• Functional principles of brass instruments

• The function of air in brass instruments
• Air stream and the standing wave
• The analogy between the human voice and brass instruments

Functional principles of brass instruments

The breathing organs of a brass player (lungs, inhaling and exhaling muscles) form a compressor, which can give excitation energy to the lip vibrator in a well controlled way. The instrument itself consists of a cup shaped mouthpiece, a partly cylindrical, partly conical tube and the bell. The instrument works as an amplifier and its inner geometry determines certain resonance frequencies. The instrument amplifies these frequencies and they make the characteristic brass instrument sound.

The lip vibrator

The part of brass players’ lips, which stays inside the mouthpiece rim, forms a vibrator (primarily the upper lip). It is human tissue exactly like the vocal cords. The lips are very soft and especially for a beginner it’s difficult to set and keep them in a static state for the desired note.

Before the note starts the lips should touch each other very lightly (not pressed together) or have a very small aperture between them. After increasing the air pressure sufficiently in the mouth cavity the pressure will open the lip aperture and the air will start streaming. The elasticity forces of the lip tissue work as returning forces and the lip aperture starts to close again. This cycle will be repeated as long as there is an air stream. The fundamental frequency of this vibration (the inverse number of the cycle time) determines the pitch. Every time the lip aperture opens a pressure impulse will be sent inside the horn. This impulse travels and reflects at the speed of sound (~340 m/s) independently of the air stream speed. The impulse travels to the bell of the horn, where part of it reflects and meets the lips of the player after travelling all the way through the tubing and back. Provided that the player has adjusted his or her lips in a suitable frequency (the duration of the opening-closing cycle is the same as or a multiple of the time used by the pressure impulse to travel back and forth through the tubing) a resonance state will arise. This is called a standing wave with nodes and antinodes. This will be heard outside the instrument as the typical brass instrument sound.

Co-operation between the vibrator and resonator

The coupling of the lips and air column

The vibrating air column inside the instrument has a coupling with the player's lips, but the mere length of the air column doesn’t guarantee the right note (desired pitch). With the same length of tubing many different notes can be played (harmonic partials). Baroque trumpets and horns are based on this principle. Modern brass instruments have valves or a slide used to adjust the length of the tubing, however, this is not enough. Players’ lips must be adjusted correctly for the desired note. In the lower register the intervals between partials are fairly big, but around the eighth and higher partials they are smaller, which makes it more difficult to hit the right note in the high register. For good tone quality and attack it’s very important to adjust the lip vibrator as accurately as possible to the desired frequency.

Tuning the vibrator to the right frequency

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