The only instrument truly allowed in a cappella music is the human body and the sounds created by that body, and the most powerful and basic of those sounds are those created by the voice. Lifting the voice in unaccompanied song is the main element of a cappella music. Singers can be solo, or harmonies can be layered by adding more singers. Examples of a cappella music in modern culture include soulful street corner doo-wop singers, old-timey barbershop quartets and full-fledged vocal choirs with scores of members.
Mimicking the sounds of instruments with the voice is a tradition that goes back much further than the beat boxers adding their rhythms to hip-hop music. Jazz singer Louis Armstrong was said to invent "scat singing" when he dropped a lyric sheet during a performance, duplicating the melody and rhythms of his instrument with his voice, and the bass man in 50s doo wop groups would copy the sound of a bass guitar with his deep voice to provide a fat bottom to the vocal harmonies. Contemporary beat boxers add the sounds of scratching records, horns, guitars and drum machines with nothing more than the sound of their voice.
Another sound made by the mouth used often in a cappella singing is the whistle, where performers simply pucker the lips and blow air through the small opening in the mouth. By moderating the airflow with the lips, teeth, tongue or fingers, performers can provide melodies and harmonies to accompany singers. The voice can also be used in conjunction with whistling to alter and deepen the sound. Whistling can be both melodic or rhythmic, becoming a lead or rhythm instrument at will.
While not created by the human voice, sounds provided by stomping feet, clapping hands or snapping fingers are widely-used elements in a cappella music, especially in rhythmically intense doo wop groups and barbershop quartets. Jazz vocal artist Bobby McFerrin often accompanies himself during a cappella performances by rhythmically tapping and thumping on his chest, providing an audible beat beneath his sung lyrics and vocal sounds and also causing his voice to bounce and pulse to the beat.
A case can be made that using a digital sampling device to record the human voice, then cut it apart and paste it back together again to form harmonies, melodies and rhythms would somehow be cheating. However, numerous artists have done just that to create entire works with nothing more than the human voice. Icelandic singer Bjork Guðmundsdóttir recorded her fifth solo album "Medulla" almost entirely a cappela, with beatboxing, choral arrangements and throat singing adding layers of complexity to the mix. Hip-Hop artist Rahzel, who was also featured on "Medulla," has created numerous albums by sampling his own voice and provided beats, rhythms, sound effects and mimicked instruments to form musical collages hard to distinguish from full bands and orchestras.