Environmental Stability and Timpani Pitch
The consistency of timpani pitch depends on stable environmental conditions. A timpano does not produce a true harmonic series, but its preferred modes can cooperate well enough to create a stable principal tone and a convincing pitch center. A crucial part of that cooperation is the way the vibrating head interacts with the air above and below it, a phenomenon known as air loading.
Environmental stability does not make timpani pitch perfect; it helps preserve the compromise that was achieved when the drum was last tempered. The head, bowl, enclosed air, surrounding air, mechanism, room, and player’s ear all contribute to the final result. Ideally, the internal and external air conditions should be similar to the conditions present when the heads were last tempered in a stabilized environment. Since ideal conditions are not always possible, tempering may become necessary when environmental conditions change.
Caveat: Temper only when the drums will remain in a consistent environment for a substantial amount of time. Temper only after the drums have acclimated to the new environmental conditions. Do not clear a drum immediately after a major environmental change. Let the drum acclimate, then test the principal tone with soft and loud strokes before deciding whether the head truly needs tempering.
The Three-Part System: Head, Bowl Air, and Room Air
Both the vibrating head and the volume of air inside the bowl have specific modes of vibration. The air inside the bowl, the air outside the drum, and the moving head all interact. This interaction influences how the head vibrates, how its modes decay, and how clearly the listener perceives the drum’s pitch center.
Preferred Modes (1,1) (2,1) (3,1) (4,1) (5,1) of the vibrating head.40
Vibrational modes of the air enclosed within the bowl 41
The frequencies of the vibrational modes of the internal air enclosed within the bowl are higher than the frequencies of the membrane modes to which they couple.41 Since the motion of these air modes is affected by the viscothermal characteristics of the enclosed air, changes in temperature, humidity, and pressure can alter how those air modes interact with upper partials of the vibrating membrane. Consequently, certain notes may seem to sing more on some days than others, or the overtones may sound more in tune depending on environmental conditions.
Viscothermal characteristics include the viscosity of the internal air, meaning its resistance to flow, and thermal conduction, meaning the transfer of heat energy through the air. These losses influence how internal air modes decay and how they couple to the head. N.B. The viscothermal characteristics of a gas are different from those of a liquid. The viscosity of liquids generally decreases with an increase in temperature, while the viscosity of gases generally increases with an increase in temperature. Thus, upon heating, liquids flow more easily, whereas gases flow more sluggishly. When a gas increases in temperature at atmospheric pressure, its viscosity increases. A decrease in temperature lowers its viscosity.49 57
For the player, the practical point is simple: changes in temperature, humidity, and pressure alter the air/head system, and the drum may respond differently until it acclimates. When environmental conditions change, the head may still seem close at individual lugs but fail as a whole membrane. The practical test is whether the principal tone remains stable across the playing area, dynamic range, and decay.
With the head as the primary vibrating system, the air mass outside the drum and the air enclosed inside the bowl both influence the motion of the head. This is especially important for the preferred modes. When these modes are aligned well through proper head tensioning and air loading, they produce a near-harmonic pitch structure that gives the drum its sense of pitch, harmonicity, and projection.
Because the head is clamped at the bearing edge, or lip, the bowl acts as a baffled radiator and an acoustically active enclosure. It is not simply a passive container. The bowl separates much of the front and rear radiation of the head, supports internal air modes, and changes how the membrane modes radiate and decay. It should not be treated as a simple resonator, but the enclosed air is still an active part of the system.
The vibrating system, assuming significant tension on the head, is influenced by three main factors:
1) the diameter, tension, and mass of the membrane
2) the volume and viscothermal characteristics of the air modes inside the bowl
3) the density and pressure of the air inside and outside the bowl, including the effect of air loading
All three are important, but they do not contribute in the same way. Head tension, diameter, and mass establish the basic membrane behavior. Air loading and the enclosed air modify that behavior, helping the preferred modes move toward the near-harmonic relationships that make timpani pitch possible. When the internal and external air conditions differ significantly from the conditions present when the head was last tempered, the preferred modes may not respond in the same way, and the sense of harmonicity, projection, and pitch stability may be affected.
Graphic courtesy of Gordon Reid
Three-Part System Defining
the Harmonicity of Timpani Pitch
1) internal air modes
2) external air pressure
3) vibrating membrane
What Air Loading Actually Means
Air has mass. Air density describes how much mass is contained in a given volume of air, while air pressure describes the force that air exerts on a surface. Both density and pressure influence how the head couples to the surrounding air. Air density decreases with altitude. Temperature, humidity, and barometric pressure all affect the density of the air. Cold air is generally denser than warm air, and warm air is less dense and therefore more buoyant.13
Unlike a vibrating string, which requires a carefully designed coupled surface to propagate its sound efficiently, a vibrating timpano head couples strongly to the surrounding atmosphere. Compared with a string, a timpano head is much larger, and the surrounding air mass, both inside and outside the drum, interacts substantially with the vibrating modes of the head. This phenomenon is called air loading and is one of the main factors responsible for establishing the near-harmonic relationship among the preferred modes.
Air loading is the effect of the surrounding air mass on the moving head. As the head vibrates, it must move air above and below the membrane, and that moving air changes the frequencies, decay rates, and radiation behavior of the membrane modes. Air loading lowers the natural frequencies of vibration from those of an ideal circular membrane. This effect is strongest for the lower modes, especially mode (1,1), mode (2,1), and related preferred modes, and it plays a significant role in adjusting otherwise inharmonic partials into relationships that sound more nearly harmonic.
Since air density is not constant, factors that affect air density, barometric pressure, temperature, and humidity, also affect how a timpano head vibrates. The most noticeable effects are slight fluctuations in pitch, changes in response, and changes in the color of the sound when air density changes.
Temperature, Humidity, and Pressure
Even subtle changes in air temperature, barometric pressure, and humidity can affect the actual pitch and perceived response of timpani. It is well documented that humidity affects timpani with natural skin heads. Synthetic PET/Mylar heads are much less sensitive to moisture than natural skin heads, but they are not completely isolated from environmental change. They still respond to temperature, tension history, creep, collar formation, impact damage, and the air-loading conditions of the drum.
A synthetic head may slow heat exchange compared with an open bowl, but the larger issue is that the whole drum, head, bowl, internal air, external air, metal parts, and mechanism, needs time to reach a stable condition. The vent-hole in the bottom of the bowl helps balance the internal and external air masses. Playing the drum can also help move air through the vent hole, gradually replacing internal air with external air from the room.
It can be detrimental to the harmonicity of the spectrum to attempt to clear or temper timpani immediately after the drums have been moved from one environment to another, such as from a dry air-conditioned room to a warm humid outdoor venue, or from a cold storage room to a warm stage. The best thing to do in this situation is to let the drums sit for at least forty-five minutes to an hour and acclimate to the new environment. Larger changes in temperature, humidity, or altitude may require more time.
Playing on the drums will not hurt them. In fact, it can help equalize the air mass in the bowl with the outer air, but it may also play tricks with your ears while the system is still changing. Pressing gently near the center of the head or playing a few loud center strokes can help move air through the bowl, because center strokes strongly excite the lowest physical mode and other concentric modes of vibration. This can encourage air exchange faster than striking only in the normal playing area, but it should be done carefully and should not replace acclimation time. It is always beneficial to move the timpani first when moving percussion equipment so the drums have as much time as possible to acclimate.
If a drum is sounding good and has a long sustained principal tone with clear near-harmonic overtones present at each tuning lug, the head does not need clearing. In general, it is best to temper or fine-tune the heads after they have been played on, for example after a rehearsal, rather than before, giving the drums time to adjust to the environment. In situations where the player is unfamiliar with the instruments, it is recommended that the drums be in position two hours before the service so the player can arrive early, check the drums, and make any necessary adjustments. Requests for early arrival of timpani are standard in the business.
Why Synthetic Heads Still Change
When intonation issues occur with timpani that have been moved from one location to another, improper equipment handling during transport is often blamed. More often than not, a substantial change in environmental conditions, especially 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 described by its manufacturer as retaining its physical properties over a wide temperature range, –70 to 150 °C or –94 to 302 °F, which is well beyond normal playing conditions. However, when under tension, Mylar® can still expand and contract slightly when subjected to abnormal heat or cold, which can affect the tension of the head. Extreme temperature conditions can also affect the flexibility of the head and how the head vibrates, which will influence the frequency of the pitch-producing modes.
The main factors affecting the dimensional stability of Mylar® film are strain relief, thermal expansion, hygroscopic expansion, and creep. While these influences may be minimal under normal playing conditions, they should not be disregarded. One should also consider the dimensional stability of metal parts, such as a copper timpani bowl, which are also subject to slight expansion and contraction when undergoing abnormal temperature changes.
Even when timpani have not been moved, but the weather changes, many players notice that drums with Mylar® heads may change pitch slightly compared with 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, flattening, sharpening, and other perceived pitch deviations often occur because of fluctuations in air temperature, barometric pressure, and relative humidity, all of which affect air density.
At the same temperature and pressure, humid air is slightly less dense than dry air. In practice, temperature usually has the stronger immediate effect on air density, while humidity has a much stronger direct effect on natural skin heads. All three factors—temperature, humidity, and pressure—affect the viscothermal characteristics of the air mass inside the drum and the air mass outside the drum.
Because air loading is a primary factor contributing to the perceived harmonicity of timpani pitch, changes in air density should concern the timpanist. Just as predicting the weather is difficult, it is difficult to predict exactly how temperature, humidity, and pressure will interact with a vibrating timpani head in every situation. However, changes in air temperature often have the most noticeable immediate effect in performance situations.
There are three main factors that contribute to the pitch of the instrument:
1) diameter, tension, and mass of the membrane, including the dimensional stability of the bowl material and head material under abnormal temperature conditions
2) the volume and viscothermal characteristics of the air modes inside the bowl
3) the density and pressure of the air inside and outside the bowl
Addressing air temperature alone, a significant lowering of air temperature will increase air density. The air becomes heavier, which can lower the frequencies of the preferred modes, especially modes (1,1) and (2,1), and make the internal air less viscous. A significant increase in air temperature will decrease air density. The air becomes lighter, which can raise the frequencies of those same preferred modes and make the internal air more viscous. N.B. The viscothermal characteristics of a gas differ from those of a liquid. When a gas increases in temperature, so does its viscosity. A decrease in temperature lowers its viscosity.49 57
If the tension of the head remains the same, a lowering of the initial preferred modes will cause the drum to sound flat with respect to where the tuning indicator was initially set, and the feel of the head may seem less responsive or more spongy. When the air temperature rises and the head tension remains the same, the raising of the initial preferred modes will cause the drum to sound sharp with respect to where the tuning indicator was initially set, and the increased viscosity of the internal air may make the feel of the head seem more responsive or more firm. These are tendencies, not absolute rules, because the drum is always responding as a complete system.
Cooler and drier conditions tend to produce a lower base pitch, with less resistance from the internal confined air and more resistance from external air.
1) At the same temperature and pressure, less humidity increases air density; dry air is heavier than humid air.
2) Cooler temperatures increase air density; the air is heavier and can lower the preferred modes.
3) Internal air viscosity is lower, which may make the head feel less responsive or even spongy.
Warmer and more humid conditions tend to produce a higher base pitch, with more resistance from the internal confined air and less resistance from external air.
1) At the same temperature and pressure, more humidity decreases air density; humid air is lighter than dry air.
2) Warmer temperatures decrease air density; the air is lighter and can raise the preferred modes.
3) Internal air viscosity is higher, which may make the head feel more responsive or even stiff.
Gauges as Warning Signs, Not Answers
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. Gauges record mechanical position, not acoustical truth. A gauge can show that the mechanism returned to the same position; it cannot prove that the head, air, room, and ear are producing the same pitch center.
Not only can the pitch of the ensemble change when the air in the room fluctuates, but so can the pitch of timpani, even those with plastic heads. If you are accustomed to using tuning indicators, use them only as a reference and remember that they reflect the conditions of the drums when they were set. Fluctuations in head tension, room temperature, ensemble pitch, and room response can all affect how you perceive the pitch of the drum relative to the gauge setting.
Transport, Altitude, and Long-Term Relocation
When transporting timpani from one location to another where the air density differs drastically, such as from sea level to high altitude, a re-tensioning or re-tempering of the head may be needed to accommodate the new environment. For long-term relocation to a significantly different altitude or climate, the heads may need to be re-tensioned or re-tempered after arrival. Do not fully de-tension or disturb the seating of a head unless you intend to re-seat and re-temper it, and follow the instrument maker’s or technician’s recommendations for transport.
Once the drums reach their destination, let them acclimate fully. Then check head centering, seating, principal tone stability, and the agreement between soft and loud strokes before making major adjustments. Any previous tensioning done to accommodate the air density of the original environment may no longer produce the best result in the new environment.
Final Practical Takeaway
For the best long-term results, you should mount and temper the heads in the environment in which the drums will be consistently played. Drums that are never moved can still change in clarity if the atmospheric conditions of the room change, such as cold, dry winter air compared with warm, humid summer air in the same room. The fine tuning of the upper partials depends on the atmospheric conditions of the physical space where the heads were last tempered. Unfortunately, those conditions may even change during a performance.
There is no magic bullet or perfect tool that will automatically temper timpani heads. At best, the result will always be a compromise because timpani do not vibrate with a true harmonic series. But with good tempering, a stable environment, and clear listening, 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 elusive missing fundamental can sometimes be detected.