The Mishna in Masekhet Rosh Hashanah (27b) addresses the case of somebody who sounded the shofar on Rosh Hashanah in a cavern, giving rise to the question of whether this satisfies the obligation of hearing the sounding of the shofar. The answer to this question, the Mishna establishes, depends on whether one heard the actual sound of the shofar, or “kol havara” – the sound of the echo. The Gemara clarifies that this refers only to those standing outside the cavern, whereas those who are inside fulfill their obligation.
Rashi understands the Gemara’s remark to mean that those situated inside the cavern fulfill the obligation under all circumstances, whereas those situated outside the cavern fulfill the obligation only if they hear the actual sound of the shofar, and not an echo. This is the ruling of the Rambam, in Hilkhot Shofar (1:8). The Rosh, however, disagrees. He questions why sometimes those standing outside the cavern would hear the actual sound, and at other times, they would hear the echo sound. And, he adds, if this depends on the factors such as the depth of the cavern and the people’s distance from the cavern, then Chazal should have established specific guidelines, rather than leave it to people to determine whether or not they heard the actual shofar sound. Therefore, the Rosh understood the Gemara to mean that in such a case, when a shofar is sounded inside the cavern, those standing outside do not fulfill their obligation, even if they feel that they heard the actual sound of the shofar. The Rosh explains that when the shofar is blown inside a cavern, the sound heard outside is the combination of the actual sound and the echo sound produced by the walls, and so anyone who hears the sound outside the cavern does not fulfill the mitzva.
The Tur (O.C. 586) follows the view of his father, the Rosh, whereas the Shulchan Arukh (586:1) accepts the Rambam’s position, that those standing outside the cavern fulfill the mitzva if they determine that they heard the actual shofar sound.
The Taz (587:1) expands the Shulchan Aruch’s ruling beyond the specific case of a cavern. He writes that even if a shofar is sounded in a synagogue, and people are standing outside, at a distance from the synagogue, they might not necessarily fulfill their obligation by hearing the shofar blasts. In this instance, too, according to the Taz, a determination will need to be made that the actual sound of the shofar was heard, and not an echo reverberating off the walls of the building.
Interestingly, the Taz further asserts that although the Shulchan Arukh rules that those who stand outside the cavern fulfill their obligation if they determine that they heard the actual shofar sound, we should not rely on such a determination. The Taz notes that there are numerous areas in Halakha regarding which we do not presume expertise to definitively identify certain conditions, and thus in this case, too, those who stand outside a cavern in which the shofar is sounded should hear the shofar blowing again. Rav Asher Weiss questions the Rosh’s claim, noting that although there are, indeed, ten different areas in Halakha regarding which we do not presume expertise, nevertheless, when it comes to the shofar sound, this does not require expertise. There is no skill needed to distinguish between the actual shofar sound and the sound of the echo, and so a person standing outside the cavern may rely on his assessment that he heard the actual sound of the shofar.
Curiously, the Taz does not combine his two claims vis-à-vis this halakha. We would have assumed that according to the Taz, those who stand outside the synagogue and hear the sounding of the shofar do not fulfill their obligation, since, in his view, this is akin to the situation of a shofar sounded in a cavern, such that one cannot rely on his own assessment. However, the Taz stops short of this conclusion. He writes that in the case of people standing outside the synagogue, they can rely on their assessment that they heard the actual shofar sound, even though such an assessment cannot be trusted when hearing the sound of the shofar blown in a cavern.
As Rav Weiss notes, this discussion is particularly relevant to shofar blowing this year (Rosh Hashanah, 5781), when, due to the coronavirus pandemic, many will be listening to the shofar blowing outside synagogues, or in different “capsules” within the same building. Even according to the stringent view of the Taz, one fulfills the mitzva in this fashion if he assesses that he heard the actual shofar sound. (Rav Weiss adds that in any event, this year, the shofar is not sounded on the first day of Rosh Hashanah, because it falls on Shabbat, and shofar blowing on the second day is required only by force of Rabbinic enactment, allowing greater room for leniency.)