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Why the Clarinet Overblows at the 12th

Author: Ken Shaw
Date: 2001-07-24 21:03

Suzanne asked a few days ago why the clarinet overblows at the 12th instead of the octave.

Here's a fairly long piece I wrote for the Klarinet list a few months ago on the subject.

In a wind instrument's low register, the air is vibrating as single entity. In an "open-tube" instrument such as the flute, oboe or saxophone, it's as if you tied one end of a rope to a tree and swung the other end, holding that end as still as possible. The whole rope swings as a unit, with the widest part of the loop at the center and the rope curved in, I think, a "cycloid" shape (the curve traced by a point on the outside of a rolling circle, such as a car tire). (I know the air in a wind instrument doesn't swing around, but the principle is the same.)

In a "closed-tube" instrument such as the clarinet, it's as if you took half the length of rope, stood at the midpoint of the "open-tube" distance and swung your arm in a circle, making the rope swing in the shape of half a cycloid, with your hand at the widest point of the loop. (That's why a clarinet with the same tube length as an oboe plays a much lower pitch.)

When you swing a rope as an open loop, it's quite easy to get it into "second mode," with two loops and a "node" (where the rope is nearly stationary) in the center. With a little experimenting, you can get it into the third and fourth modes, with two and three nodes, respectively. (The easy change of modes is why flutes, oboes and saxophones go easily into the second register and are hard, at least for beginners, to hold in first mode at the bottom of the low register.)

When you swing the rope in closed-loop style, it's also possible to get it into a higher mode. In this case, you have 1-1/2 full loops, with the node 2/3 of the way down the rope. Physically (with the rope) and acoustically (with the clarinet), you're in third mode, with one loop in the rope and (half of) a second loop ending with your hand. That's why the clarinet can't overblow in the second mode, but skips to the third. Just as it's more difficult to get to third mode with a closed-loop rope than it is to get to second mode with an open-loop rope, the clarinet is more stable in its low register than a flute, oboe or saxophone, and it's more difficult to play in its second register (third mode) without opening a register key.

When you open the octave/register key, it interferes with the first mode vibration and forces the air to vibrate in a higher mode. Second mode (on flute, oboe and sax) or third mode (on clarinet) is an easier way to play than a higher mode, so even though the vent is at the wrong nodal position, the instrument plays in second/third mode.

Acoustically, the octave/register vent should be exactly at a node point for every upper register note. Ideally, then, there should be a separate octave/register vent and key for each note. Mechanically, of course, this is impractical (not to mention that all the extra holes would make the bore so irregular that the instrument would be unplayable). Fortunately, wind instruments play pretty well with the octave/register vent placed rather high, where the node points are close together, even if the placement isn't perfect.

On the open tube instruments, however, the vent position for the lower second-mode notes has to be comparatively low. (Remember that the node for second mode is at the midpoint of the tube.) As you get higher in the second-mode register, that vent becomes too low and acts acoustically like the next open hole that gives the next note in the scale. Therefore, oboes and saxophones need a second octave vent, placed much higher, which is used above G#.

The acoustically perfect register vent position on the clarinet is 1/3 of the way down, rather than 1/2 as it is on closed-loop instruments. Therefore, it doesn't interfere as much with third-mode vibration as the tube gets shorter. Also, closed-loop third mode vibration is more stable than the open-loop second-mode vibration on flute, oboe or sax.

Soprano clarinets therefore don't need a double register vent and the accompanying complex mechanism. It becomes necessary with larger instruments. Alto clarinets split about 50/50 between single and double register key mechanisms. It's almost essential for bass clarinets. Today, only the lowest grade student instruments have a single key. Leblanc, in the 1950s and 60s, offered a top-of-the-line bass with a single, compromise vent just below the neck, but it really didn't work, and even Leblanc offered a double-vent mechanism "for those who prefer it" (i.e., those who want a decent upper register). Contras need a double-vent mechanism to play at all. And of course before the automatic register key mechanism was adopted, all bass clarinets (and even alto clarinets) had two register keys for the left thumb, which you had to switch back and forth on between clarion D# and E. (This is possible. Take a look at the left thumb keys on a bassoon.)

When you open the throat A key and press the register key, the reason you get a throat Bb, and not an extension of the clarion register (as high F) is that the register vent is too far down the tube to force third-mode vibration. Instead, the air column "sees" the hole as the one for the next note up the scale from throat A.

However, it's possible to extend the clarion register with practice. Everyone does it by using the two bottom trill keys for the C to C# trill, and maybe in Shepherd on the Rock. The clarinetist in the 1960s band The Dukes of Dixieland almost never went into the altissimo register. Instead, he continued the clarion fingerings up to high F, using the throat keys. I've tried to learn it, but the tone thins out too much to be really useful. It would make second half of the Saint-Saens sonata slow movement a lot easier.

There has been at least one attempt to extend the altissimo register down to Ab, just be continuing to finger down beyond the C#. It was designed by a man named Marca, and it had a second register vent mounted near the top of the barrel and operated by a key for the left thumb. The vent was high enough to keep the instrument in fifth mode. The great technician Ted Planas applied the keywork to a Selmer Bb instrument and showed it at the Clarinet Congress in London in 1984. I tried it and thought that the fifth mode was difficult to "find," though once I found it, the altissimo really did go all the way down to Ab very well. Several other people tried it, and they all had trouble getting into fifth mode reliably -- even Marca himself. Marca printed up flyers and fingering charts (I have one), but as far as I know the instrument never made it to the market, probably because you really don't need the extra fingerings.

Best regards.

Ken Shaw