How “The Weather” Affects Timpani Pitch – Environmental Considerations
Air, and consequently our atmosphere, does have weight. The weight of the air is referred to as air density. This weight (air density) decreases as you go up within the atmosphere. When gravity acts on the air, the air exerts a force upon the earth called pressure. Air pressure varies according to temperature. Cold air is more dense than warm air, i.e., it weighs more. As a result, it tends to sink. Warm air, on the other hand, is less dense. Therefore, it weighs less and tends to rise. Meteorologists say that warm air is buoyant. 13
Unlike a vibrating string, which requires a carefully designed coupled surface to propagate its sound, a vibrating timpano heads strongly couples to the surrounding atmosphere, which in turn propagates the sound. Since air does have weight, air density plays an integral role in affecting how a timpano head vibrates. Compared to a vibrating string, a timpano head is considerably larger in size and the surrounding air mass (both inside and outside of the drum) interacts with the vibrating modes substantially. This phenomenon is called air loading and is the main factor responsible for establishing the near harmonic relationship among the preferred modes.
Air loading is the effect the weight of the surrounding mass of air has on the motion of the timpano head. It lowers the natural frequencies of vibrations from those of an ideal circular vibrating membrane. This effect is strongest for the lower modes especially mode (1,1), (2,1) and plays a significant role in adjusting the inharmonic partials, making them sound more harmonic. Since air density is not a 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 and changes in the color of the sound when air density changes.
Even subtle changes in air temperature, barometric pressure and humidity (factors which determine air density) can affect the actual pitch. It is a well documented fact that humidity affects timpani with natural skin heads. It is also purported that Mylar heads are virtually impervious to the moisture content of the air, but the irony is that since plastic heads were never alive like natural skin heads were, they consequently don’t breathe like natural skins do. Mylar has a very low heat exchange coefficient, which contributes to a slower exchange of air density (temperature mostly) between the inside and the outside of the bowl, which is critical when differing air masses need to be equalized. The vent-hole in the bottom of the bowl is there to help with the balancing of the internal and external air masses. Playing the drum forces the internal air to escape through the hole thereby replacing it with external air from the room.
It can be detrimental to the harmonicity of the spectra to attempt to clear or temper timpani after the drums have been moved from one location to another where differing conditions in the air exist, e.g. a dry air-conditioned room to a warm/hot humid outdoor venue or a cold storage room to a warm stage. The best thing to do in this type of situation is to just let the drums sit for forty-five minutes to an hour and acclimate to the new environment. Playing on the drums won’t hurt them, in fact it will actually help equalize the air mass in the bowl with the outer air, but it will play tricks with your ears. To help expel the internal air from the bowl, press down directly on the center of the head or strike the center of the drum loudly a few times. This excites the drum’s actual fundamental and the other concentric modes of vibration, which will force an exchange of air faster than if you strike the drum in the normal area. It is always beneficial to move the timpani first when moving percussion equipment so that the timpani can acclimate to the new environment.
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, e.g. 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 that the player can arrive early and check the drums over and make any necessary adjustments. Requests for early arrival of timpani are standard in the business. There should be no excuses as to why the drums sound bad if prep work is done ahead of time and the instruments are of reasonable quality.