903. The Length of the Shofar Sounds

129:12 When removing the Torah from the ark, the practice is to recite the Thirteen Attributes of Mercy; one should start from "And G-d passed before him and proclaimed, 'Hashem, Hashem....'" On Shabbos, some places recite neither the Thirteen Attributes of Mercy nor "Ribono shel Olam" ("Master of the World").

129:13 The measurement of the shofar blasts are optimally as follows: Teruah is nine short sounds. Shevarim is made up of three, one after the other. Each one should be as long as three of the short teruah sounds, making the three of the shevarim the same length as the nine of the teruah. One must be very careful not to extend the shevarim until each sound is as long as the nine of the teruah because if this is the case, the mitzvah of shofar will not be fulfilled, even after the fact. The tekiyos are straightforward sounds. In the sequence "tekiah shevarim-teruah tekiah," each tekiah should be as long in duration as the shevarim and the teruah combined, which would be 18 beats in length. In the sequence "tekiah shevarim tekiah," each tekiah should be as long as the shevarim, i.e., nine beats. (A little bit longer, actually - Mishnah Brurah 590:13.) The same is the case for the sequence "tekiah teruah tekiah."

In the shofar blowing before Musaf, the shevarim and the teruah should be sounded in one breath, so the one calling out the notes should announce them together, i,e., "shevarim-teruah." (The one blowing should pause slightly in between the two types of sounds but not long enough to draw another breath - MB 590:18.) In the shofar blowing during the repetition of the Amidah, they should be sounded in two breaths but one should not make a break between them.  Rather, one should follow the other in succession, so the one calling out the notes should still announce them both together. (The one blowing shofar should only pause long enough to take the second breath - MB 590:21.)