Slide Mechanisms on Wind Instruments: Basic Principles

A musical wind instrument that consists only of its pipe, or length of tubing, such as a natural horn or trumpet, can play only its fundamental tone and some of its higher natural tones (overtones). To obtain the many missing tones between the natural tones, the instrument needs some means by which to be temporarily shortened (to raise the pitch of the pipe) or lengthened (to lower the pitch of the pipe).

On some instruments, the device used to effect such changes is a slide mechanism. A slide instrument consists of two separate sets of tubing, one sliding in and out of the other to shorten and lengthen the sounding tube.

From the 15th to the 19th centuries, a slide mechanism was applied to some trumpets.

However, the main instrument based on a slide mechanism is the trombone. The trombone was, in fact, the first modern orchestral instrument to appear in its present form. Since the 15th century, it has had the same basic shape and has produced tones through the slide technique.

The trombone has a cylindrical bore for its top two-thirds of length and then flares out into a conical shape toward the bell (opening at the lower end). The left hand holds the instrument by a brace while the right hand operates the slide.

The slide is a U-shaped cylindrical pipe that slides back and forth over two straight cylindrical pipes and thus varies the effective length of the instrument.

The trombone can be played with the slide in seven different positions. The reason the instrument has seven positions is that the largest gap in the series of overtones is the interval of a fifth, which contains seven semitones, or half steps. Therefore, by combining the tones of seven series of overtones whose fundamentals are separated by seven adjacent semitones, a complete chromatic scale can be obtained over a range of almost three octaves.

When the slide is held closest to the player and the total effective length of the pipe is at its shortest, the slide is said to be in first position. Each successive position, with the slide extended farther and farther away from the closed position, lowers the fundamental pitch by a semitone. The seven fundamentals are called pedal tones.

The tenor trombone, for example, has the following fundamentals, or pedal tones, in its seven positions (conventionally indicated by roman numerals): B-flat in I, A in II, A-flat in III, G in IV, G-flat in V, F in VI, and E in VII.

The distances between adjacent positions increase as the slide extends to higher positions. From first to second position is 3 3/16 inches, while from sixth to seventh position is 4 3/4 inches.

Trombones have been made in many different sizes, ranging from soprano to contrabass. The most common modern trombones are the tenor, bass, tenor-bass, and contrabass.

The slide mechanism makes the execution of a true legato difficult because slurring two tones that require a change of position tends to force a break in continuity to avoid an undesirable portamento (sliding) effect.

On the other hand, a glissando, an extended continuous sliding of pitch from one pitch to another, is easy.

Intonation problems that other kinds of instruments struggle to solve are relatively easily solved on the trombone because pitch adjustments can be made quickly by slight movements of the slide.
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The New Grove Dictionary of Music and Musicians. 2nd ed. London: Macmillan, 2001.