Why Copper?
Timpani with hardened copper bowls are purported to sing more than other types of bowl materials, and are preferred by most professional timpanists. The hardness, weight and physical mass affect how much of the mechanical energy generated by the displaced internal air mass will be absorbed by the bowl walls. Most of the measurable energy output from the bowl and frame/parts (up to 16% of the overall instrument output in some cases, as measured by Fleischer & Fastl) is in the 1k-2k frequency range and does not support the preferred modes. The measurable energy output in the frequency range of the pitch is <1%, and is negligible for frequencies below that. Fleischer & Fastl
Timpani can be designed (as well as physically placed on the stage) so that some of the energy generated by the motion of the head and internal air will be transferred to the bowl and frame, as well as to the floor/risers if so desired. This is called coupling. This is often why some European timpanists use heavy timpani, often placed on hollow on risers, or heavy timpani that couple directly to the floor; it is believed that the transmission of energy to the risers or floor enhances the sound of the instrument.
Within the current laws of physics, the law of conservation of energy states that the total energy of an isolated system remains constant—it is said to be conserved over time.56 Energy can neither be created nor destroyed; rather, it transforms from one form to another. The transformation of kinetic energy when you strike a timpano is as follows:
1) you strike the head with X amount of force generating energy
2) this energy transforms into mechanical energy via the various modal vibrations of the head
3) this energy is also transformed into mechanical energy via the displacement of the various air modes inside of the bowl
4) any of the above mechanical energy that is transferred to the bowl and frame/parts/floor (coupling) won’t get used by the head and the internal air modes to generate and sustain the pitch producing audio signal. The vibrating head is where the only viable pitch portion of the sound is generated.
In essence, how much of the overall energy that is transferred or coupled to the bowl, frame or floor in turn affects how much of the overall energy will then be available to influence the internal air modes that support the vibrations of the head. “It is suspected that resonances of distinct parts of the instrument stand can convert vibration energy into heat which, in consequence, is no longer available to the generation of sound.” Fleisher & Fastl
The influences of the volume of air inside the bowl (air modes), and the air mass outside of the bowl work together with the vibrating modes of the head as a single process to fine tune the pitch so that it has a sense of harmonicity. The higher thermal mass (transfer/conduction and retention of heat) of a 32 oz. bowl allows the bowl to retain a more consistent air density, and reach and maintain thermal equilibrium more efficiently, which keeps the internal and external air masses more in balance. The somewhat lower thermal mass of a 24 oz. bowl may not be as efficient as a 32 oz. bowl, but it offers a different function.
A thinner, (lower physical mass) copper bowl (< 24 oz./ 20 gauge/ .0323 inches/ .820 mm) has the potential to respond more efficiently to the displaced internal air causing the bowl to vibrate. These vibrations can add more inharmonic resonant frequencies (desired collateral color) to the first few hundred milliseconds of the sound. A 24 oz. bowl can absorb more of the energy of the displaced air and vibrating head than can a 32 oz. bowl (of the same hardness), transferring it (coupling) to the frame/external parts/floor, which tends to shorten the length of the decay time of the vibrating head to some degree. This is often a desirable trait in a drum when you are playing repertoire where the volume and sustain of the sound is not as important as the response time, pitch and articulation of the sound.
Early to mid twentieth century timpani produced by Dresdner Apparatebau (makers: Jähne & Boruvka and Spenke & Metzl) are often characterized as having this sound trait because of their (somewhat inconsistent) thin, lead washed “red leopard” bowls and very round lip (bearing edge). Not a lot of resonance or sustain from the player’s perspective, but exceptional pitch, articulation and blend from the listener’s perspective. The combination of the thin copper (<.820 mm) and the heavy lead content of the wash tends to absorb the energy from the internal displaced air and vibrating head (usually calf heads) more than do thicker untreated bowls. It was perhaps a design of economy, rather than one of acoustics, but none the less, it has become a coveted sound among many timpanists today.
Dresdner Apparatebau: Spenke und Metzl
circa late 1950s early 1960s
Courtesy of Tom Freer