Why Copper?
The amplitude of sound diminishes over time because energy is lost through various damping mechanisms. The resulting decay in amplitude is the audible effect of this damping. For timpani, there are five possible reasons for energy loss resulting in the damping of the sound emitted from the instrument. 47 48 50
1) radiation of sound
2) internal mechanical losses within the membrane material
3) viscothermal loss in the confined air [inside of the bowl]
4) mechanical loss in the kettle [bowl] walls
5) mechanical loss in the frame and external parts
The sound of a modern timpano (whether you are using a natural or a synthetic head) is shaped by the density and temperature of the enclosed air, as well as by viscous and thermal losses in the boundary layers inside the bowl (thermoviscous effects), which influence the decay behavior of individual modes. These air masses work together to create a single balanced air system. Air loading from the surrounding atmosphere—and especially the interaction between the enclosed air and the vibrating membrane—is what gives timpani their characteristic near-harmonic spectrum and produces a clear, stable sense of pitch. Changes in temperature, humidity, and air pressure alter air density and the speed of sound, which in turn shifts the cavity resonances and the air–membrane coupling. These changes can subtly shift the alignment of the near-harmonic partials. Such shifts can be perceptible to trained listeners.
Copper being an excellent conductor of heat tends to equalize and maintain the temperature of the internal/external air masses more efficiently than other popular bowl materials, which can actually act as insulators – e.g., fiberglass or aluminum. The best conductor of thermal energy is silver. Silver bowls in court settings were primarily chosen for visual impact and ceremonial prestige; any thermal or acoustic advantage would have been secondary, though players may have perceived differences in sound.
Franz Peter Bunsen (German, ca. 1725–1795). Kettle Drums, 1779. Silver, iron, calfskin, textiles; Diameter: 53 cm (20 7/8 in.); Height: 41 cm (16 1/8 in.); 24 kg (52.9 lbs.). The Metropolitan Museum of Art, New York, Purchase, Robert Alonzo Lehman Bequest, Acquisitions Fund, and Frederick M. Lehman Bequest, 2010 (2010.138.1-.4)
In the United States, copper sheet is traditionally bought and sold by the pound. The thickness of the sheet is measured in ounces per square foot rather than by gauge, which describes the thickness of many other sheet metals.
16 oz copper weighs 1 pound per square foot. Therefore a 3′ x 10′ sheet of 16 oz copper would weigh 30 pounds. 20 oz copper weighs 1.25 pounds per square foot and is 25% thicker than 16 ounce copper. 24 oz copper weighs 1.5 pounds per square foot and is 50% thicker than 16 ounce copper. 32 oz copper weighs 2 pounds per square foot and is twice as thick as 16 oz copper. Therefore a 3′ x 10′ sheet of 32 oz ounce copper would weigh 60 pounds. Modern artisan timpani bowls in the U.S. are generally produced from either 24 oz or 32 oz copper sheet.
With respect to practical bowl materials in use today, 32 oz. (17 gauge/ .0431 inches/ 1.09 mm) hand-hammered copper bowls are preferred by most professional American and European timpanists. Why? Hand-hammered artisan copper timpani bowls, in general, function in two ways that most players are perhaps unaware. Both deal with the exchange and loss of energy; heat and mechanical (potential and kinetic), which results in affecting the instrument’s voice. The first is because thermal mass. Copper is an excellent material for heat transfer, and it can also hold more heat than other common bowl materials. The second is the ability for copper to absorb mechanical energy.
The objective of the hand-hammered (whether by hand or machine) artisan method of timpani bowl production is to produce a bowl with a uniform hardness and consistent thickness that:
1) helps to evenly balance the temperature of the air inside and out side of the bowl
2) helps to evenly balance the temperature of the internal air from the bottom to the top of the bowl
3) helps to maintain a consistent temperature, volume and density of the internal air (viscothermal)
4) influence the amount of energy loss through the bowl walls
The even distribution of air temperature makes the drum more efficient at reaching and maintaining a steady viscothermal state (thermal equilibrium). Maintaining a consistent temperature of the volume and density of internal air and surrounding external air helps keep the upper preferred modes in harmonic alignment. The harmonic alignment of the upper preferred modes contributes significantly to the strength of the virtual pitch the instrument can create. Adjusting the harmonic alignment of the preferred modes is what one does when tempering or clearing a head.