The amplitude of sound will diminish over time due to resistance. The decay of amplitude over time is called 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) mechanical loss in the membrane
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 heavily influenced by the air density and the viscothermal characteristics (viscosity and thermal conduction) of the volume of air inside the bowl, and the air mass out side of the bowl. These air masses work together to create a single balanced air system. It is in fact the weight or the earth’s atmosphere (air loading) that makes it possible for a drum (timpani) to produce a sense of pitch. When the temperature (as well as moisture content and air pressure) differs between the volume of air inside of the bowl and the air outside of the bowl from when you tempered the drums, the structure of the overtone series changes. A spectral frequency shift occurs resulting in an in-balance of the near harmonic partials. Even slight environmental differences are noticeable to the trained ear.
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. Perhaps this is why many of the court timpani from the 16th and 17th centuries which were made of silver as they were thought to sound as well as look better than their copper counterparts.
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.
One aspect of why the copper bowl (24 oz. or 32 oz.) is a critical component for the tone of the timpani is because of its thermal mass. Copper is an excellent material for heat transfer, and it can also hold more heat than other common bowl materials. This is called thermal mass.
The other aspect 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 thermal equilibrium in a steady viscothermal state. Maintaining a consistent temperature of the volume and density of internal air and surrounding external air helps keep the preferred modes in alignment.
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) 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 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.
In 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 vibrations of the head
3) this energy is also transformed into mechanical every via the displacement of the air inside of the bowl
4) any of the above mechanical energy that is transferred to the bowl and frame/parts (coupling) won’t get used by the head and the internal air modes to generate and sustain the 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 and frame 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, 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 volume and 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 and external parts, 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
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 Fleisher & 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, 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. This is because the mechanical energy loss through the bowl walls has the potential to add more collateral color (percussive attack) 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 and the drums are said to “bark” more than those with 32 oz. bowls because the vibrating head does not have the mechanical energy needed to support the volume. 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 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 better thermal mass (transfer/conduction and retention of heat) allows the bowl to retain a more consistent air volume, and reach and maintain thermal equilibrium more efficiently, which keeps the internal and external air masses more in balance. The air inside and outside of the bowl work together as a single process to fine tune the partials of the vibrating head.