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.
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 waves determines the volume. Volume is a measure of the sound’s loudness. The frequency 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 many modern orchestras is “A”, at a frequency of 440, or 440 cycles per second. Higher tones have higher frequencies.
A brass instrument’s sound is produced by the vibrations of a 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 the 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 instrument’s tubing determines the finished sound. The overall length of the tube determines the fundamental pitch of the instrument. Each note that is played for a particular length of tubing is either the fundamental note or, more commonly, one of its overtones. Overtones are secondary vibrations of the air column, at higher frequencies than the fundamental tone. Overtone frequencies are multiples of the fundamental frequency. The player uses the tension of the lips inside the mouthpiece to “select” a particular overtone from those available in the set belonging to the fundamental note. In this way, it is possible to play different notes without changing the length of the tube.
Changing the Length
The earliest brass instruments were metal reproductions of animal horns. They had a fixed length, with no mechanism for changing the tube length. Skilled players could produce different notes through lip tension and breath control, but true chromatic playing was difficult and was only possible at the extreme upper range of the overtone series. Instruments such as the post horn and the simple bugle are examples of brass instruments with fixed-length tubes.
The difficulty in playing instruments with fixed-length tubes led designers to devise means of changing the length while the instrument was in use. Lengthening the tube lowers the fundamental pitch. Most modern brass instruments use valves to redirect the air through loops of tubing. The trombone uses a large, freely-moving slide instead of valves.