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.
When a brass instrument is played, the mouthpiece is placed against the player's lips. Three 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 sound made by the lips and channels it in to the body of the instrument.
- Shank – This part of the mouthpiece is inserted into the instrument. It is tapered to ensure 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 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.
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.
The main tube extends from the leadpipe to the bell. On an instrument with valves, the valve assembly splits it into two portions. The main tube gradually widens into a flare at the bell. Some sections are cylindrical, especially on the trombone, with its large slide. The bigger proportion of the main tube that is cylindrical, the brighter the tone of the instrument.
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 are attached the tubes for the air path, and a cylinder that contains ports (air passages) that line up with the openings of the tubes in the casing. The cylinder either slides vertically in the casing (a piston valve) or rotates (a rotary valve), causing the ports to line up in a different manner, connecting or disconnecting the air passages from the tubes. In order for a valve to work properly, is is machined to extremely close tolerances – the valve must be able to move freely but must not allow air to leak around the cylinder. Also, the ports in the casing and the body of the valve must line up precisely or the diameter of the air column will be affected, which can change the tuning and sound quality of the instrument.
The graphic above is a schematic representation of a typical piston valve. In the “up” 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 around instead of moving in a linear fashion, but the concept of ports opening and closing to lengthen the air column is the same.
The basic modern brass instrument has three valves which are tuned approximately a half-tone apart. There are seven different combinations that produce different notes. The trombone slide is constructed so that each position of the slide is a half-step, so that the seven positions on the trombone slide each correspond to a valve combination. Valves are numbered beginning with the one closest to the leadpipe (in terms of the air path). Valve combinations are named according to the numbers of the valves that are “closed” (in the “down” position). The term “open” refers to the situation where all of the valves are in the resting position.
|Open||1 (slide fully in)||None|
|2||2||0.5 tone lower|
|1||3||1.0 tone lower|
|1 + 2 (or 3)||4||1.5 tone lower|
|2 + 3||5||2.0 tone lower|
|1 + 3||6||2.5 tone lower|
|1 + 2 + 3||7||3.0 tone lower|
All brass instruments are constructed out of tune. The physics and mathematics are simply too complex for an instrument to be perfectly in tune on all valve combinations in all registers at all times. Some of the bad tuning has achieved the status of a standard – for example, the 1 + 3 valve combination is almost always quite sharp. Instrument designers have come up with a number of different schemes to compensate for these problems.
On smaller instruments, such as cornets, mechanisms can be provided that allow the player to move on or more of the tuning slides provided in the valve loops while playing. Various rings, levers and brackets are fitted to the instrument that permit the third valve slide (and often the first valve slide also) to be moved with the player's left hand (the right hand is busy on the valves). Larger instruments are sometimes designed with overall tuning slides that can be operated with the player's free hand (this is common in orchestral tuba designs). However, a much more common solution is to add one or more extra valves. The fourth valve (lowers the sound 2.5 steps, equivalent to 1 + 3) is a de facto standard on the euphonium and is often seen on other instruments. This valve compensates for the most commonly out-of-tune note and also provides additional low-end range. Some orchestral tubas are designed with five or six valves.
Every brass instrument has a number of slides which are used to make small adjustments to the length of the air column in order to tune the instrument. There is always one slide in the main tube, for overall pitch (even on a trombone). There are also slides on each major valve loop. Some instruments, most often flügelhorns, have a moveable leadpipe rather than a main tuning slide.
The bell is the “big end” of the main tube. Different models and manufacturers of the same type of instrument have widely varying amounts of flare. The flare has a small influence on the overall sound of the instrument.