QUOTE (Stephen Barber @ Oct 15 2009, 12:45 PM)

Since I'm at home with a cold:
I've wanted to ask this for years, but was embarrassed to show my ignorance. Getting too old now to worry!
There are often references to the chorus on a manual being "based on" a certain stop. Does this involve a subjective judgement or is there a technical definition.
Sorry about spelling of topic title. I don't know how to edit this.
Sorry to hear about the cold - there's a lot about (and I've succumbed too)
In a nutshell, the ranks of pipes in a diapason or principal chorus (e.g. 8, 4, 2 2/3, 2, etc) should relate to each other in pipe construction, design and scaling (the relationship of the circumference of the pipe to it's speaking length). The reason for this is so the sound from all the pipes blends into a single, unified sound and makes the chorus sounds homoegous and pleasing to the ear.
The most simple type of chorus is the "straight" chorus. In a straight chorus, a pipe of a given length (e.g. 2' C) will be the same, whether it sounds middle C in the 8' stop, tenor C in the 4' stop, bottom F in the 2 2/3 stop or bottom C in the 2' stop. In a perfectly straight chorus, they will all produce the same power and sound. The effect of this chorus is very strong, with a lot of harmonic interest. It can make a bold, ringing sound which is very effective in a large space, where the air will attenuate the higher frequencies.
In a smaller, intimate space, the effect of a straight chorus can be rather wearing on the ears. So many builders would make the higher pitched pipes in the chorus quieter. The easiest way to do this is in the voicing: an easy way is to make the hole at the bottom tip of the pipe smaller , while means the pipe gets less wind and so sounds quieter. This type of chorus can sound very appealing, with a lot of homegenity and excellent blend. But if the pipe is made to speak too quietly, the sound of the higher pitched pipes can become bland and unappealing, with problems starting the pipe speech.
Another way to make the pipes to sound quieter is to make them smaller scale - so they're narrower. This allows the higher pitched pipes to keep a harmonically interesting sound while at a lower level of output and gets around the problems listed above.
Builders have experiemented with many different relationships between the scalings of the pipes in the choruses over the years. One of the simplest methods is to rescale the pipes one or 2 notes small each time you go up. So the 4' stop at bottom C will have the same diameter as the pipe in the 8' rank at tenor D (i.e. not tenor C), bottom C of the 2' stop will be the same scale as middle e of the 8' stop (i.e. not middle C) and so on and so forth.
One further factor to throw into this is halvings. You would imagine that the diameter/length relationship of the pipes remains constant throughout their lengths so a 4' pipe will have a diameter exactly half that of an 8' pipe because it is half the length. This doesn't happen. Lower pitched pipes will need more hamonics in their sound so the human ear can detect their pitch more quickly but the higher pitched pipes would sound too shrill and thin if they were the same scale. There's also some technical details of the pipes to take into account as well as they get smaller. But basically, most ranks of organ pipes halve their diameter somewhere around every 15th note (i.e. so tenor D# will be 1/2 the diameter of bottom C).
Now, if you apply that to re-scaling of the higer pitches above you'll find that bottom C of the 4' rank is the same diameter as tenor or F in the 8' rank and the gap widens as you go to the 2' stops - Bottom C of the 2' rank will be around the same diameter as middle G or A of the 8' rank.
Different builders had slightly different flavours to this. In the earliest organs, the relationship between the pipe circumference and length was geometric. If you plotted a graph of pipe length against pipe circumference for a single rank of pipes on a graph you'd get a straight line, with the length halving every octave (every 12th pipe), the circumference halving whenever the organbuilder had chosen (typcially somthing like every 15th pipe). Dom Bedos describes a method where the builder would use a dulcimer to determine the scale of the pipes. Basically, you had a big board with a diagonal line on it. The builder would put a piece of pipe metal against it at the length the pipe was to be and measure off from the side of the board to the diagonal line the width of the pipe (this would determine the pipe's circumference). Hey presto, the organ builder has his pipe metal of the correct dimensions and he doesn't even need to be able to read.
It is highly likely that this method was used until surprisingly recently. The evidence from the choruses of the Victorian organs of Hill, Willis, Gray and Davison and J.W.Walker seem to conform to this geometric progression (or variants thereupon) until the years between WW1 and WW2, where the demand for new organs dried up and the majority of the work turn to rebuilding. It's a bit difficult to be sure of this - organ builders in this period jealously guarded their scaling plans in a way we find difficult to understand today.
In the 1830s, theorists like Toepfer proposed a new method of scaling pipes, using logarithmic scales. The standard scale was born but the majority of well-established builders (listed above), brought up in the noble tradition of organ builders passing down their skills and knowledge through the generations (and probably without the knowledge of how to wield a slide rule), did not adopt these new principals as quickly as has been suggested. Schultz and Lewis were early adopters and it tended to be the provincial builders and supply houses that seem to have been the early adopters of Toepfer's ideas, using the (slightly spurious) argument that the (supposedly) scientific principals of this scaling method were superior to the older, geometic-based, methods. Today, organs are predominately scaled to a variation of Toepfer's ideas, with the ideas of scaling to alternative methods confined to a very few small pockets.
If we were to compare the difference in the scalings between these 2 methods we would see something quite interesting. If we were to plot the pipes in the Toepfer standard scale as a straight line, we would see our geometrically-scale pipes whizzing off to a hugely wide scale in the extreme bass, being proportionately narrowerer in the mid-range of the keyboard (between somewhere in the tenor octave in the 8' rank and about an octave above middle c) before broadening out to be wider scale in the extreme treble.
As you can imagine, the problem comes if we try to combine pipes scaled from different builders with different schools of thought. Ralph Downes, with a little knowledge of Toepfer standard scales, really couldn't make head of tail of the scales employed in old contiental organs which were never designed using these principals - hence free variable scaling was invented.
However, consider adding a 2' rank scaled to toepfer-based logarithmic scales to a geometrically-based chorus. This is fraught with risk and danger. Even if the new rank is balenced to give the right amount of power to fit in with the pipes beneath, in some places of the keyboard the pipes will have to forced to speak very loudly compared to the other pipes on the same note and so they won't blend together comfortably - the new material will stick out. In other parts of the keyboard, the new pipes would have to quietened down and so the nature of the sound would be different again - the new material will sound muddy and indistinct. Now try playing a scale across the entire compass and see how the nature of the sound changes! Of course, at a basic level, it'll work if the regulation is OK but to someone who can objectively appreciate the difference of the sound of the choruses of different builders, the results will be horrible. This is why sticking a modern mixture made to standard modern scales from a trade supplier on top of your 1860s Hill or Walker Great principal chorus is to be discouraged.
So if a chorus says "it is based on a certain 8' stop" it means that the higher pitched pipes of the same family of stops are designed relate to that stop to form a homogenous chorus, as the organ builder envisaged and designed it.