Technically, we must hear the Shofar 9 times on Rosh Hashana, for the Torah uses the word "terua" 3 times in reference to RH, & each terua is preceded & followed by a tekia sound. But exactly what sound is intended? Is it a wailing tone, such as women would cry when they lamented? Is it a kind of sigh (or kretsch), as one might repeatedly emit in a state of acute sorrow? Or is it a combination of both sighing and lamenting? So, Rav Avahu ruled we sound all 3 of these shofar tones, & to differentiate between them we call the wailing sound "terua;" the sighing sound "shevarim;" & the combination of the two "shevarim-terua." To resolve all doubt as to the original terua sound, we sound all 3 possibilities, each preceded & followed by the straight tekia.Some suggest that the sound of the terua is a form of ululating, a variable shout traditional to the Middle East & still widely done, both at happy occasions (e.g. a wedding) or sad events (e.g. funerals, lo aleynu).