The Torah mentions the word "Terua" three times, in three different contexts, and the Gemara understood these three references as indicating that three Terua sounds must be blown with the Shofar on Rosh Hashanah. The Gemara also established that each Terua sound must be preceded and followed by a Tekia sound. In principle, then, the Torah obligation of Shofar requires blowing nine sounds – namely, blowing the sequence of Tekia-Terua-Tekia three times.
However, the Gemara notes, it is uncertain what precisely the required Tekia sound is, as different opinions exist. All agree that a Terua resembles a crying sound, but there are various ways of crying. According to one opinion, the Terua is "Yeluleh Yelil" – a series of very short sounds, whereas according to another opinion, the Terua is "Genuheh Ganah" – a series of longer sounds, like groans. Yet a third opinion maintains that a Terua is a combination of both sequences. As we deal here with a Torah obligation, the Gemara concludes that one should satisfy all three opinions. And thus we end up with a series of 30 sounds – three series of "Tashrat" (Tekia, Shebarim-Terua, Tekia), another three series of "Tashat" (Tekia, Shebarim, Tekia), and then three series of "Tarat" (Tekia, Terua, Tekia). This way, we fulfill the requirement according to all opinions. The sound we call "Terua" fulfills the requirement according to the view of "Yeluleh Yelil"; the sound we call "Shebarim" fulfills the requirement according to the view of "Genuheh Ganah"; and the "Shebarim-Terua" fulfills the requirement according to the view that both sounds must be blown.
How long must these sounds be?
The Gemara instructs that the length of the Tekia must be the same as the length of the Terua, and that a Terua must be three "Yebabot" ("wails"). Rashi explains the word "Yebaba" to mean a very short sound, the shortest sound one can make. It turns out, then, that according to Rashi, a Terua is a sequence of three very short, rapid sounds, and a Tekia is to be at least the duration needed to make these three sounds, or about one-third of a second.
The Shulhan Aruch (Orah Haim 590:3) brings Rashi’s opinion as one of two views. He notes that according to Rashi’s view, one must be extremely careful when blowing the Shebarim sound to ensure that it is shorter than one-third of a second. If it is even slightly longer than this duration, then it becomes a Tekia, and therefore, according to Rashi, the one sounding the Shofar must see to it that the Shebarim is precisely the duration of two brief sounds, so it is longer than a Terua and shorter than a Tekia.
The Shulhan Aruch then proceeds to cite a different view – that of Tosafot (Medieval French and German scholars) and the Rashba (Rav Shlomo Ben Aderet of Barcelona, 1235-1310). In their view, a "Yebaba" is three brief sounds, such that a Terua – three "Yebabot" – is nine brief sounds. A Tekia, then, is the length of nine brief sounds, or about one second – three times the duration of a Tekia according to Rashi’s view. According to this view, the Shebarim-Terua is the length of 18 brief sounds. Now Halacha requires that the Tekia must be at least the same length as the Shebarim, Terua, or Shebarim-Terua in that sequence, and therefore, when blowing the sequence of "Tashrat," one must ensure to blow a long Tekia – the length of 18 brief sounds, or two seconds.
There is a basic rule that when the Shulhan Aruch brings two different opinions, introducing each as "Yesh Omrim" ("there are those who say"), Halacha follows the second opinion. In this instance, therefore, Halacha follows the view of Tosafot and the Rashba, that a Terua is nine brief sounds. Hence, we do not need to ensure that the Shebarim is no longer than the duration of two brief sounds. On the other hand, we must ensure when blowing "Tashrat" that the Tekia is at least two seconds long.
However, Hacham Bension Abba Shaul (Jerusalem, 1924-1998) proves that although we follow the second opinion when the Shulhan Aruch presents two views in this fashion, it is preferable to satisfy also the first view. It is for this very reason that the Shulhan Aruch brings the first opinion, even though he sides with the second opinion – because he maintains that even the first opinion should be taken into account and preferably abided by. Hacham Bension proves this point from the Shulhan Aruch’s comment in this context, after presenting the view of Rashi and the Rashba. The Shulhan Aruch writes that if one extends the Shebarim and does not sufficiently extend the Tekia, then he does not fulfill the Misva according to either opinion – neither according to Rashi, nor according to the Rashba. If Halacha does not take Rashi’s view into consideration at all, then the Shulhan Aruch should have written simply that the person in this case does not fulfill the Misva according to the Rashba, as Rashi’s opinion is of no consequence. The fact that the Shulhan Aruch emphasizes that the person in this case fulfills the requirement according to neither view demonstrates that although he sides with the Rashba’s view, he still takes Rashi’s view into account.
For this reason, some have the practice to return to the synagogue on Rosh Hashanah to hear the sounding of the Shofar a second time, with a very brief Shebarim sound, in order to fulfill Rashi’s opinion. Hacham Bension himself followed this practice. In our community, however, this is not customary.
Summary: According to the accepted Halacha, the Terua is nine very brief, rapid sounds, and the Tekia is a straight sound which extends at least for the duration of a Terua, or about one second. The Tekia sounded before and after a "Shebarim-Terua" must be at least twice this duration. Some have the custom to hear an additional series of Shofar blasts in order to satisfy the other opinion, according to which the Terua and Tekia are much shorter, thus necessitating sounding a shorter Shebarim.