These pages are a simplified explanation of the workings and parts of a brass instrument. The actual science behind an instrument is quite complex, and this is not intended to be an exhaustive treatment.
Producing Sound
The Basics
Waves of compression and expansion, in a medium such as air, are detected by the ear and perceived as sound. The amplitude, or “height”, of the wave determines the volume. Volume is a measure of the sound’s loudness. The frequency, or wavelength, determines the pitch (highness or lowness) of the sound. Frequency is a measure of how many waves are detected over time, usually expressed in cycles per second. For example, the standard tuning note used in most modern orchestras is A 440, a note with 440 cycles per second. The higher the frequency, the higher the tone.
A brass instrument’s sound is produced by a vibrating column of air contained within the tubing. The origin of the sound is the lips of the player. A “buzz” is produced inside the mouthpiece of the instrument when the player pushes air through their lips. This sound is amplified and modified by the tubing of the instrument. Although the player blows into the mouthpiece in order to produce the necessary lip vibrations, the movement of the air through the tubing contributes little to the overall sound. It is the vibration of the air, not its passage through the instrument, that produces the sound.
The size and shape of the tubing produces the finished sound. The overall length of the tube determines the fundamental pitch. For a given length of tubing, each note that can be played is either the fundamental note or a note in the overtone series of the fundamental note. Overtones are secondary waves that arise from the fundamental note at higher frequencies. By adjusting the tension of the lips inside the mouthpiece, the player can select a particular overtone as the sound produced. This is why it is possible to play different pitches with the same length of tube.
Changing the Length
The earliest brass instruments were metal reproductions of animal horns. They had tubes of a fixed length, with no mechanism to change the length while playing the instrument. Skilled players could produce different pitches through lip tension and breath control, but true chromatic playing was difficult and only possible in the extreme upper range of an overtone series. Instruments such as the post horn and simple bugle are examples of this type of instrument that are still used today.
The difficulty in playing fixed-length instruments led designers to devise ways of changing the tube length while the instrument was in use. Lengthening the tube makes the fundamental note lower in pitch. Two main designs are used to accomplish this: the slide and the valve. Most modern brass instruments use valves, which lengthen the instrument by adding extra loops of tubing. The trombone uses a large slide to adjust the length of the tube. In addition to the valves or main slide, there are also smaller slides that are used to tune the instrument by making small changes to the tube length. The minimum number of valves needed to produce a fully chromatic instrument is three, which allows seven different combinations. These correspond to the seven slide positions on the trombone.
