Hammered vs. Spun
Bowl Shaping: Hammered vs. Spun/Form-molding
Essential to the art of the traditional artisan timpano bowl is to take multiple pieces of soft rolled sheet copper (usually two pieces – bottom/walls) of the same thickness, connect them together and then work harden/hammer them into a shape that has an overall homogeneous hardness and thickness. This is not an exact science so slight variations do occur, especially in the seams, consequently each bowl will have its own personality and character. The more thick, homogeneous and work-hardened the copper, the more likely it will not take energy away from the vibrating head. This type of bowl also tends to have a high and consistent thermal mass.
If the hammered copper is thin enough, the separate pieces (including the seams), can have their own patterns of vibrations that can produce resonant frequencies. The result (in as much as the bowl is allowed to vibrate) is a system that can vibrate evenly yet produce a small amount of colorful overtones (<1 dB) adding to the percussive attack (collateral color). This type of bowl, however, does take energy away from the vibrating head and has a low, yet consistent thermal mass. (see Fleisher & Fastl)
As timpani began to be mass produced the mid-twentieth century spun/form-molded copper bowls (Ludwig, Slingerland, Premiere etc.) generally were made from a single piece of soft copper. The spinning/molding process tended to stretch the copper, which usually resulted in a bowl with a thicker bottom and thinner walls. Bowls made in this fashion do not have as much of a homogeneous thickness, hardness or as consistent a thermal mass. When compared to a traditional artisan timpano bowl, the result is a system that:
1) unevenly absorbs/deflects mechanical energy generated by the displacement of the internal air
2) does not maintain a consistent thermal equilibrium of the internal/external air masses (thermal mass)
3) does not sustain potential colorful vibrations as well
All of these factors influence the overall production of timpani tone/sound to a small extent.
Consequently, the mid-twentieth century, commercially mass-produced spun/form-molded bowl is generally not able to generate as much collateral color, provided it is allowed to do so, or adhere to the preferred physical/thermal characteristics of traditional homogeneous hand-hammered copper timpani bowls. The cosmetic hammering of a traditional spun bowl does not necessarily contribute to a homogeneous thickness or hardness as much as it does on a bowl made from multiple pieces of rolled sheet copper of the same thickness. Cosmetic hammering on the older commercially mass produced spun bowl is more likely to add to the aesthetic value of the instrument, rather than to its acoustic value.
However, due to the overwhelming desire for hand-hammered copper bowls (how much of this is media/market generated is curious), some modern timpani manufacturers e.g., Adams, Yamaha and Majestic do have bowl forming processes that purport to maintain a uniform thickness without stretching the copper. These bowls are then uniformly machine hammered to harden them. Adams offers a 1.1 mm hammering on their higher-end bowls. The Majestic Grand Classic timpani are offered with three hammered finishes (coarse, standard or fine). Jaap Ruisch of the Majestic corporation states: “Our hammering is a precise computerized process, which will harden the copper. Because during the process the kettle will fully be supported it will prevent widening the kettle. The finer the hammering, the harder the copper will become and will make sound more open and bright.”52
Is a copper bowl an important ingredient in the production of timpani tone? Yes, but not for the reasons most people are led to believe. Contrary to popular timpani culture, energy output from the vibrating bowl (and frame) doesn’t add all that much to the actual sound output of the instrument. In the frequency range of the preferred modes (65 Hz- 520 Hz), at most, it is less than one percent (1%) in terms of actual output level. Most of the SPL energy output (<1dB) from the bowl/frame occurs in the 1K -2K level, which adds a small amount of collateral color to the sound, but it doesn’t support the preferred modes. One of the important things that the makeup of the bowl material does influence is the amount of mechanical energy loss through the bowl walls. (Fleisher & Fastl)
Whether hand-made and hand-hammered or spun/form molded and machine hammered, factors that have the most influence on the sound are:
1) the volume of air the bowl contains
2) how the bowl material reacts to the energy created by the displacement of the internal air modes
3) its thermal mass properties
4) how much surface area of the bowl is coupled to the frame, external, and mechanical parts
These aspects of the bowl affect the resonance and sustain of the vibrating membrane, and can help with the alignment of the partials into more of a harmonic order. It is the vibrating head and the vibrating head only that produces the viable sound output from timpani. It is questionable whether the finer subtleties of timpani design such as Hammered vs. Spun copper timpani bowls is even noticeable to the general audience or the other musicians in the context of the sound of a full symphony orchestra, but it is certainly a matter of concern to contemporary timpani culture (perhaps market driven hype) as it is debated incessantly.