The percussion group is in turn divided into a series of subfamilies, the main ones being: membranophones (a membrane, usually stretched over a resonance box, is set vibrating), metallophones (a metal object is set vibrating, with or without a resonator), and wooden percussion instruments (instruments built entirely of wood, hollow or not). Within each of these subfamilies a further distinction must be made, between

1. percussion instruments of definite pitch (that is, whose pitch is clearly perceptible),
2. percussion instruments of indefinite pitch (that is, with a timbre so inharmonic that it does not allow a clear distinction of pitch).

The king of the membranophones of definite pitch is undoubtedly the timpani (J. Brahms – Symphony no. 1 – first movement), essential in the orchestra for building up (with rolls) and delivering great sonorities, but also (in a less conventional but more evocative way) in gloomy pianissimos full of tension and expectation. Among those of indefinite pitch we mention at least the bass drum, the snare drum, the tom-tom and the bongos; but the family is truly boundless, considering the valuable contribution of non-European (above all African) cultures. Among the metallophones of definite pitch the vibraphone, the tubular bells and the glockenspiel stand out above all (the glockenspiel has a silvery, sparkling timbre, and is often used like a music box, in the theatre too, as in Papageno's famous aria in W. A. Mozart's The Magic Flute); while those of indefinite pitch include the very wide variety of gongs and tam-tams, suspended cymbals and small cymbals of the most diverse shapes, sizes and materials. The ensemble of these instruments in the folk groups of Bali and Java is particularly striking. The wooden percussion instruments of definite pitch are the marimba and the xylophone (made of wooden bars arranged horizontally on a frame and struck with special mallets); those of indefinite pitch include woodblocks, castanets, claves and the guiro, all suited to giving an "exotic" character or to evoking a dance rhythm belonging to a particular geographical area, as the following examples show:

• bongos in African folk music,
• marimba in W. Disney's The Little Mermaid,
• castanets in the Malagueña from Ravel's Rapsodie espagnole.

Acoustic characteristics

There are a great many percussion instruments, and each has its own acoustic characteristics. Moreover, the general classification reflects a classification by physical source of the sound (generated by the vibration of membranes, bars or plates). So, for an analysis of the acoustic characteristics of (at least one member of) each family, see the pages on the individual instruments:

• Timpani
• Snare drum
• Other percussion instruments: triangle, tambourine, castanets, woodblock

Why do some percussion instruments have a definite pitch and others not? In a string or a pipe the frequencies of the modes of vibration are (almost) integer multiples of the fundamental, that is, harmonics, and the ear fuses them into one note. In membranes, bars and plates this is not the case: in an ideal circular membrane, for example, the first modes have frequencies 1, 1.59, 2.14, 2.30, … times that of the lowest mode, and in a free bar 1, 2.76, 5.40, …^[1] These "inharmonic" sounds do not give a precise note. To obtain a definite pitch, makers modify the instrument so as to make some modes almost harmonic: in timpani the air in the kettle and the surrounding air shift the frequencies of the membrane; in the bars of the marimba and the xylophone the central part is thinned until the second mode falls at 4 or 3 times the frequency of the first^[1].

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Further reading and links

• Related questions and answers:
  • Why don't all percussion instruments produce a precise note the way string instruments do?
  • Why are some percussion instruments, such as the timpani, tuned while others, such as the cymbals, are not?