Brass wind instruments consist essentially of a tube whose diameter is variable over its length (Illustration bellow).
The air column "enclosed" within functions as an oscillator and the position of its resonant frequencies correspond mainlyto 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 the Horn. The peaks of the curve represent the frequencies of playable natural tones.
The Natural Tones of a Brass Instrument.
The note value is an indication of congruity with tempered tuning. The shorter the note value, the poorer the agreement between the natural tone and the musical tuning.
To produce chromatic notes between the natural tones, valves are used to engage cylindrical tubes, lengthening the acoustically active tube by a whole tone (1st valve), a half tone (2nd valve) or 1 ½ tones (3rd valve).
The experience of professional musicians shows that valves have a remarkable influence onto the structure of musical slurs. 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 for the first time in the 1980's as a part of FWF Project[ ] on the special characteristics of the Viennese horn. In the course of the studies 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 clearly that the Viennese horn creates tones which 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 band of noise 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 define 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 departure tone to the fundamental of the arrival tone) were cut out and successively combined.
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 occurring at the player's lips and 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 in later experiments.
Campidell showed that, regardless of valve type, the half-open valve "cuts off" the instrument at the point where the valve sits. 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 reply of the instrument with half-open valve
pulse reply when the tube is cut after the first valve
It is clear that there are no differences in the acoustical characteristic.
This means that during every slur, there is a moment in which the tube behaves as if it were terminated at the valve. Because the various valve types are positioned differently on various trumpets, the next tests were devoted to finding out how the valves' position influences sound.
Valve positions over the length of the tube
The Rotary- and Perinet 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 that 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 lies with in a pressure antinode of the frequency in question.
Impedance during the slur c''-Bb' (Valve at antinode)
The player "wanders" 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.
If the valve is 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 the node.
A high impedance like a mountain ridge allows for a smoother slur between two tones.
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 a "aprupt, 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 apply, though they have less practical significance because the player has to surmount 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 parts of Germany produces more smoother slurs than the American model with Perinet valves.
The cause is not the character of the valves themselves, but the position of the Valve section!
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 suit specific needs.