An Engineering Approach to the Historically Underdamped Nature of Flexing Wind

This is the seventeenth post in a blog where I conduct a design study focused on building a conceptual yet physically credible organ for a Minneapolis church. The study uses pipes from their existing mid-industrial era gallery organ to explore possibilities for a 43-note continuo suited to this space. I creatively draw on my familiarity with both the materials and the space, having served as an organist there for eight years.

If you are new to my project, start here and work your way up.

I introduced the concept of flexible winding in my last post. The image above is taken from the treatise "L'art du facteur d'orgues," written in the eighteenth century by the French organ builder Dom Bédos de Celles. It is still widely considered an authoritative work today. The winding system shown contains several wedge-shaped bellows, which are raised by hand and allowed to fall under gravity. Weights placed on each determine the wind pressure.

The bellows should fall at a constant velocity depending on the volume of air released from each pipe speaking. But the abrupt opening of the pipe valves as one plays the organ often induces a short pulsation in the bellows if not somehow damped. This vibration dissipates to a steady state rather quickly, but it is noticeable to the ear. In other words, an early wind system is an example of an underdamped dynamic system, one that exhibits a steady-state response along with a short initial transient response.

Stay with me here. I promise to make it easy.

A course in dynamic systems was part of my mechanical engineering curriculum while I was an undergraduate. I won't get into the math here. I won't develop equations of motion or make you solve them using differential equations that require Laplace transforms. But I want to include the graph here, attributed, showing a transient response decaying to a steady state over time. In our case, this time interval is relatively short but importantly apparent to the sound of the organ.

In addition to the graph, you can hear the transient responses generated in this audio recording I made on the first organ I built as an independent builder. The recording is one verse of the Advent hymn, "O Come, O Come, Emmanuel", ideal because the season of Advent is upon us.

The winding systems for the organs I built for two Lutheran churches that have since closed were not a series of wedge-shaped bellows. I modeled their behavior using a dynamic systems approach, creating an underdamped wind supply with a transient response that quickly decayed to a steady state response, just as some of the old organs still do.

An organ intended to play Romantic or Orchestral-Symphonic music needs steady wind, free of the transients often typical of authentic Baroque instruments, and requires an overdamped wind supply rather than an underdamped one. Steady wind is currently favored among the organ community, and I was ready to build my 43-note continuo with a steady wind supply. But lately, I see no reason to use a go-along-to-get-along approach to something that adds a beautiful dimension to the instrument's sound and the music it plays.

My innovative wind reservoir design, supplied by an electric blower, imitates the wedge bellows. It's compact and easy enough to build. I don't have to think of it as historically aligned either. It's another design element that defines my 43-note continuo.

One more thought. If you're still lost after seeing the graph and listening to the audio recording, you're not going to like the problem set I'm about to assign. Did I say, no worries? Good afternoon, class. It's due next Tuesday.

An update: Flexing wind is apparent in this recording, also made on the first organ I built, of the hymn written by Marty Haugen, "Awake, Awake, and Greet the New Morn", which is also used during Advent. The recording is an interesting interpretation of a contemporary composition played on an instrument with design roots dating to the eighteenth century. I used the Flaut 4' stop, a set of recycled open wood pipes with inverted or "German" mouths, dating from the late nineteenth century.

And it all works very well.

Posted November 25, 2025 18:25