In the wind . . .

February 9, 2011

John Bishop is executive director of the Organ Clearing House.


Expressly expressive
I once heard an orchestral conductor state that the pipe organ is not an expressive instrument because the player cannot alter the volume of a single pipe. This ignorant statement was part of his argument against including an expensive new organ in an even more expensive new concert hall.
One might respond that most of the instruments of the symphony orchestra are unmusical because they can only play one note at a time. By saying “most” I’m excepting the strings of course, which can play two notes at time—maybe three under special circumstances. So an orchestra (by definition) needs many instruments to play music, expressively or not.
Aha! In order for the organ to be an expressive instrument, it comprises thousands of pipes. And big groups of those pipes are enclosed in wonderful expression machines that give the organist all sorts of control over dynamics.
The first Swell boxes were pretty simple affairs made of light wood with a few shutters in front that were operated by a lever near the floor. You could push the lever down and a little sideways with your foot to latch it open, you could let it slam closed, or you hold it halfway open, calf muscles a-trembling. Rigs like this are found on very old English organs, and there are quite a few nineteenth-century American organs that still have expression boxes like that. In 1996 I restored an organ built by E. & G.G. Hook in 1868 that had a “ratchet” Swell pedal. There was a sort of stationary wooden gear whose teeth could arrest the motion of the pedal in five or six different places. You could push the pedal a certain way to release the ratchet or you could leave the shutters partially open at any of those positions. And it was a good idea to release the ratchet as you opened the shutters—otherwise they said “click-click-click” as they opened.
The development of the mechanical balanced Swell pedal was a pretty big deal. Most American organs built between 1870 and 1900 have them. A sturdy mechanical linkage connects the pedal to the shutters. Because gravity works on horizontal shutters, balanced Swell shutters are almost always vertical. You can take your foot off the Swell pedal and the shutters stay still right where you left them. The only problem is that you have to remember to leave the shutters open when you’re finished playing to allow the temperature inside the Swell box to stay as close as possible to the ambient climate of the organ. Leaving the shutters closed typically results in a different temperature inside the Swell box so the Swell won’t be in tune with the Great. That’s not too big a deal because as soon as you open the shutters the temperature will moderate and the pitches will come back together—so if you’re halfway home and realize you’ve forgotten to leave the Swell pedal open, don’t worry about it too much!
If you get halfway home and wonder if you’ve left the blower running, then you’d better go back to the church.
And by the way, in most electro-pneumatic organs, the shutters are held open by springs, so when the organ is turned off the shutters open, no matter what position the pedal was left in.


During the Great Revival of classic styles of organbuilding in the second half of the twentieth century, many of us got used to playing organs that had no expression enclosures. Twenty years into that movement, shutters started finding their way back into organs, and today new organs are built with very sophisticated collections of expression chambers including double expressions—those fancy divisions in which an expression box that encloses ten stops might also enclose another expression box with five or six stops. It’s mighty effective when either very powerful voices (Tuba) or very soft voices (Unda Maris) are double-enclosed. The Tuba can start from nothing and Swell to a roar, and the Unda Maris can start from a whisper and vanish into thin air.
I often write about the organ as the most mechanical of instruments. (I’m glad that opinionated ignorant conductor didn’t wade into this pond!) A large organ, especially with electro-pneumatic action, can look like a mysterious mechanical monster inside. It’s no wonder that the sexton of your church mistakes it for a furnace room and piles it full of folding chairs. (You shouldn’t be storing chairs in the furnace room either.)
The organbuilder is forever challenged by the conflict between the organ’s mechanical identity and its artistic purpose. If the music is interrupted by too much mechanical noise, the effect is diminished.
The expression shutters can be the biggest culprit. Who among us has not sat through a recital or a worship service marred by a squeaking Swell shutter? I once attended a choral concert in a conservatory concert hall in which several pieces were accompanied on the organ. The Swell shutters were exposed as part of the façade, they squeaked, and the organist had an annoying habit of beating time with the Swell pedal. Flap-flap-flap, squeak-squeak-squeak was all we could hear.
I’ve made lots of service calls to correct squeaking shutters. Often enough a little squirt of oil or silicone is all that’s needed—that’ll be $200 for the travel and time and four cents for the squirt.


For the organist, the ideal expression shutters can silence the division when closed and allow it to roar when open. They can open or close in a nano-second, and if you operate the pedal slowly they provide infinite gradation of volume —no jerking from one stage to the next. OK, we’ll see what we can do.
In order to achieve really effective expression, the box and its shutters must be massive. If you build a Swell box and shutters out of three-quarter-inch-thick wood, you’re building more of a soundboard than an enclosure.
Let’s start with the fabric of the box. The walls and ceiling of the box should both deaden and reflect the sound of the organ. Deaden—so when the shutters are closed there’s no resonance going on. Reflect— so no sound is lost or absorbed by the interior surfaces. In other words, the sound should be effectively contained when the shutters are closed and when the shutters are open the sound should be propelled out through them.
Organbuilders have experimented with all sorts of construction styles. The simplest is heavy soft wood. Use two-inch-thick pine for the walls and you’re doing pretty well. Try two one-inch-panels with an airspace between. Just as massive, but the airspace cuts down the transmission of vibration. How about fill the airspace with sawdust? That works great—the sawdust really absorbs sound so the box is most effective when closed. But it’s a real drag when you’re surprised by fifteen cubic feet of sawdust pouring out by accident when you’re dismantling an organ.
There’s a material called MDF (maximum density fiberboard). It is manufactured in 4′ x 8′ sheets like plywood. It’s made from a sophisticated recipe, but it can be described simply as sawdust and glue cast into sheets. A sheet of three-quarter-inch plywood weighs about 65 pounds, heavy enough. But the same size sheet of MDF weighs 96 pounds. We have built a number of expression boxes using double-thicknesses of MDF. It’s hard work because the stuff is so heavy, and because it’s so dense it’s hard to cut—it burns up saw blades like kindling wood. But it sure makes an effective tonal enclosure.
My first work in organbuilding shops focused mostly on classic-style mechanical-action organs. It was from that bias I heard or read that E. M. Skinner had built cement swell boxes. Cement swell boxes? How decadent. What I pictured was the newly poured foundation of a house with rebar (steel reinforcement bars) sticking up out of it. How could that be musical? But when I finally worked on an organ that had such a thing I realized that my youthful and ignorant bias was exactly that—a youthful and ignorant bias. In fact, the “cement” swell box has a structure of studs and joists something like normal wood-frame construction with heavy plaster surfaces, and a finish coat of Keene’s Cement, which is an anhydrous calcined gypsum mixed with an accelerator used as a hard finish, or more to the point, hard plaster. The heavy structure of the walls and ceiling deaden the sound and the Keene’s Cement surface reflects it—the best of both worlds. The expression chambers of the mighty Skinner/Aeolian-Skinner organ at the Cathedral of St. John the Divine in New York are built as free-standing rooms in the huge spaces some 90 feet up above both sides of the chancel. The walls are thick and heavy, and the surfaces are finished with Keene’s Cement, and those powerful reeds sure go quiet when the shutters are closed.

I shudder to think
What about the shutters? Just like the boxes, there are lots of ways to build expression shutters. They are usually made of wood, ideally an inch-and-a-half thick or more. The edges are usually beveled so they effectively overlap when closed. The edges of the shutters where they come in contact with one another usually have heavy felt or some other soft material glued to them so they close quietly and tightly. Some builders make shutters out of metal and we’ve even seen them made of glass and Plexiglas. Just like the walls of the expression chamber, the best shutters are massive and shaped and fit so they close really tight. The more massive, the more they contain the sound of the organ.
The shutters are mounted in frames—we call them expression frames. Sometimes the shutters are vertical, sometimes horizontal. As I said earlier, it’s easiest to build a balanced mechanical expression action if the shutters are vertical—that way there’s no effect of gravity on the weight of the shutters. All you have to balance is the action itself.
Shutters are mounted in the expression frames with some kind of rotary bearing to allow the shutters to pivot. Most often you find a strong steel pin (axle) that pivots in a hole drilled in hard wood. The holes and pins are greased, and if the shutters are vertical, the bottom bearing is figured out so as to keep the shutter high enough that it doesn’t rub against the wooden frame. In fact, those bottom bearings are often adjustable—if the shutter settles and starts squeaking against the frame, you can raise it with a turn of a screw.
Some organbuilders go the extra mile and use commercial ball bearings for mounting expression shutters.
It’s also ideal for the shutters to be easily removable. In many organs it’s necessary to remove shutters in order to tune, but you also want to be able to remove a shutter that has warped and needs to be planed straight.

And something to drive it
Some pneumatic expression systems feature an individual pneumatic to operate each shutter. Each contact on the expression pedal opens one shutter. (Most Möller organs work that way.) But it’s more common for the shutters to be linked together by an action that is in turn operated by a single machine. The machines can be electro-pneumatic or all-electric. But what you’re looking for is a combination of expression machine, linkage, and shutters that have a large enough travel so the shutters can close tight and open really wide, move silently when operated either fast or slow, and that have plenty of gradation between stages so that the range of expression seems infinite.
Most electro-pneumatic or electric expression machines have eight stages. It’s generally agreed that for most organs eight-stage expression are sufficient. I think it was Ernest Skinner who built the first sixteen-stage machines. (Dear reader, if you know otherwise please share it.) Those machines are elegant, fast, and powerful. Dividing the travel of the console expression machine into sixteen stages really gives a smooth operation.
Mr. Skinner called his expression motors Whiffle-trees. The term Whiffle-tree was originally used to describe the system of harnesses and reins that tied a team of horses together, allowing the weight of the load to be distributed between the horses according to their individual strength. Mr. Skinner used that principal to harness a row of pneumatic motors together so that each motor (or stage of the machine) contributes to the motion of the shutters and collectively they equal the total motion of the machine. Skinner’s Whiffle-tree expression motors were installed in thousands of Skinner and Aeolian-Skinner organs and in my opinion set the standard for electro-pneumatic pipe organ expression.
There are several suppliers to the pipe organ industry that have developed and market all-electric expression motors. The best of these use the powerful, compact, and quiet electric motors developed for wheelchairs. They are equipped with solid-state controls that translate the contacts on the console expression pedal into stages of expression. The organbuilder can adjust them for different distances of travel and adjust the amount of travel and the speed of each stage separately. So, for example, you can make the first step from fully closed be fast on opening (so it responds instantly) and slow on closing (so it doesn’t slam shut). Mr. Skinner handled this by using a small exhaust valve for the first stage, which choked its speed, keeping the shutters from slamming.

A rose by any other name
You’ll notice that I’m saying expression box, pedal, or shutter rather than Swell box. It’s true that most organs with expression are two-manual organs, and on a two-manual organ the expressive division is usually a Swell. But keeping the language clean, I’d rather not put a Choir division in a Swell box—so expression is the word.


In a large organ, the shutters of one division might collectively weigh close to a ton. It takes a lot of thought and skilled engineering to get that amount of stuff to move quickly and silently in response to the artistic twitch of an organist’s ankle. But when an expression chamber is working well, it can produce breathtaking effects. As familiar as I am with all that gear, I love to think of that big mass of stuff on the move when I’m sitting in the pews listening to an organ. It’s difficult to express.

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