Construction
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 typically consist of a skin 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, also known as a log drum as it is 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 placed over the opening of the drum, which in turn is held onto the shell by a "counterhoop" (or "rim"), which is then held by means of a number of tuning screws called "tension rods" which screw into lugs placed evenly 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, counterhoop material, type of drumhead used and tension applied to it, position of the drum, location, and the velocity and angle in which it is struck.
Prior to the invention of tension rods drum skins were attached and tuned by rope systems such as that used on the Djembe or pegs and ropes such as that used on Ewe Drums, a system rarely used today, although sometimes seen on regimental marching band snare drums. The head of a talking drum, for example, can be temporarily tightened by squeezing the ropes that connect the top and bottom heads. Similarly, the tabla is tuned by hammering a disc held in place around the drum by ropes stretching from the top to bottom head. Orchestral timpani can be quickly tuned to precise pitches by using a foot pedal.
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