Mechanical Pipe Organ Actions and why Expression is Achieved with Rhythmic Variation Rather than Transient Control

This paper is a progress report on a project funded by the Arts and Humanities Research Council at the University of Edinburgh.

Whether mechanical organ actions allow organists to control the way in which they move the key and thus influence the transients has been discussed for many decades. This is often given as their main advantage. A number of large organs built during the 1990's in the UK had dual mechanical and electric actions resulting in compromise and expense. In every case the electric action is reported to be used almost exclusively.

The original work leading up to this project analysed the mechanics of the standard bar and slider windchest and showed that certain characteristics worked against the player controlling the airflow into the pipe. The main ones were pluck, the initial resistance felt as the pressure difference across the pallet valve is overcome, and flexibility in the action resulting in the pallet not starting to open until the key had moved a significant distance - up to half its travel. Initial measurements of organists playing confirmed that variations in key movement were not reflected in the pallet movement. There were, however, significant variations in rhythm and timing of which the player was not always aware.

The current project has taken this further with work at the Göteborg Organ Art Centre (GOArt) in Sweden and The Eastman School of Music (ESM) in Rochester NY. One technique, demonstrated at GOArt, used to ensure expressive playing throughout a performance is "Rhetorical Figurings". These were shown to introduce distinct and consistent rhythm and timing variations. Measurements of the pressure in the groove and under the pipe foot showed very strong grouping that indicated that there were just two different ways in which the pressure increased. Measurements of students at ESM also showed strong groupings of the pressure rise profile and wide variations in rhythm and timing.

These variations in pressure profile did not always result in audible differences, but this will depend on the voicing of the pipes, and highly trained organists may be more sensitive to these differences.

Organists clearly like mechanical actions, and it appears that tactile feedback is an important if not the most important characteristic, but there is a limit to their size if the key forces are to remain comfortable.