The Drum Circle symbolizes the heart beat of the earth. Thunderous powwow drums, sometimes called community drums, are made of wood and have heads of elk, moose, buffalo, horse, or cow. Native American drumming is also accompanied by singing. The words may be in English or a Native American language, or they may be "vocables." A vocable is a chant like the familiar "wey-ah-hey-a" sound. Drumming songs vary from tribe to tribe and can be hundreds of years old. Because of the difficulty of a Native American drum circle, many powwows have drum contests.