Thursday, 12 April 2012

One thing: Physics of fluting

This is my flute.  Nothing too impressive to look at, sitting in its box, but I love to play it.  This one is made by Stephen Wessel, a clarinetist, model aircraft enthusaist and metalworking artisan who lives in the South-West of England.

Stephen has chosen a selection of materials to make his flutes.  The body is in solid silver, although you are not allowed to say so, since it's not hallmarked.  The mechanism is in stainless steel for lightness and strength, and the keys are made of resin, again for lightness.  The idea is that the lighter the mechanism, the easier it is to make it move and stop.   I also like the combination of colours; silver, grey and black, altough they're not easily differentiated in the photo.

The flute is not a very efficient way of converting the energy of breath into sound.  With a reed or brass instrument, all the energy of your breath ends up in the instrument, and is, at least in principle, available to be turned into sound.  With the flute, you blow across the hole, and a lot of energy just dissipates into the surrounding air.  Therefore the flute mechanism has to be very accurate; there is no room for leaks.  Stephen hand-makes the screws on this flute with a fine pitch to their threads, so that the mechanism can be adjusted very precisely.

The flute makes a sound by being made to resonate as a tube that is open at both ends (see figure A below).  This means that the lowest frequencey (the "fundamental") that the tube will resonate at corresponds to a sound wave twice as long as the tube.  (The tube contains half a wavelength)  Higher frequency resonances are possible, where the tube length is 2, 3, 4, etc half-wavelengths long.  Figure B shows the tube resonating at the next highest available frequency (twice the frequency of the fundamental) where the tube is one wavelength (i.e. two half-wavelengths) long.

The range of the flute is three octaves, and this simple relationship between the length of the tube and the wavelength of the notes available means that the fingering for middle octave is almost the same as for the lowest octave.  The fingering for the highest octave has broad similarities to the lower two.   By contrast, reed and brass instruments resonate as a tube closed at one end and open at the other, so that the tube is 1/4 wavelength long, and the first harmonic available is an octave-and-a-half above the fundamental (the tube is then 3/4 of a wavelength long).  (Figure C).   The fingering for these instruments is correspondingly more complex, but the reflection of sound waves off the closed end of such an instrument is more efficient, making it easier to get them to resonate than the flute that lacks a closed end.

When you play a 3-octave flute, blowing a given length of tube will therefore, (with minor adjustments to fingering) give you three possible notes, each one an octave higher than the adjacent lower one.   In between these octaves, the pitch of the note you play is set and adjusted by changing the effective length of the tube.   The body of the flute is equipped with holes at various intervals, that are covered, as necessary, by keys.  Holes are closed from the blown end progressively, creating a correspondingly longer and longer tube, generating lower and lower notes.  The first hole that is open to play the note "C" is called the "C" hole, and the key that you use to close it to play a "B" is called the "B" key.  Thus the "B" key covers the "C" hole, a confusion that is inevitable, like 19th century dates starting with 18.

The action of closing a hole is fundamentally different from opening it.  If you tap a tube it will ring, (see, for example, tubular bells) and if you tap a key hard down onto the flute, the flute will ring to the corresponding pitch, even if you are not blowing it.  One of the aspects of learning to play is to get a descending scale (where mostly you close holes) to sound as smooth as an ascending scale (where you mostly open them).  What makes it worse is that if you slowly close (or open) a hole whilst blowing the flute, at one point the flute simply stops playing and you get a mish-mash of noise instead of a note.  You have to close the hole quickly enough to avoid this, to get a smooth transition between notes, but not so fast that the key 'taps' the flute.  The flute is not naturally a legato instrument.

The flute has an almost constant diameter along its length, a constraint that arises from the need to achieve good tuning across its three-octave range.  A tube of a given diameter will resonate differently as its length changes.  Another aspect of learning to play is to achieve a uniform tone across all the notes whether they are generated by a short tube or a long one.

Since it is a human trait to play musical intruments to the limits of human capability, and since flutes are less efficient than other wind instruments at converting energy into sound, it follows that they don't play as loud.  They just about hold a place in a modern orchestra amongst much louder intruments, but the flutists often complain that they can't be heard.  The flute doesn't play especially softly either, (most of the sound escapes the two ends of the tube rather than being reflected back down it to create the resonance) unlike say a clarinet that can make a sound on the threshold of hearing.  It is a common complaint levelled at flutists that their music lacks dynamic contrast.  Or any contrast at all in fact.  Fair cop.  It's hard to do, but can be done.

The flute has one key per finger, and you hold the flute such that each finger is right over its corresponding key. (Except the right-hand thumb that is used to balance the instrument, and the right-hand little finger that has three keys)  With a few exceptions, every note on the flute can be achieved by moving one or more fingers no more than 2-3 mm, the distance necessary to move a key to close a hole.  This is why the flute can give a flurry of rapid notes that is difficult for any other instrument to match.  One aspect of learning the flute is the elimination of finger movement beyond that which is strictly necessary.

So why do I play this thing?  It's hard to play loudly or softly, hard to play legato, hard to play in tune (more later), wastes breath and needs regular servicing to keep the delicate and precise mechanism in good order.  Because I love the sound it makes.  I make no special claims to skill, and it doesn't happen very often, but I have silenced a room full of talking people, and stopped people in their tracks, when making a noise with this thing.

1 comment:

the fly in the web said...

That was a fascinating exposition!
Thank you!

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