In Search of the Missing Fundamental: by Richard K. Jones
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How “The Weather” Affects Timpani Pitch – Environmental Considerations

When intonation issues occur with timpani that have been moved from one location to another, improper equipment handling during transport is often the scapegoat. More often than not, a substantial change in environmental conditions (in particular, extreme fluctuations in temperature and atmospheric pressure) can be the cause of erratic timpani pitch. Extreme changes in temperature can also affect the dimensional stability of Mylar.®  Mylar® film is purported by is manufacturer to retain its physical properties over a wide temperature range (–70 to 150 °C or –94 to 302 °F), which are well within normal playing conditions. However, when under tension Mylar® can expand and contract slightly when subjected to extreme heat or cold, which will affect the tension of the head. The main factors affecting the dimensional stability of Mylar® film are strain relief, thermal expansion, hygroscopic expansion, and creep. While minimal, these influences should not be disregarded. One should also consider the dimensional stability of metal parts, e.g. a copper timpani bowl, which are also subject to slight expansion and contraction when under going abnormal temperature changes.

Even when timpani haven’t been moved, but the weather changes, many players often notice that their drums with Mylar® heads change in pitch slightly compared to where the gauges had been set. This is not necessarily because the Mylar® itself is expanding or contracting. When timpani (with well seated heads) are used under normal/stable playing conditions, the flatting, sharping and other perceived pitch deviations likely occur because of fluctuations in air temperature, barometric pressure and relative humidity, all which affect air density. All three of these factors impact the viscothermal (viscosity and thermal conduction) characteristics of the air mass trapped inside of the drum, and the air mass outside of the drum.  Since air loading (weight of the atmosphere pressing down on the head reacting against the pressure of the mass of air inside of the drum) is the primary factor contributing to the harmonicity of timpani pitch, changes in air density should be of concern to the timpanist. Just as predicting the weather, it is difficult to predict how all three will always interact with a vibrating timpani head since there is both an internal and external air mass affecting how the drum makes pitch. However, changes in air temperature have the most direct effect on air density and are probably most noticeable by the timpanist in performance situations.

There are three main factors that contribute to the pitch of the instrument:

1) diameter, tension and mass of the membrane (subject to dimensional stability of the bowl material and head material when under abnormal temperature conditions)
2) the volume and viscothermal characteristics (i.e., stiffness and thermal conduction) of the air modes inside of the bowl
3) the density of the air (air pressure) inside and outside of the bowl

Addressing air temperature alone with regard to air density, a significant lowering of the air temperature will increase air density (the air gets heavier), which in turn will lower the frequencies of the preferred modes, in particular modes 1,1 and 2,1 as well as and make the air modes inside less viscous. A significant increase in air temperature will decrease air density (the air gets lighter), which will raise the frequencies of those same preferred modes and make the air modes inside of the bowl more viscous. (N.B. The viscothermal characteristics of a gas are just the opposite of those of a fluid. When a gas increases in temperature, so does its viscosity. A decrease in temperature lowers its viscosity)49

If the tension of the head (dimensional stability) remains the same, a lowering of the initial preferred modes will cause the drum to be flat with respect to where the tuning indicator was initially set and the “feel” of the head may seem less responsive. When the air temperature rises and the head tension remains the same (dimensional stability), the raising of the initial preferred modes will cause the drum to be sharp with respect to where the tuning indicator was initially set and the increased viscosity of the internal modes may make the feel of the head seem more responsive. In either case, if your tuning indicators are accurate, they can be a great assets in determining how the pitch of the instruments is fluctuating and how your drums are responding when the air temperature changes in the performing venue under normal conditions (i.e. not extreme heat or cold).

Cooler & more dry conditions =  a lower base pitch with less resistance from the internal confined air, more resistance from external air
1) Less humidity increases air density, i.e. the air is heavier (dry air is heavier than humid air)
2) Cooler temperatures increase air density, i.e. the air is heavier and weighs on head yet sinks in bowl
3) Internal air viscosity is lower making the head feel less responsive or even spongy

Warmer & more humid conditions = a higher base pitch with more resistance from the internal confined air, less resistance form external air
1) More humidity decreases air density i.e. the air is lighter (humid air is lighter than dry air)
2) Warmer temperatures decrease air density, i.e. the air is lighter and rises inside of the bowl
3) Internal air viscosity is higher making the head feel more responsive or even stiff

It is very important to remember that even when the most accurate tuning indicators are set properly, they are still nothing more than the proverbial canary in the coal mine. They serve only to indicate that some sort of change has occurred. Not only can the pitch of the ensemble change when the conditions of the air in the room fluctuate, but so does the pitch of timpani, even those with plastic heads. If you are accustomed to using tuning indicators (gauges) use them only as a reference and be cognizant of the fact that they only can reflect the conditions of the drums when they were set. Fluctuations in the tension of the head, temperature of the room, moving pitch of the ensemble can all affect how you perceive the pitch of the drum relative to how you set the gauges.

When transporting timpani from one location to another where the air density differs drastically, e.g. sea level to high altitudes, a re-tensioning/tempering of the head should be done to accommodate for the air density of the new environment. Before the timpani leave the original location, block the pedals and remove all of the tension from the heads without removing the counterhoops/heads from the drums. Once the drums reach their destination, center, tension/temper the heads based on the new air density.  Any previous tensioning done to accommodate for the air density of when the drum was originally tempered will negated and the new tensioning will reflect the current environment (air density) only.

The bottom line is that for the best long-term results, you should always mount and temper the heads in the environment in which the drums will be consistently played. You may even notice that drums, which are never moved, will change in the clarity of sound if/when the atmospheric conditions of the room change, e.g. cold dry winter air vs. warm humid summer air in the same room. The fine tuning of the upper partials is dependent upon the atmospheric conditions of the air in the physical space of where the heads were last tempered. Unfortunately, that may change even during a performance. Timpani are like humans and function best when the temperature is between 23.5 °C (74.3 °F) and 25.5 °C (77.9 °F), airflow velocity of 0.18 m/s and when the relative humidity is in the range of 30-60%.

There is no magic bullet or perfect tool that will automatically temper timpani heads. At best, the results will always be a compromise since timpani do not vibrate with a true harmonic series as do all other standard orchestral instruments, but with good tempering you will hear clear near harmonic overtones and a strong principal tone at all dynamic levels throughout the range of the drums. When the drums are well-tempered, the illusive missing fundamental can be detected.

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