THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

18.10 Wind Instruments 549Figure 18.35. Different types of organ pipes (from Olson, 1967).The pedal organ features the largest flue and reed stops. The great organ, thechoir organ, and the swell organ feature decreasing steps of flue and reed stops.The swell organ is placed in an enclosed space equipped with shutters in the wallbetween the wall and the audience. The opening and closing of these shutters arecontrolled by the swell pedals in the organ console. The solo organ, if there is one,is also enclosed in a swell box.Just above the top manual, a row of tilting tablets serve as couplers. This couplingsystem allows the actuation of a mechanism associated with more than one keyby simply pressing one key. The different manuals can be interconnected by thiscoupling arrangement; and the pedal keyboard, for example, can be tied to anymanual by this coupling procedure.In the early organs, keys were connected directly to valves that controlled theairflow to the pipes. This required considerable strength on the organist’s part topress the keys. The action was improved later by the introduction of a pneumaticsystem, in which the key operates a small pneumatic valve requiring a lighter force,which in turn operates a valve connected with the pipes. But this resulted in slowaction owing to the slowness of the propagation rate of air impulses from the key tothe valves at the organ pipes. As a result the console had to be located very near thepipes. Except for small organs, modern organs now use either electropneumatic orelectric action to actuate the pipes. In the electropneumatic action, pressing downon the key energizes an electromagnet to move a set of valves in an air chamber,causing bellows to expand and move a linkage that opens up a larger valve whichin turn lets in the air supply into the wind chest to activate the organ pipes. Theadvantages of the electropneumatic system are: a light force is required to pressthe key, the action is swift, the keys can be interconnected by merely flipping aswitch, and the console can be located almost any distance from the organ and evenmoved about, because the wiring connecting the console to the different organscan be grouped together inside a reasonably small, flexible cable. The all-electrical