Different individual shamans and different local traditions may use different sounds. For example, in the south of Tuva and in Mongolia the "khomus", or jaw harp is commonly used in shamanising. Particular sounds, like bells, may be used for purifying the place in which the ritual is to be performed. This is because a ritual involving contact with the spirits is always potentially dangerous, and one of the dangers is that of pollution. A bell may also be used for calling or sending back spirits. Shamans will also imitate the sounds of birds and animals in order to call spirits. Sami shamanic singing, called Joik, is also about summoning, for example, animal spirits, rather than singing about them or representing them: the spirit is experienced as being present. Within shamanic ritual, sound can also be used as a healing power, conceived as a way of directing spiritual energy from the shaman into an afflicted person. In Tuva sick persons are said to have been healed by the sound of a stringed instrument made from a tree struck by lightning. The shaman's song - or "algysh"  in Tuvan - is personal to the shaman and tells of her or his birthplace, initiation, ancestral pedigree, special gifts and special connections to particular spirits. The melody and words are composed by the shaman and generally remain the same throughout the shaman's professional life. The "algysh" is often sung near the beginning of the ritual and accompanied by drumming on the dungur drum. It serves to remind the shaman of their shamanic identity and power. It proclaims the shaman's abilities and announces the shaman to the spirits. In some traditions the shaman's song may be broken up into short sections, varied and recombined in different performances. The single headed frame drum is widely used in shamanic ritual, often with metallic ritual objects dangling inside, held by an interior wooden cross-piece, and played with a special beater that may also itself be a rattle. The drum will be made of consecrated materials, a specially chosen tree as a tree struck by lightning  and the skin of a carefully chosen animal, sometimes by a specialist drum-maker. It will also have been 'enlivened' usually by a more powerful shaman so as to give it its spiritual properties. The head is sometimes decorated with a graphic typically representing the division of the cosmos into upper, middle and lower worlds. A shamanic ritual often begins with heating the drum head over a fire to bring it up to the desired pitch.