The teru’a is made up of a series of short, broken-off sounds, like a sob. In halakha, these sounds are referred to as “tromitin.” Some maintain that a teru’a is made up of three tromitin (Rabbeinu Ḥananel; Rashi), while others maintain that it is made up of nine (Rivam; Riva; Smag). In practice we blow nine, but be-di’avad the obligation is fulfilled with three. One may blow more than nine tromitin for a teru’a, as long as the sounds are continuous, with no break (SA 590:3; MB ad loc. 12; for the Yemenite custom, see the note).
The shevarim is made up of three medium-length blasts, like a sigh or groan. Each individual blast (shever) is about the length of three tromitin. Le-khatḥila, we do not add to the three blasts, but bedi’avad, one who does has fulfilled his obligation (SA 590:3; MB ad loc. 11). Some blow each shever with a rise, like a sigh in a broken voice. This is the Lithuanian custom.
If one blows a shever which is the length of two tromitin, he fulfills his obligation, for it is clear that the blast is similar to sighing rather than crying, since it is still twice as long as a teru’a’s tromit. If, however, a shever lasts for less than the length of two tromitin, he has not fulfilled his obligation. One who blows each shever for four tromitin fulfills his obligation, as this is very similar to the normal shever. Even one who extends a shever up to six tromitin fulfills his obligation be-di’avad.
. Some say that a shever that is 3 tromitin long is invalid. Why? Because a shevarim is a type of teru’a, and we know that the teki’a and the teru’a must be the same length. According to Rabbeinu Ḥananel and Rashi, a teru’a of 3 tromitin is acceptable. Therefore, 3 tromitin is the length of a teki’a within the set of tarat. True, within the set of tashrat, the teki’a is longer, to match the length of the shevarim-teru’a. Nevertheless, since a teki’a within tarat is 3 tromitin, a shever that is 3 tromitin can be confused with a tekia (Tur’s understanding of Tosafot and Rosh; first opinion in SA 590:3). According to Rivam, Riva, and Smag, a teru’a is 9 tromitin (as is a teki’a). Accordingly, be-di’avad, a shever can be longer or shorter than the prescribed 3 tromitin, as it will not be confused with a teru’a or tekia (second opinion in SA).
Others maintain that there is no connection between the different sets; in each set, each teki’a must be the same length as the adjacent teru’a. Thus, within the set of tashrat, the teki’a must be the length of a shevarim-ter’ua, while within tashat it must be the length of a shevarim. Since there is no connection between the different sets, even according to Rabbeinu Ḥananel and Rashi a shever that is longer than 3 tromitin is acceptable (Mordekhai; Hagahot Asheri; Rema). This is the common practice (MB 590:15).
Some maintain that we must take into account the first opinion of SA, so for at least thirty blasts we should make sure that each shever is less than 3 tromitin (MA ad loc. 2; see Kol Teru’a 8). In my humble opinion, this is not necessary. First, it is a case of a triple doubt, where we can be lenient: (a) It may be that those who maintain that a teru’a is 9 tromitin are correct; (b) even among those who believe that it is 3 tromitin, it is possible that Mordekhai and Hagahot Asheri are correct that there is no connection between the different sets; (c) there are other opinions as to the length of a teki’a: Rambam says that it is half the length of a teru’a, while Raavad says that a teki’a is always 9 tromitin. Second, it is very difficult to guarantee that each shever be less than 3 tromitin, as the difference between a shever of 2 tromitin and 3 is approximately a quarter of a second. It is next to impossible to discern this difference, and we have a principle that the Torah was not given to the ministering angels (who have superhuman capabilities). Third, if the shofar-blower tries to blow a shever of 2 tromitin, he may in fact end up blowing it a little shorter. That would make the shever into a teru’a sound, which all would agree does not fulfill the obligation. Therefore, it would seem that as long as the shever sounds like a sigh, it is acceptable le-khatḥila. Some say that people who follow the Lithuanian custom when blowing have removed themselves from any doubt; since the blast they blow has a rise, there is no way to confuse it with a teki’a. (See Hilkhot Ḥag Be-ḥag 12:64.)
How long may a shever be? Those who say that a teru’a is 9 tromitin say that a teki’a is the same length, and a shever must be shorter than this (SA 590:3, second opinion). However, this requires further inquiry, because if a shever is too long, it approximates the joyful sound of a teki’a. I therefore wrote that a shever, to be acceptable be-di’avad, must be no longer than 6 tromitin. (See Mateh Ephraim ad loc. 11; Elef La-mateh ad loc. 14.) According to Rambam’s view that a teki’a is half as long as a teru’a, after 4.5 tromitin a shever cannot be distinguished from a teki’a. Thus, a shever must be less than 4 tromitin. Le-khatḥila, one should defer to Rambam’s view.