Why Can Timpani Produce a Definite Pitch?

Ordinary drums differ in height of sound, yet we usually hear the impact and a complex spread of overtones first. Timpani can play recognisable written pitches such as D or A. This is not simply because their heads can be tightened. A circular membrane on its own vibrates in many modes whose frequencies are not neatly arranged as integer multiples, unlike the harmonic pattern of an ideal string.

A timpano stretches one membrane over a deep, bowl-shaped cavity. As the head moves, the membrane, enclosed air and outside air interact. This coupling rearranges the frequencies and decay of some modes, bringing several strong components close enough to harmonic relationships for the ear to group them around one pitch. The pedal changes the tension of the entire head, shifting that group higher or lower. Bowl size sets a useful register, which is why orchestras normally use several drums of different diameters.

When listening, separate the initial mallet strike from the sound that follows. The pitch often becomes clearer after the attack. Harder or softer mallets change which upper components stand out, but they do not tune the instrument for the player. A good timpani sound has both the edge of percussion and the direction of a bass instrument: it can reinforce a harmonic root, create semitone tension, or glide towards another pitch under a sustained roll.

Definite pitch therefore does not mean fixed keys like a piano. A timpanist must listen, tune and correct during performance. The instrument provides conditions in which pitch can emerge; the ear, pedal and striking position complete its stability.

https://www.yamaha.com/en/musical_instrument_guide/timpani/mechanism/mechanism002.html
https://philharmonia.co.uk/resources/instruments/percussion/