The drum is a member of the percussion group of musical instruments, which is technically classified as the membranophones. Drums consist of at least one membrane, called a drumhead or drum skin, that is stretched over a shell and struck, either directly with the player's hands, or with a drum stick, to produce sound. There is usually a "resonance head" on the underside of the drum, these are usually tuned to a slightly lower pitch than the top drumhead. Other techniques have been used to cause drums to make sound, such as the thumb roll. Drums are the world's oldest and most ubiquitous musical instruments, and the basic design has remained virtually unchanged for thousands of years.
All types of drums such as timpani for example are always tuned to a certain pitch. Often, several drums, other than timpani drums, can be arranged together to create a drum kit.