Two of the three Shofar sound patterns are mentioned in the Bible: The Tekia and the Terua.  In the Mishna, the Tekia is described as a long blast, and the Terua as three yevavot, or wavering sounds.

Actually, the exact nature of these sounds was never firmly established.  In the third century, Talmudic scholars debated the exact nature of the Terua, and they finally agreed that the Tekia was to be one single, long blast.  Some thought the Terua was to be a moaning sound, while others felt it should consist of nine staccato blasts. 

A compromise was reached: The Terua would consist of nine staccato notes and the shevarim would be introduced.  The shevarim was to fall in-between the single straight blast of the Tekia and the nine staccato notes of the Terua, and its pattern became three undulating notes.