The Smoothflow Tone Hole

I've never felt that the keyed notes on wooden flutes sounded quite as well as the unkeyed notes, the ones produced through open finger holes.  It's perhaps not surprising.  When you open a finger hole, you lift your finger quite a way above the hole.  When you open a key, the pad rises only a little way, and many obstacles remain to inhibit the free flow of air.

Wait a minute, you cry, gesticulating wildly at the open keyhole in front of you.  Surely air is thin, and will have no difficulty getting out of a hole that big!  Air might be thin, but it does have some mass, and in a flute those air molecules have to move pretty fast to make the full rich sound we want.  And anything with mass moving quickly resents sharp corners.

Take a close look at a typical old-fashioned flute key hole.  The hole itself is set at 90 degrees to the bore, so there's the first crunch point for speeding air molecules.  Once they negotiate that bend, they slam into the pad looming above the hole, before scraping through the narrow gap between the pad and the very sharp edge of the pad seat.  Still not free, they now slam into the wall of the seat, before finally scraping over its sharp edge to the safety of the outside world.

(It's actually a little more complex than that, as individual air molecules don't have to move very far in a flute.  It's as if the flute tube were full of ping-pong balls, being rocked violently back and forth by the sound waves you're producing.  So while no particular ping-pong ball suffers all the indignities I've mentioned above, at any one time many ping-pong balls are suffering one or more of them.  So the net result is the same - pain for ping-pong balls, err, I mean air molecules.)

Now the effects of this chaos are easy to visualise and hear.  Firstly, efficiency is down, as some of the energy we want to radiate as sound is being turned into the  heat of impact.  Secondly, noise is up - wind whistling over sharp edges always produces noise, ask the banshee.  Thirdly, clarity is down as the noise acts to partially mask the note, and to intermodulate with it, creating a litter of inharmonic by-products.  Articulation also suffers, as resistance decreases the "quality factor" (Q) of the resonance, increasing the attack time.

Now, none of this is new, although not commonly known.  Oboe makers in particular have to deal with the problem, because the tiny holes used in the oboe really make it hard for the air to move freely.  It affects small hole flutes in particular, but is noticeable on big hole flutes too.  It's particularly aggravating in the case of the note D on 8-key flutes.  Interestingly, it is less noticeable on G# notes, probably because of the very excellent venting provided by the normally large G and F# holes separated by only a semitone each.  Conversely, the note F enjoys only very poor venting from the small hole E - the nearest help comes from the D hole, three semitones away.


Some 19th century flute makers clearly realised it and incorporated one or more strategies to reduce the effect.  An example is this chamfered edge hole employed by Potter on an 1851 model Clinton flute.  Interestingly, not all the holes on the flute are dealt with in the same way.

After some examination of the issue and quite a bit of thought, I've come up with a new tone-hole design which deals with all these issues.  I'm calling it the Smoothflow tone hole as that epitomises the aims.  The Smoothflow tone hole features:
  • rounding at the base of the leading wall to improve the bore to hole transition
  • rounding at the base of the trailing wall to improve bypassing the hole when the key is closed
  • shaping of the tone hole to reduce resistance
  • rounding at the top of the hole
  • a half-round rather than pointed pad contact area
  • a flat seat floor rather than downward sloping
  • a sloping rather than vertical seat wall
  • rounding of the top of the seat wall

The half-round profile of the pad contact area is harder to make cutters for, but brings a number of benefits compared to the normal conical type:

  • much more aerodynamic
  • far less likely for the edge to crumble or get chipped
  • reduced pad wear
  • reduced seat wear

I don't claim originality in all the strategies outlined above, but I do claim originality in their combination.  I look forward to the Smoothflow tone hole or something similar becoming the norm in wooden flute construction.

Terry McGee, 9 September 2001.

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