How Does a Valve Work? Adding Tubing to Change the Notes Available

A length of tube played by buzzing the lips produces a series of notes determined by its length, with the player selecting between them by changing lip tension, and those notes are spaced widely at the bottom and progressively closer together higher up. That means a natural instrument can play only a handful of pitches low down and a nearly complete scale only in the upper register, which is why old fanfare writing sits high and why low parts are limited to a few notes. Adding length lowers the whole series, so a longer instrument produces a different set of available pitches. A valve provides a way of adding a fixed length of tubing instantly while playing, which shifts the series down by a known interval and lets the player fill in what the open instrument cannot reach.

Three valves in a settled configuration cover everything needed:

• •The second valve adds tubing lowering the pitch by a semitone
• •The first valve lowers it by two semitones
• •The third valve lowers it by three semitones
• •Used in combination they cover every interval down to six semitones
• •Six semitones is enough to reach the next note of the natural series from any starting point
• •A fourth valve, common on lower instruments, extends the range downwards and improves tuning

Two designs dominate and the difference is audible as well as mechanical. A piston valve is a cylinder moving up and down inside a casing, with passages bored through it that align differently depending on its position, so pressing it routes the air through the extra loop and releasing it restores the direct path. A rotary valve turns rather than slides, with a rotor operated by a linkage and a lever, and it is standard on horns and on many instruments in central European traditions. Pistons change the airway abruptly and give a crisper articulation, while rotaries change it more smoothly and are generally described as producing a rounder transition between notes, which is part of why the choice between them is regional and traditional rather than simply technical. Both were developed in the first decades of the nineteenth century and both were being argued about within a few years.

Valves are a nineteenth century invention and the instruments existed for centuries beforehand, so the workarounds are worth knowing. Players of natural horns learned to place a hand inside the bell, which lowers the pitch by a controllable amount and fills in missing notes at the cost of a muffled tone, and the technique was developed to a high art with composers writing for the difference in colour deliberately. Crooks were interchangeable lengths of tubing fitted before playing, which set the instrument into a different key for a movement and could not be changed quickly. Trombones solved the problem entirely with a slide, which varies the length continuously and is why they needed no valves and were writing full chromatic parts long before other brass instruments could. Keyed instruments with holes along the tube were tried and abandoned, since the holes spoiled the tone.

The arrangement has a built-in tuning problem that every player has to manage. Each valve adds a length calculated as a proportion of the instrument's open length, so the tubing for the first valve lowers the open instrument by the right amount. When two valves are used together, however, the second one is now lengthening an instrument that is already longer, so its fixed tubing is proportionally too short and the combination plays sharp, with the error growing as more valves are combined. Notes requiring all three valves are noticeably sharp on an uncorrected instrument. The remedies are movable slides operated by the left hand while playing, a compensating system that routes the air through extra tubing automatically when certain combinations are used, an additional valve providing an alternative fingering, and lipping the note into tune, which is what most players do most of the time.

A tube plays only the notes its length allows, spaced widely low down, and valves add fixed lengths of tubing to shift that series and fill the gaps. Three valves lowering the pitch by one, two and three semitones cover everything needed in combination. Combinations play sharp because each added length is calculated for the open instrument, which slides and compensating systems correct.