Boehm Head Bore




No one event probably changed the course of in flute history more than Boehm's invention of his new cylindrical bore.  The new instrument wasn't to everyone's satisfaction of course, then or now, but it has to be recognised as a remarkable feat.  But while everyone knows that the flute itself as being cylindrical, the finer details of what happens in the head are not so well known.  Despite the head being the engine room of the new design, it has received very little discussion, and much of that small amount of discussion has been vague, confusing and sometimes downright misleading.  I thought a bit of actual data from real-life examples might be helpful.

The data

The data is presented in graphical form to illustrate the old maxim - there is more than one way to skin a cat, or ream a flute.  Unfortunately for such an interesting topic, I haven't a lot of time on my hands at the moment, so I'll confine myself to the bigger issues, in the eternal hope that one day I'll have time to get back to the finer details.

Most of the data presented below is taken by me of flutes that either I own or that have passed through my hands for repair.  Any others probably came from published sources.

General observations

It's fair to say we can break the head up into three segments:

  1. The tuning slide and region directly behind that

  2. The tapered central section, and

  3. The stopper and region beyond that. 

Useful to note that the stopper region starts about when the bore gets down to around 17mm.  The embouchure is about 17mm left of that.

Tuning slide region

Boehm flutes always seem to have a tuning slide, and by definition, tuning slides have to be cylindrical.  We can expect therefore to find the first 50mm or so (starting at the open end of the tuning slide) to be cylindrical.  The graph above confirms that in most cases, although there are a couple that appear otherwise.  I suspect these tubes were simply warped a little, and that whoever measured them did not take that into account. 

Stopper region

Jumping to the other end, we can see three different approaches:

  1. Continue to taper downwards (the yellow trace).  This requires the stopper to be tapered also, and means it cannot be freely adjusted over a wide range.  Not that that presents much of a problem.  It does mean that the stopper has to be installed and ejected out via the tuning slide end of the head.

  2. Become cylindrical at around 17mm.  Probably the most popular approach, and the most practical.  Stoppers work well and can come out either way.

  3. Backreamed.  Two examples show the end of the head backreamed using probably the same reamer as used for the central section.

Tapered Central Section

Now we come to the crux of the matter, and I'll repeat the chart for convenience:

You can see most of the flutes measured followed the same form with only very minor variation.  Only three stand out dramatically:

  • the sudden reduction in diameter of Rudall Carte #2443 at the end of the slide section.  this might be due to a wood shrinkage issue.

  • The Moon flute which leaves its run about 20mm later, but then follows the same curve, and

  • The Clinton flute that seems the only one to have a different taper.  I probably need to investigate why.

The taper

Of these three sections, it's the taper that is the most significant acoustically, and the least well understood.  As you can see, it is not a straight taper, but neither does it really look like a parabola.  If you hold a piece of paper up to the screen such that it crosses the end of the tuning slide section and the start of the stopper section, you'll see that the bore forms a curve between those two points, with the centre of the curve passing through a point about 0.2mm higher than a straight line would do.  Using the University of New South Wales flute model, I can see differences in tuning when I move the middle of the taper by this amount, so we can probably assume Boehm could too. 


Hopefully, you'll find the data and explanations above of interest and maybe help in your work.  Clearly, there are some unanswered questions that hopefully one day we can get back to.


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Created: 2 April 2013