message dated 1/24/03 4:26:28 PM Pacific Standard Time, email@example.com
Ok, we now have an idea of how big the line needs to be. And maybe defined the number of elbows to put in the line. Now I'd like to ask the question, does anyone have a good rule of thumb or reliable test for how much spring tension vs. weight should be used on a "manual" chest regulator. Remember, we are trying to make this thing tremulate.
Piedmont Theatre Organ Soc.
The amount of weight depends on the regulator size and the rank to be tremulated. A Vox regulator will not likely need any weights at all. The most weight addition will be on a Tibia regulator; but, unlike idiots who like to "fool the regulator" with a ton of pig iron, the weight of the top of the regulator is fairly substantial to begin with and *not much* weight is needed to tune it to the correct rate of tremulation. All that the weight is good for is inertia and on a theatre organ like you are constructing, weights should not provide but a small fraction of the downward force needed to balance the regulator top against the wind pressure. In order to compute the weight you first have to state the size of the regulator, the number of springs, and the estimated volume of the wind system around the regulator. Only on classical non-tremulated instruments would you consider weights for force on reservoirs or regulators (they are different devices).
Use the formula that I posted to compute the spring tensions in a previous post then when all the winding is done you can add weights incrementally until the regulator reacts to the correct trem rate. I use my hand to move the regulator top to get a feel for the correct resonance and you can develop the same feel with time. However, if you get to weights that are more than a small percentage of the downward spring force then something is wrong. I rarely have seen a regulator top that needed more than 20-30 pounds, 40 is not unheard of, and anything greater means something somewhere is not right. I personally prefer less than 20 pounds.
The balance you are trying to achieve is that the pipes play musically with the trem on *and* off. You want to avoid "pitch sag" at all costs. This is when you have a large chord suddenly drawn that the pitch detectably drops, comes back up, rises too far, and creates a bounce effect. What that means is that the tremulated sound will be whippy with the pitch peaking quickly, overshooting the ideal trem pitch curve, and generally giving a less than pleasing trem effect. Properly done a trem should be tight and near a sine wave for a pleasing effect. Normally this means 25 cents sharp maximum and no more than 50 cents flat on the deepest Tibia tremulation.
So, push down on the regulator top and hand trem it. If it wants to be slower than your hand rate, you have too much weight; and, if it wants to be faster then you need to add weights. Remember to add weights equally from side to side of the regulator top center of mass or the top will wobble. All of this process is shortened from about the three chapters of a book that would give an explanation but it should start you out on the journey toward a pleasing tremulation.
Having hit an age where his whole body tremulates...
So he's been eating chocolate to add weight to the midsection to tune it...
©reserved - 2003 - to keep little kids from getting their hands on subversive ideas –
Good to hear from a man who knows what Starret tools are! If you really want formulas you cannot beat Dennis Hedberg's, *The Physics of Tremolo," in the ATOS Shop Notes.
All that my formulas do is put me into the ball park. They give me an idea of how to select springs and what weights to try. Then you go with experience and a step by step approach until you are happy with the results. Since each "system" is so different you cannot put it all down to one set of pat formulas. Everything from the blower static to the leaks in the system must be included to arrive at a happy outcome. I don't think a single mathematical expression would ever cover the gamut of different builders, different installations, different leakage coefficients, different chests, etc., and the best one can do is to put things in then do what organbuilders have done for the last 10 centuries, experiment. All that I do is get into the ballpark with a little math.
One of my ballpark formulas is simply for spring tension. I may have posted this on another list but I was fairly sure that way back I did the formula for doing spring tensions. Spring tension = (regulator top Width x Length inches) x (desired wind pressure/27.68) The reason you divide the desired wind pressure by the number 27.68 is that number is 1 pound per square inch. So, if you have a 20 x 30 regulator which is 600 square inches then you multiply that by say 10"WC divided by 27.68 and you get a total downward force of about 216 pounds. You could subtract the weight of the regulator top and any iron placed on it for the at rest final figure. If you are using four springs then each spring must be one forth of that final figure. Let us say the regulator top weighs 20 pounds and you have 16 pounds of iron on it then the springs will have to pull 180 pounds between them OR roughly 45 pounds per spring. There are a number of reasons this would not be good and more springs, like say 6, would be better. It has to do with Hooke's Law and Young's modulus. One must remember that a spring proportionally varies its tension when pulled. A weight will not. A 20 pound weight with a force of gravity, an acceleration force, will not give a steady 20 pounds while traveling downward or upward, it is only 20 pounds at rest. Now a spring may require 30 pounds to start expanding at 4 inches and be at 45 pounds when stretched to 6 inches then to 60 pounds when further stretched to 8 inches. Turn buckles or links are placed in series with the springs on most regulators so that the stretch can be adjusted to the sum of 216 pounds downward force that will just have all the regulator valves close and give you the desired wind pressure. All springs must be equally spaced from the center of mass and equally stretched.
Going back to the Physics there is Hooke's Law and Young's modulus which is the numerical value derived from Hooke's Law computations. Essentially Young's modulus is the stress divided by the strain. Stress is defined as force divided by the amount of change in shape such as the lengthening of a spring. Strain is defined as the amount of deformation in the object under stress. All of this verbiage simple means that special springs must be used to attain the values of stretch so that you have a correct range of spring expansion. When buying from Organ Supply or any other organ source you get industry standard springs designed to have a value of tension that is midway between closed and when the spring reaches its elastic limit (when it is stretched so much that it does not return perfectly to the original closed length). We've all seen springs with open loops caused by idiots who have pulled them out so much that a weakened, permanently deformed, series of coils exists. These are the *dangerous* springs that have a tendency to break; but, since springs are cheap, just replace them. They are also nonlinear and introduce distortion in the tremulation wave form.
One thing to do is to try the regulator first with no weight except for the intrinsic weight of the regulator top itself. Too fast and harsh? Add 5 pounds sitting on felt with a plumbers steel tape to temporarily hold it down. Try the trem and see if the system sounds better. No? Then use a temporary weight of 10 pounds, 15, 20, etc., until you get the thing tuned to where you want it and it sounds good to your ear. Readjust the windpressure each time using a calibrated gauge or "U" tube manometer. Do this with the spring turn buckles. To restate what I said before, the weight provides inertia that makes the trem waveform more of a sine wave. To little weight or too much weight and the trem wave can be what I call whippy (not a standard term).
I hope some of this makes sense since I was not happy with my previous post upon rereading it. I guess the anesthesia from my stay in the hospital this week has not yet worn off. As I write this I think I have had a triple Martini...
Best wishes to all,
Attempting to write down what is on his alleged mind...