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The modern orchestral percussion section can vary from a few standard instruments to a whole arsenal, depending on the demands of the music. Percussion instruments made their way into the orchestra relatively late, as references to European military music or, for a more exotic flavor, in imitation of the Ottoman janissary bands which terrorized Europe for centuries. The most standard percussion instruments today are the xylophone or marimba, the snare or side drum, the cymbals, the gong or tam-tam, the tambourine and the timpani.
The timpani, sometimes called kettledrums, are the most important of the orchestral percussion instruments, mainly because they are capable of producing notes of definite pitch and so can take part in the harmony of the orchestra. They are tuned, each to a given note, according to the composer’s directions in the score, and these notes may be altered as required during the performance of a work, by tightening or slackening the drumhead by means of screws or other mechanisms. Timpani are played with a pair of drumsticks, color-coded in a wide variety of shapes and textures according to the effects required.
The kettledrum dates from antiquity, when it was made from tree-trunks, clay or tortoise shells. During the 13th century the use of a pair was adopted in Europe from the East for martial occasions. In the 17th century, Lully was the first composer to use them in the orchestra. Late romantic composers made especially heavy demands on them, which led, in part, to greater sophistication in the tuning mechanism. Several concertos and sonatas have been written for timpani in this century.
Source: The Norton/Grove Concise Encyclopedia of Music, edited by Stanley Sadie. London: Macmillan Press Ltd.,  ISBN 0-333-43236-3