Brass wind instruments consist essentially of a tube whose diameter is variable over its length (Illustration below).
The enclosed air column functions as an oscillator and the resonance frequencies of the standing wave correspond mainly to the well-known partial series. Therefore, only a limited number of "notes" can be produced with such an instrument, namely those whose frequency is identical with the resonant frequencies of the tube. These are called the "natural tones" of the instrument. (Illus. below)
Resonance of a Horn in F. The peaks of the curve represent the frequencies of playable natural tones.
The Natural Tones of a Brass Instrument.
The note value indicates the congruence with the equally tempered scale. The shorter the note value, the poorer the agreement between the natural tone and the musical scale.
To produce chromatic notes between the natural tones, valves are used to add cylindrical tubes, lengthening the acoustically active tube by a whole tone (1st valve), a semitone (2nd valve) or 1 ½ tones (3rd valve).
Musicians think that valves have a remarkable influence on the structure of a musical slur. The sound qualities of slurs are attributable to different valves developed in varying musical traditions. There are currently two types of valves in use today: the rotary valve and the piston valve. The Viennese horn is constructed using a special type of piston valve, the " Viennese double piston valve". The function of the Viennese double piston valve is illustrated below.
The sound characteristics of various valve types was documented the first time in the 1980's as a part of FWF Project on the special characteristics of the Viennese horn. In this study it was shown that a series of slurs sounded smoother using the Viennese valves compared to the rotary valves of the double horn.
The illustrations below show that the Viennese horn creates slurs where the tones seem to flow into one another, where the double horn shows a band of noise (approx. 15 ms) which interrupts the slur for a short time.
This noiseband is not consciously audible to the listener, but does define the sound character of a specific instrument or type of valve.
Illustration: Radiated sound of an octave slur f'-f''(left, Viennese valve, right, rotary valve.) Frequency is shown from left to right, time from front to back.
In order to investigate the acoustical behavior of the instrument, a mechanical process was developed to depress the valve in eight steps, measuring the input impedance at each step.
The relevant frequency range (from the fundamental of the starting note to the fundamental of the target note) was cut out and arranged like a waterfall spectrum.
The Path-impedance-frequency diagrams created in this way correspond to the waterfall spectrum of the sound radiated from the instrument (illustration below) and represent the situation for the player's lips inside the mouthpiece during the slur.
Similar measurements on trumpets with various valve types achieved the opposite results, however, and some results which showed no connection between valve type and the instrument's acoustical characteristics. Further knowledge of the acoustical developments within the instrument during slurs was achieved through the thesis work of Stephan Campidell.
Campidell showed that, regardless of the type of valve, the half-open valve "cuts off" the instrument at the point where the valve is positioned. Diagrams of the pulse response show no difference between a half-open valve, and the same instrument with the rest of the tube removed at that point (see illustration below).
pulse response of the instrument with half-opened valve
pulse response if the tube is cut after the first valve
It is clear that there is no difference in the acoustical characteristic.
This means that during each slur, there exists a position where the tube behaves like being terminated at the valve. As the position of the valve section along the tube axis differs between trumpets with rotary valves and piston valves, we took a closer look to the conseuences.
Valve positions over the length of the tube
The Rotary- and Perinet/Piston valves were exchanged to each other's position, and tested on additional positions as well.
Results conclusively showed that different types of valves at the same positon on the tube produce the same characteristic sound.
The acoustical characteristics of an instrument during a slur is not primarily determined by the type of valve, but by it's valve's position on the axis of the instrument's tube!
Because each frequency produces pressure nodes and antinodes in different positions within the instrument, the position of a normal instrument's valve section relative to the changing internal pressure systems varies with the notes played.
Example of pressure distribution in the test instrument for the 6th natural tone: e''.
Latest studies have shown that a clear seperation between two legato notes of a slur occur when the valve in use is positioned at a pressure antinode of the frequency of the starting note.
Impedance during the slur c''-Bb' (Valve at antinode)
By changing the tension of the lips from the starting frequency to the target frequency, they "move" from left foreground to right rear. After the first third of the slur a steep impedance trough occurs, which is heard by the listener as a "tone separation" caused by a very short noise band.
If the valve is positioned at a node, smoother slurs are easier to play, regardless of valve type (illustration below). In a position between antinode and node, a mixture between the two extremes occurs.
Same as the previous illustration, but the valve is positioned at a node of the standing wave of the starting frequency.
A high impedance like a mountain ridge allows a smoother, glissando-like slur between the two notes.
The following three illustrations show how the positions of the valves influence the acoustical structure of a slur. Slur of c''-Bb'' on a b flat trumpet with rotary valves.
|Valve section at the Pressure Antinode. Slur shows a short band of noise. (usually perceived as an "abrupt, clear" slur).||Valve section at the Pressure Node. Slur shows no noise between the two nodes (smooth).||Valve section between antinode and Node. Mixed form.|
The illustration below shows an experimental trumpet specially made for this purpose by Romeo Adaci, with the connected BIAS measuring head and the moveable rotary valve section. The couplings necessary for various valve positions and the Perinet valve section can be seen at right.
With slurs over larger intervals, for example g1-f2 (also on the b-flat rotary trumpet), the same principles are valid, though they have less practical significance because the player has to surmount several natural tones (= impedance peaks- see illustration below).
|Slur g1-f2, valve at the pressure antinode||Slur g1-f2, valve at the pressure node|
The table below shows clearly that the rotary valve trumpet used in Vienna and in Germany produces more smoother slurs than the American model with perinet/piston valves.
The reason for is not the type of the valves themselves, but the position of the valve section along the tube!
These new findings indicate that, depending on the taste of the player, the optimum position of the valves can be found, regardless of whether it's an existing, theoretical, or newly built instrument.
To further this project, a statistical study of which "smooth" or "clear" slurs are most often used in the practical musical situation for the various instruments (horn, trombone, etc.) would be necessary.
This information offers new perspectives to instrumentalists and instrument makers.
For example, a player may avoid a long search among the endless number of instrument models available to find his or her ideal. Instrument makers may offer instruments with special valve positioning to match specific needs.