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.
Mouthpiece
The mouthpiece is the part of the brass instrument that rests against the player’s lips. It is the beginning of the tube that produces the sound. There are three basic parts to a mouthpiece:
- Rim – This part of the mouthpiece rests against the player’s lips.
- Cup – This part of the mouthpiece collects the vibrations of the player’s lips and funnels them into the instrument.
- Shank – This is the part of the mouthpiece that is inserted into the instrument. It is tapered to insure a tight fit. The opening of the shank should be circular and the shank must make an airtight seal with the leadpipe.
For many players, the mouthpiece is the most important part of the instrument, because it is the point of contact. Subtle differences in the shape of the rim or the design and depth of the cup can profoundly influence the quality of the sound.
Leadpipe
The leadpipe is the starting point of the main tube, where the mouthpiece is inserted. The opening of the leadpipe is often reinforced, because any deviation from a circular cross-section could cause air to leak between it and the shank of the mouthpiece.
Main Tube
The main tube is the body of the instrument, extending from the leadpipe to the bell. On an instrument with valves, the main tube is divided into two portions by the valve assembly. Some portions are cylindrical, especially on the trombone, with its long slide. The larger portion of the main tube is conical, widening into a wide flare at the bell. Instruments with a larger portion of cylindrical tube are brighter in tone.
Valve
Mechanics
In general mechanical terms, a valve is a device that opens or closes a pathway for the passage of a liquid or a gas. Musical instrument valves are compound devices, where a single movement opens one passage and closes another. This has the effect of changing the length of the vibrating column of air within the instrument, thereby changing the fundamental pitch. This is analogous to an organ pipe, where larger, longer pipes produce lower tones.
There are various designs of brass instrument valves, but all work on the same general principle. The valve consists of a casing, to which the tubes attach. Inside the casing is a cylinder with ports bored through it for air to pass through the valve. These ports line up with the tubes attached to the casing. The cylinder either slides (a piston valve) or rotates (a rotary valve). The motion causes the ports to align in a different manner, connecting or disconnecting the passage from the tubes. When the valve is activated, the ports line up with a loop of tubing, which increases the length of the tube.
The graphic above is a schematic representation of a typical piston valve. In the “up”, or resting position, the air column passes through one passage of the valve, and the loop is bypassed. When the player presses down the valve core, this passage is closed and other passages align with the ports in the casing, diverting the path through the loop. This lengthens the air column and lowers the fundamental note. The larger the loop, the more the pitch is lowered. When the player releases the valve, a spring (not shown) returns the valve to the up position. Rotary valves spin instead of moving in a linear fashion, but the concept of ports opening and closing to lengthen the air column is the same.
In order for a valve to work properly, it is machined to very close tolerances. The valve must be able to move freely inside the casing, but must not allow air to leak around the cylinder. The ports in the casing and the body of the valve must align precisely. Any change in the diameter of the air column can change the tuning and sound quality of the instrument.
Usage
The basic modern brass instrument has three valves which are tuned approximately a half-tone apart. With three valves, there are seven different combinations that produce different notes. Valves are numbered beginning with the one closest to the leadpipe (in terms of the air path). Combinations are named based on the numbers of the valves in the activated (“closed” or “down”) position. If all of the valves are in the inactivated resting position this is termed “open”. Trombones have seven slide positions which correspond to the valve combinations, with each position lowering the fundamental note by a half-step.
| Valve Combination | Trombone Position | Effect |
|---|---|---|
| Open | First (slide fully in) | None |
| Second | Second | 0.5 steps lower |
| First | Third | 1.0 steps lower |
| First + Second (Third) | Fourth | 1.5 steps lower |
| Second + Third | Fifth | 2.0 steps lower |
| First + Third | Sixth | 2.5 steps lower |
| First + Second + Third | Seventh | 3.0 steps lower |
Compensation
All brass instruments with valves are constructed out of tune. The physics and mathematics are simply too complex for an instrument to be perfectly in tune for all valve combinations and all registers at all times. Although different designs have varying tuning characteristics, there are some things that are nearly universal. For example, the valve combination first + third is almost always sharp. Instrument designers have come up with several schemes to compensate for these problems.
On some instruments, mechanisms are provided that allow the player to move one or more of the tuning slides while playing the instrument. For example, on many cornets, rings or levers are provided that allow the player to lengthen the slides on the first and third valve slides, in order to correct the sharpness of the first + third valve combination. Another approach is to add more valves. For example, almost all euphoniums used in brass bands have a fourth valve, which lowers the sound 2.5 steps, or the same amount as the first + third combination. The fourth valve also allows more chromatic notes at the extreme low range of the instrument. In recent years, manufacturers have produced instruments with both extra valves and slide levers, for maximum flexibility in keeping the instrument in tune throughout the octave. A third method is to add extra tubing which comes into use only when certain valve combinations are played.
Slides
Every brass instrument has one or more slides that are used to make small adjustments to the length of the air column in order to tune the instrument. These are commonly referred to as “tuning slides”. There is always one slide somewhere in the main tube for the overall pitch of the instrument, even on trombones. Usually, there is also a slide on each major valve loop. As mentioned in the Compensation section above, sometimes these slides are connected to systems which allow the player to move them while playing the instrument.
Bell
The bell is the flare at the end of the main tube opposite the mouthpiece. The amount of flare varies widely with different instrument designs. This part of the instrument as a small influence on the sound of the instrument.
