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. (In practice, what matters most is not only the material’s conductivity, but also the bowl’s thermal mass and construction: thickness, physical mass, surface treatments/coatings, and how quickly the bowl can exchange and retain heat. Fiberglass can behave as an insulator; metals conduct well, but differences in mass and construction can still lead to different thermal behavior in real instruments.) 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 of 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) influences 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.
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 wood 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 bowl and frame do not radiate significant pitched sound directly; their role is to shape how much energy the head and enclosed air modes can use to sustain a tonal signal.
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.”Fleischer & 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 tends to accept a greater share of the available vibrational energy from the air/head system, so more of that energy ends up as bowl/stand vibration instead of staying in the head + air modes. 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
A heavier 32 oz. copper bowl has a greater physical mass than does a 24 oz. bowl, which tends to create a weaker coupling making it more resistant to movement from the energy of the displaced internal air mass. This lengthens the decay time of the vibrating head because the energy from the displacement of the trapped air is not as readily lost as mechanical energy through the vibrating bowl walls, frame and external parts; consequently, this energy can be used to influence the vibrations of the head potentially generating more volume and sustain (see Fleischer & Fastl).
The actual aural differences between drums with 32 oz. bowls and those with 24 oz. bowls to someone in the audience is debatable, but many professional timpanists report that the most noticeable differences in sound happen during the attack portion of the envelope (attack transients), and when the instruments are being played at sustained high dB levels. During the attack, drums with 24 oz. copper bowls are purported to have more of an immediate response to the sound (more attack transients). This is because the mechanical energy loss through the bowl walls has the potential to add more collateral color (percussive attack transients) to the sound than drums with 32 oz. bowls. However, at sustained higher dB levels e.g., a loud roll, drums with 24 oz. copper bowls are purported to distort the sound to some degree, and the drums are said to “bark” more than those with 32 oz. bowls. This occurs because more of the available vibrational energy is directed into high-frequency structural motion during the attack, leaving less energy to support the sustained membrane modes. Consequently, the vibrating head does not have the mechanical energy needed to support the necessary volume, and the drum is often said to be “over played.” Needless to say that in the hands of a master, either will sound fine.
Which should I buy? 24 oz. or 32 oz. hand-hammered copper bowls?
32 oz. copper bowls have more physical mass and better thermal mass than do 24 oz. copper bowls.
1) The greater physical mass tends to lengthen the decay time (sustain) of the head because not as much energy is lost via transmission through the bowl walls; this conserved energy can therefore be used by the head to produce sound.
2) The greater thermal mass (transfer/conduction and retention of heat) helps the bowl retain a more stable internal temperature, resisting rapid environmental changes and helping maintain stable air density inside the bowl. The air inside and outside of the bowl work together as a single process to fine tune the partials of the vibrating head.