harmful pressure transients in the stack as a result of air leakage while the valve element is in motion, traveling from one seat to the other. While a single playing note might work OK, large numbers of notes being played simultaneously or nearly so might suffer from the regulator's inability to overcome the effect of the transients.
To keep on the safe side, I'll keep on setting Duo-Art valves in the .032 to .035 range.
Dave Saul
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(Message sent Mon 24 Feb 1997, 17:08:39 GMT, from time zone GMT-0600.) |
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