Course:PHYS341/Archive/2016wTerm2/How the Quality of Brass Affects the Tone of Brass Instruments

How the Quality of Brass Affects the Tone of Brass Instruments

Brass instruments are musical instruments that produce sounds when a player vibrates their lips, which results in sympathetic vibrations of air in a tubular resonator. Many factors affect how the instrument will sound, such as the type of brass used to make the instrument, the flare of the bell, the bore size and shape, as well as the type of plating that is on the instrument.

Bore sizes and shapes

Fig. 1 Cylindrical and conical brass instruments stretched to length

Brass instruments are made with two main types of bores: cylindrical and conical, and almost always a combination of both. Although a brass instrument will never perfectly resemble the shape of a cylinder or a cone, its general shape would resemble the aforementioned shapes if the instrument was stretched to its full length (with flares at one end for the bell).

Examples of cylindrical bore instruments include the trumpet and trombone, both of which eventually flare out into a conical bell.

Conical bore instruments include the horn, tuba, and euphonium.

The type of bore will affect the tone and timbre of the instrument. Cylindrical bore instruments will generally produce, as musicians would call it, a "brighter" tone, whereas a conical bore instrument will produce a "mellower" tone. Brighter, from a physics standpoint, refers to having more of the higher frequency harmonics, while mellower refers to having less.^[1] This is especially apparent when you compare a trombone with a tuba. The trombone will have a much more piercing tone, while the tuba resonates more with lower frequencies, which is demonstrated here. ^[2]

How brass instruments are made

Sheets of brass are cut into certain shapes for the different parts of the instrument, then rolled into a cylindrical or conical shape (depending on the instrument) and soldered at the seams. The rough shapes of each part of the instrument are often hand-hammered to smooth out any imperfections, cut to different lengths, then machined down to precision. The instrument is then buffed to shine, so they can later be coated in different types of metal plating which protects the instrument from corrosion. Smaller moving parts like the valves and adjustable tuning slides are prepared and buffed separately, then assembled later.^[3]

Types of brass and how they affect tone quality

Fig. 2 Trumpet made of yellow brass

Fig. 3 Trumpet made of gold brass

Fig. 4 Trumpet made of rose brass

Fig. 5 A trumpet without any plating

There are three standard grades of brass used to make brass instruments: yellow brass, gold brass, and red brass.^[4] Each type has its own advertised characteristics in the type of sound it can produce, depending on the manufacturer and the type of instrument made.

Yellow brass, industry standard for brass instruments, contains 70% copper, 30% zinc. This combination creates a very resonant alloy, which helps produce a "bright" and projecting "direct" tone, that can "cut through" the sounds of an orchestra very easily, making it very effective for instruments like the trumpet and trombone.^[5]

Gold brass is another alloy that contains more copper content than yellow brass: 85% copper, 15% zinc, giving it a darker colour. This combination of materials creates a "darker, broader, fuller" tone without sacrificing projection.^[6]

Red brass, also known as rose brass, contains the highest copper content, with 90% copper, 10% zinc. It produces a tone that is "warmer and mellower," although it does not project as well as the other alloys due to it being a softer metal with lower zinc content.^[7]

In an experiment conducted by Robert W. Pyle, Jr., he had a tenor trombone with two detachable lacquered bells made of "standard weight" yellow brass, and one made of "lightweight" red brass. The bells were made on the same mandrel with identical contours. The experiment was to find the differences between the two types of bell material through spectral analysis. It was found that at lower frequencies, the yellow brass tended to be "brighter," while at high frequencies, they were essentially identical. ^[8]

In general, the more copper content (the softer the metal), the warmer and mellower the sound, but less projection of the sound, although the differences are not as noticeable to the untrained ear.

Vibrational qualities of the bell

An important characteristic is the treatment of the bell of the instrument. According to Renold Schilke, a renowned trumpet player, designer, and manufacturer, stated that "the more inert [the bell] is to vibration, the better it is." A way that this can be tested is by physically tapping the bell with a finger or knuckle. In one of two extreme cases, Schilke had made a very hard steel bell which, when struck, rung very much like a bell. When the instrument was played, however, the quality of sound was extremely dead. Upon inspection, it was found through an oscilloscope that the sine pattern was very faint, but "the distortion pattern, coming from the vibration of the bell itself, [was] going through at a very jagged and rapid rate, killing the brilliance of sound of the true tone." In the second extreme case, a bell was made of lead, which emitted an extremely dead sound "like rapping on a piece of wood." The sound that was created from it, however, was extremely brilliant, "brilliant to the point of being mechanical." When examined with an oscilloscope, it produced a perfectly true sine pattern, with no distortions in either the above or below harmonics. Because the sound was absolutely pure, it was not usable musically. When determining whether a brass instrument meets a manufacturer's standard, they generally need to be able to emit harmonics above and below the produced sound, as these "distortions" are what the human ear deems acceptable as a musical sound. ^[9]

How the finishes (metal plating) on the instrument affect tone quality

Fig. 8 Gold lacquered Bb trumpet & silver plated C trumpet

Fig. 6 Spectral analysis of a lacquered trumpet playing a written middle G4 (concert F4)

Fig. 7 Spectral analysis of a silver plated trumpet playing a concert F4

According to Renold Schilke, metal plating does not significantly affect the sound quality of a brass instrument. However, when lacquer is applied to an instrument, it can significantly alter how the instrument plays. In his experiment, he crafted three (near) identical trumpets: one without any plating, one with silver plating, and one with lacquer. He had these trumpets play tested by professional symphony players and found that the unplated and silver plated instruments played virtually identically, while the lacquered one had an "impaired tonal quality." His explanation was that silver plating is only .0005 inches thick, while lacquer is .007 inches thick. Because lacquer is not as compatible with brass as silver, and is as hard as glass, it essentially "doubles the thickness of the bell." This would clearly alter the tone of the instrument. ^[10]

In figure 6 and 7's spectral analysis graphs, a lacquered and silver plated trumpets were tested playing the same note. The lacquered trumpet was found to be slightly quieter with less variations in harmonics, while the silver plated trumpet was slightly louder with more distortions.

In another experiment, Robert W. Pyle, Jr. also found similar results, with the unlacquered instrument playing slightly louder (fig. 9).^[11]

Overall, the differences are definitely measurable, but through a layperson's untrained ears, may not be as easily distinguishable.

Fig. 9 Robert W. Pyle Jr.'s experiment: frequency vs sound pressure level in lacquered and unlacquered horn

References

[1] UIUC Physics 406 Acoustical Physics of Music

[2] Fountains of Rome excerpt w/ bass trombone & tuba

[3] Making of Brass Instruments

[4] Yellow Brass, Gold Brass, Red Brass: What’s the difference?

[5] Yellow Brass, Gold Brass, Red Brass: What’s the difference?

[6] Yellow Brass, Gold Brass, Red Brass: What’s the difference?

[7] Yellow Brass, Gold Brass, Red Brass: What’s the difference?

[8] The Effect of Wall Materials on the Timbre of Brass Instruments, Pyle, Jr., Robert W.

[9] The Physics of Inner Brass and the Acoustical Effects of Various Materials and Their Treatment Schilke, Renold O.

[10] The Physics of Inner Brass and the Acoustical Effects of Various Materials and Their Treatment Schilke, Renold O.

[11] How Brass Instruments are Built: Art, Craft, Perhaps Even Science Pyle, Jr., Robert W.

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