Popular Version of Paper 2aMU4
Presented June 17, 1997
133rd ASA Meeting
Embargoed until June 17, 1997
Most brass players believe that the playing qualities of their instruments are affected by the method of construction and the materials used. Composition, thickness, and hardness of the metals used are all cited as important. Phrases such as ``hand-hammered bell'' are often found in advertising literature. This paper will give an overview of the tools, techniques, and materials used by brass-instrument makers today and in centuries gone by. Traditional techniques are still used along with the latest metalworking methods, so that a present-day instrument may contain some parts produced by computer-controlled machine tools and others made using processes familiar to sixteenth-century artisans.
Is there any objective evidence that the sound of a brass instrument is affected by the material from which it is made? Indeed there is. An experiment was performed using a French horn with a removable bell flare. One microphone was mounted inside the mouthpiece cup and another on the axis of the bell about one and one half bell diameters away from the bell rim. A meter connected to the mouthpiece signal allowed the player to monitor her playing level and repeatably produce the same level inside the horn with different bell flares attached to the instrument.
Figure 1 shows the spectrum of the sound outside the bell using the same bell flare with and without a a protective coat of lacquer. At the lower playing level, there is no significant difference between the lacquered and unlacquered conditions, but at the higher level, the unlacquered bell radiates more energy at all frequencies.
The fact that the effect of the lacquer is noticeable only at the louder playing level implies nonlinear behavior. A plausible hypothesis is that the lacquer damps vibrations in the bell flare, but this has not been verified.
Figure 2 compares bells made of different metals: nickel silver (an alloy of copper, zinc, and nickel) and Ambronze (an alloy of copper, zinc, and a small percentage of tin).
At the softer playing level, the nickel silver bell radiates more at the higher frequencies than the Ambronze, but at the louder level the reverse is true. However, as the player ascends the scale from the note shown, the variation between the bells decreases. When the experiment is repeated two octaves higher, at a played fundamental frequency of 520 Hz, there is virtually no difference. Again, the change in the relative behavior with the change in playing level implies nonlinear behavior.
Now let us follow the production of two instruments, a replica of a 1632 trumpet by Hannes Hainlein of Nürnberg and a modern symphonic trombone. The trumpet was made to the extent possible using the tools and techniques of the sixteenth and seventeenth centuries. First the trumpet.
The bell and all tubing are made from sheet brass, wrapped to form a seam which is brazed with an alloy having a lower melting point than the brass. Whenever metal is reshaped by bending, hammering, or stretching, it becomes harder and more brittle. To avoid cracking, it is necessary periodically to soften it by annealing, that is, by heating to red heat and cooling.
At the start, the large end of the bell is smaller than it must ultimately become. It is stretched by repeated hammering and annealing (Figure 3).
The annealing blackens the surface with an oxide layer, and at intermediate stages, the bell looks more like something that has been through a really bad accident than the trumpet it will become (Figure 4).
The bell assumes its final shape by being hand burnished onto a steel mandrel of the desired shape. "Pickling" in an acid bath removes most of the oxide. A bright surface finish is achieved by sanding and polishing with a fine abrasive. Figure 5 shows the bell on its mandrel. A garland will be added to strengthen the bell rim. This is a band of brass stiffened with a ring of heavier brass. The garland is not soldered to the bell; the edge of the garland is crimped over the bell rim.
Centuries ago, final polishing was accomplished by "ragging" with a strip of cloth or leather into which the polishing agent had been rubbed, as shown in Figure 6. Nowadays, most polishing is done on a powered buffing wheel, but ragging is still used for the nooks and crannies that cannot be reached by the buffing wheel.
Figure 7 shows the completed replica of the 1632 Hainlein trumpet. The various components of trumpets of this period were not soldered together. A wood block wrapped with a decorative cord (red, in this case) serves to tie the instrument together.
Modern brass-instrument bells are also formed on steel mandrels, but usually by spinning, as shown in Figure 8. The metal is forced to conform to the mandrel by pressing forcefully on it with a lubricated polished steel tool while mandrel and bell spin together in a lathe.
The trombone bell shown here is a "two-piece" bell, meaning that it is formed from a bell tail with a longitudinal seam, like the Baroque trumpet bell, and a separate bell flare, spun from a disk. The bell tail is smoothed onto the mandrel in a single operation by drawing it hydraulically through a steel washer that stretches as it passes over the brass.The bell tail and flare are brazed together as in Figure 9 before the final spinning. The rim of the bell is rolled around a wire to provide the stiffening that in earlier times came from the bell garland.
A somewhat tarnished completed bell is shown in Figure 10. The brazing alloy has tarnished less than the brass, so that the seams are clearly visible. Notice that the edges that are brazed together were held in position by a series of teeth cut into the metal and hammered down straddling the opposite edge. Older bells, made when labor was cheaper, typically have seams with many more teeth.
One-piece bells are also made, and seem to be preferred by professional trumpet players in the United States. A similar picture of a one-piece bell would show the longitudinal seam continuing to the bell rim. High-quality trombones are made with both one- and two-piece bells; nearly all French horns have two-piece bells.
To bend tubing, it is filled with a material to prevent buckling and collapse. Lead is the traditional filler, but nowadays a mixture of pitch and tar is often used, or an alloy of lead and bismuth. After bending, the filler is melted out. The inside surface of a bend typically experiences some wrinkling.
Figure 11 shows two partially completed trombone tuning slide bows. The one on the left shows the characteristic wrinkles. The one on the right has had the wrinkles smoothed by gentle tapping with the small hammers just above it. This process would have been very familiar to the sixteenth-century artisan.
The latest machining techniques are used where possible. Figure 12 shows some of the components of a rotary valve used in a trombone's F extension.
The rotor, casing, and cap on the left are all produced by a computer-controlled machine tool. After the tubing is hand-brazed into the ports in the casing, as shown on the right, the remaining machining is once again under computer control. To assure a close fit, the rotor is hand-lapped into the completed casing.
How could any of this matter to the player? Consider one-piece vs. two-piece bells. In a one-piece bell, the maximum stretching of the metal occurs at the bell rim, so unless the maker takes extraordinary measures, the metal will be thinnest there. In a two-piece bell, the greatest stretch occurs in the flare where it joins the bell tail and the metal is normally thinnest there. Vibrations of the bell metal will differ in two such bells even if they are of the same shape; to the extent that such vibrations color the sound, the instruments will differ.
How the various parts of the instrument are attached can affect the "feel" of the instrument to the player, even if the sound is unchanged. Consider the different bracing patterns of the three French-horn third-valve slides shown in Figure 13. Some makers think this is important, and at least two other variations are found. Too little or misplaced bracing can cause the instrument to vibrate noticeably on certain notes, which is very distracting to the player. The author has added bracing to such third-valve slides on several of his instruments.
Experienced players will often customize their instruments. They will change to heavier valve caps (or lighter), perhaps on certain valves only. They may selectively anneal portions of the bell, or solder extra mass on the outside of the bell. Any of these can noticeably alter the tone quality, but knowing what to do to produce a given change is very much a black art.
Cryogenic treatment of instruments by immersion in liquid nitrogen is a subject of controversy. Many players who have tried it believe it has improved the "feel" of the instrument without substantially altering the tone quality. The instrument is said to respond more uniformly. Proper double-blind listening tests have yet to be performed, so the jury is still out.
Although the most important factor in the design of a brass instrument is probably the shape of the bore, the difference between a good and a superb instrument is, like the devil, in the details. Changes of metal thickness, alloy, hardness, mass distribution, and surface treatment (lacquer or not) all have a demonstrated importance to the player. At present, there is only limited empirical knowledge of the relation between these physical details of the instrument and how they affect the player and listener. A player will often say something like, "I altered this to make the tone carry better." We cannot yet say what are the attributes of a tone quality that "carries better", let alone relate this to the instrument. Much remains to be learned.