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as the serpent shown in Figure 5 used holes in the instru- ment that were covered with the fingers much like a modern recorder. Slides, such as found on the trombone, originated in the 15th century as a quick and easy way to change the length of the instrument, but this is only practical if there is a significant length of cylindrical tubing in the instrument. Manually adding and subtracting lengths of tubing was common practice from the 16th through the 18th centuries, but the process is not rapid and in the 19th century, valves evolved as the most economical and practical method for changing the length of brass instruments. Today, the only commonly used brass orchestral instrument that does not have valves to change the length of the air column is the slide trombone, although there are a few uncommon instruments such as the slide trumpet that occasionally appear in concert.
All modern orchestral brass instruments have sliding pieces of pipe that can be adjusted before beginning to play, but these slides are designed to change the pitch slightly and bring the instrument into tune with other instruments. Typically, the extension of the slide is on the order of 2 or 3 cm and once set, the position remains constant, adjusted only occasionally as the temperature changes. These slides, referred to as tuning slides, are lubricated with a heavy grease so they can be moved when desired but stay in place during a performance. Slides designed to change the played note, such as those found on trombones, are fundamentally differ- ent from tuning slides. Slides used for changing the played note represent a significant portion of the length of the in- strument and are lubricated with lightweight oil to enable rapid movement.
In most modern brass instruments, valves are inserted into the cylindrical part of the tubing instead of a slide. Valves can quickly change the length of the air column and require much less space when performing. Piston valves, like those usually found on the trumpet and cornet, are the most com- mon kind of valve used in brass instruments, but some in- struments, such as the French horn, have rotary valves. Both types of valves have holes through them that correspond to two different pathways for the air to follow. In a piston valve, the piston is depressed directly, whereas rotary valves are turned by depressing a lever that is attached to the valve in a manner to change linear motion into rotary motion. Diagrams of a piston valve in both positions are shown in Figure 6, illustrating how depressing the valve results in the addition of a section of tubing to lengthen the bore.
Figure 6. Diagram of a piston valve in the open (left) and depressed (right) position. When depressed, the air column includes an extra length of tubing. The arrows indicate the direction of airflow. Draw- ing by A. J. Fijatkowski, CC SA-BY 2.5.
The choice of valve type is an engineering decision and has only a minimal effect on the acoustics of the instrument, but the placement of the valve has a considerable effect, especial- ly in instruments that are not primarily made of cylindrical tubing. Even if the bore expansion is gradual, interrupting it to insert a section of cylindrical tubing has significant acous- tical consequences.
It may not be obvious why one would choose valves over slides, or vice versa, when designing an instrument, but there are advantages and disadvantages to each. For exam- ple, slides are not viable in conical instruments and valves can be depressed much more quickly than a slide can be moved. However, in valved instruments, the choice of how much tubing to add to the air column is set by the manu- facturer. Slides, on the other hand, are slower to move and take up more space than valves, but they have the ability to change the length in a continuous manner.
It can be argued that it takes a more skilled musician to play a slide instrument than one with valves because there are infinitely many slide positions. Much like a violinist must be conscious of the tuning of each note while the guitarist merely has to press the string between the frets, the trom- bonist cannot rely on merely pressing the correct valves. This is not a valid assertion, however, because it is impos- sible to make a perfectly tuned valved instrument.
To understand the problem with adding tubing using valves, it is instructive to return to the example of the trumpet. A typical trumpet has three valves. Ideally, the second valve lowers the playing frequency by approximately a semi- tone, which is equivalent to moving down one key on a piano (Audio 2, http://acousticstoday.org/semi/); the first valve lowers the playing frequency by approximately
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