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 drumstick, 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.
A few types of drums such as timpani are always tuned to a certain pitch. Often, several drums, other than timpani drums, can be arranged together to create a drum kit.
Macaque monkeys drum objects in a rhythmic way to show social dominance and this has been shown to be processed in a similar way in their brains to vocalizations suggesting an evolutionary origin to drumming as part of social communication. Other primates make drumming sounds by chest beating or hand clapping, and rodents such as kangaroo rats also make similar sounds using their paws on the ground.
In the past drums have been used not only for their musical qualities, but also as a means of communication, especially through signals. The talking drums of Africa can imitate the inflections and pitch variations of a spoken language and are used for communicating over great distances. Throughout Sri Lankan history drums have been used for communication between the state and the community, and Sri Lankan drums have a history stretching back over 2500 years.