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Brass Musical Instruments
A good example of the difference in sound between a cy- lindrical instrument and a similar conical instrument can be heard in these two recordings by Brian Shook (Audio 1, http://acousticstoday.org/tcdemo/ ). A trumpet was used in the first recording while a cornet was used in the second. Note especially near the end of the recordings where the louder, higher, staccato notes make the differences in timbre most obvious. Although the two instruments are the same length and play the same pitch range, the difference in sound is striking.
It is rare to find a modern instrument with a purely conical bore shape. Campbell et al. (2006) note that the alphorn is an instrument that is almost purely conical, but it appears to be unique in this respect. The bugle comes close to being com- pletely conical, but even here, the bell flares at the end rather than merely ending in a cone. However, several instruments have significant sections of the bore that are not cylindrical. The cornet, flugelhorn, and tuba are good examples of in- struments that fall into the conical branch of the brass fam- ily but are not completely conical.
The conical shape in the tubing is important for two reasons. As noted, conical instruments are characterized as having a mellow sound and do not sound as brassy as cylindrical in- struments when played loudly. The conical instruments such as cornets, flugelhorns, and tubas cannot produce the clas- sic brassy sound found in cylindrical instruments such as the trumpet and trombone because this sound results from nonlinear effects that occur only in long cylindrical sec- tions of tubing (Hirschberg et al., 1996; Myers et al., 2012). Also, in contrast to the cylindrical bore profile that results in only odd harmonic components, a purely conical bore has resonances that incorporate the complete harmonic series. Therefore, the end of a purely conical instrument does not function to shift the resonances into a harmonic relation- ship. However, brass instruments that are primarily conical still have a flaring bell so the mouthpiece, bell flare, and tub- ing all must work in concert to keep the overtones harmoni- cally related.
Valves and Slides
The bugle, posthorn, and alphorn are examples of instru- ments that are played on a single fundamental frequency. These instruments have a length of tubing that cannot be changed, and, therefore, the repertoire is confined to har- monics of the fundamental frequency, which is determined by the length of the horn. However, most instruments are
Figure 5. A serpent made during the early 19th century is an exam- ple of a labrosone with finger holes. From the Metropolitan Museum of Art.
designed so that the player can produce all the notes within the range of the instrument. Naturally, the length deter- mines what notes can actually be played, so for the instru- ment to encompass the entire musical scale, the length must be changed.
Changing the length is relatively easy, but adding and sub- tracting lengths of piping quickly is not a trivial process. Much of the history of the development of modern brass in- struments involves various attempts to quickly change the length of the instrument. Some early brass instruments such
34 | Acoustics Today | Winter 2016