The Flûte harmonique is one of my favorite stops on French Romantic (and inspired) instruments. Even though it was known centuries before, it was not used with any regularity until Aristide Cavaillé-Coll used them in his instruments. In fact, his first major build for the church of St. Denis, includes 7 of them! They are an essential ingredient to the French Romantic flavor. The sound they make is sheer beauty.
But this is also one stop that has baffled me for years. I am familiar with its sound through the many recordings I own. And I have certainly read about the concept of a harmonic stop. The trouble is picturing it in my mind’s eye and understanding the physics behind it.
I’ve found several sources that try to explain how the harmonic pipes work. All agree that harmonic pipes are constructed at twice their normal length (which would cause them to speak an octave lower). But after that my understanding gets a bit fuzzy.
One source says that the pipes are then overblown to speak their first harmonic (octave higher). That same source says that a hole is then drilled at half the length of the pipe to prevent the formation of an “acoustical node.” But wouldn’t that change the pitch again? Another source simply says that it’s a pipe constructed at twice its normal length and then the hole is drilled in it to cause it to speak an octave higher.
The only Flûte harmonique I have ever encountered was on the organ at my college. But if I recall correctly, it is not a true harmonic flute. Instead, it is two flutes built into one pipe, sharing a common wall over the same foot. I have no idea what the reasoning behind this is, or why the organ builders built it this way. It is a beautiful stop, but I know my organ instructor was never quite happy with it. It wasn’t till I started listening extensively to recordings of Cavaillé-Coll instruments that I found out why.
So, like I said, there’s some fuzziness in my understanding of this particular stop, but one thing I do know that this type of pipe construction is not used for the entire stop. Often the lower notes are constructed with stopped flutes and then the harmonic flutes take over typically in that first octave above middle C. The sound they produce can be described as soaring, even penetrating, but with amazing beauty, with an increasing dynamic as you ascent the keyboard. Careful voicing ensures that the sound between the differently constructed pipes blends.
This construction reinforces the treble range of the stop, which often times starts to thin out the higher the pitch. The result is beautiful. The best demonstration I can find for you is taken from Charles-Marie Widor’s “Symphony Gothique.” The second movement, “Andante Sostenuto,” begins on a 8′ Flûte harmonique.
Besides being great as a solo flute, the Flûte harmonique is an essential stop in the formation of the “fonds d’orgue,” one of my favorite sounds that can be produced by a French Romantic Instrument. The power of that sound is carried between the Flûte harmonique and the Montre. They are supplemented by an 8′ Bordon and 8′ Gambe. When all four of these stops are combined the sound melds together beautifully, but the second you take away that Flûte harmonique – it’s lost a big ingredient.
A most beautiful stop. Wonderful as a solo stop or adds that je ne sais quoi, to mixed ensembles – it plays well by itself and with others! One of these days I’ll hopefully be able to see one of these Flûte harmonique stops in person. And maybe someday someone can explain the physics/construction behind this.