The largest part of the kettledrum is the bowl, or pan, which is made of sheet copper. Its shape ranges from hemispherical to funnel-shaped, depending on the period it dates from, its size and the acoustic demands made on it – large kettledrums have a deeper bowl. The bowl is the resonator.
The open top of the bowl is covered by calf or goat skin stretched across a counterhoop. The counterhoop is joined to the bowl by a pull ring; the skin tension can be altered by means of a screw mechanism which presses the pull ring against the vellum. On the underside of the shell there is a small aperture; this does not serve to project the sound but to maintain even air pressure inside the bowl. A closed bowl would hinder the free vibration of the vellum after it had been struck.
The bowl is mounted on an iron stand which rests on a heavy metal base. The iron bars do not touch the bowl so as not to damp its vibrations.
The most sensitive part of the timpani is the vellum, an evenly-surfaced calf skin which has been smoothed during production with a scraper or pumice-stone. To attain a pure tone it is vital that the skin tension is absolutely even over its entire surface. In addition, the vellum is very sensitive to changes in temperature and humidity; during a concert or an opera the instrument’s tuning must be checked several times because the change in temperature and humidity caused by the audience quickly affects the vellum and therefore the pitch. A high level of humidity slackens the vellum. Since the middle of the 20th century plastic skins have also been used, and these have the advantage of being impervious to changes in the atmospheric conditions.
Modern timpani are tuned with a tuning pedal which operates the pull rings by way of rods on the outside (and occasionally on the inside) of the shell. Activating the pedal increases the tension of the vellum, which raises the pitch. In this way a kettledrum’s pitch can be altered by as much as a sixth. The purest tone is produced in the middle of the compass. A tuning gauge with a scale of pitches is mounted on the rim and is used as a rough check on the tuning. For fine-tuning, which is done with a handle, the so-called fine-tuning handle, the timpanist must rely on his own exceptionally sensitive sense of hearing.