The drum is a member of the percussion group, technically classified as a membranophone. Drums consist of at least one membrane, called a drumhead or drum skin, that is stretched over a shell and struck, either directly with parts of a player's body, or with some sort of implement such as a drumstick, to produce sound. Drums are the world's oldest and most ubiquitous musical instruments, and the basic design has remained virtually unchanged for thousands of years.[citation needed] Most drums are considered "untuned instruments", however many modern musicians are beginning to tune drums to songs; Terry Bozzio has constructed a kit using diatonic and chromatically tuned drums. A few such as timpani are always tuned to a certain pitch. Often, several drums are arranged together to create a drum set that can be played by a musician.
The shell almost invariably has a circular opening over which the drumhead is stretched, but the shape of the remainder of the shell varies widely. In the western musical tradition, the most usual shape is a cylinder, although timpani, for example, use bowl-shaped shells. Other shapes include a frame design (tar, Bodhrán), truncated cones (bongo drums, Ashiko), goblet shaped (djembe), and joined truncated cones (talking drum),
Drums with cylindrical shells can be open at one end (as is the case with timbales), or can have two drum heads. Single-headed drums normally consist of a skin or other membrane, called a head, which is stretched over an enclosed space, or over one of the ends of a hollow vessel. Drums with two heads covering both ends of a cylindrical shell often have a small hole somewhat halfway between the two heads; the shell forms a resonating chamber for the resulting sound. Exceptions include the African slit drum, made from a hollowed-out tree trunk, and the Caribbean steel drum, made from a metal barrel. Drums with two heads can also have a set of wires, called snares, held across the bottom head, top head, or both heads, hence the name snare drum.
On modern band and orchestral drums, the drumhead is affixed to a hoop (also called a "rim"), which in turn is held onto the shell by a "counterhoop", which is then held by means of a number of tuning screws called "tension rods" (also known as lugs) placed regularly around the circumference. The head's tension can be adjusted by loosening or tightening the rods. Many such drums have six to ten tension rods. The sound of a drum depends on several variables, including shape, size and thickness of its shell, materials from which the shell was made, type of drumhead used and tension applied to it, position of the drum, location, and the velocity and angle in which it is struck.
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