The Gemara in Masekhet Rosh Hashanah (27b) addresses the status of a shofar that was plated with metal, such as gold, and establishes that if the interior of the shofar is plated, then the shofar is invalid for the mitzva on Rosh Hashanah. If, however, the plating is along the shofar’s exterior, the Gemara rules, then the shofar is invalid only if the plating has the effect of altering the shofar’s sound. Despite the fact that, as the Gemara there notes, the shofar sound is valid regardless of its quality, this applies only if the sound is natural. If the sound was manipulated in any way, such as by a substance attached to the shofar’s exterior surface, the sound does not fulfill the mitzva.
The Ritva understands the Gemara’s ruling to mean that if a shofar has plating or any sort of decoration on its exterior, then it is presumed invalid for use for the mitzva unless experts have determined that the metal or decoration does not change the shofar’s sound. Intuitively, one might have reasoned that if the foreign substance is on the shofar’s exterior, the shofar retains its presumed status of validity unless it is determined that the sound has been altered. But the Ritva argues that since we deal here with a Torah obligation, such a shofar may not be used until it has first been conclusively determined that the sound is not altered by the presence of the foreign substance.
Rav Asher Weiss cites the Ritva’s discussion in reference to the question that arises this year (Rosh Hashanah, 5781) as to whether the opening the shofar may be covered by material to obstruct the germs of the one blowing the shofar. As part of the efforts to avoid the risk of spreading coronavirus, some have recommended wrapping material around the end of the shofar in order to prevent the germs from exiting through the shofar and scattering throughout the room. Would this be allowed, or does this affect the shofar sound, thus invalidating the blowing?
Rav Weiss notes that the Gemara speaks of material that alters, rather than muffles, the sound. The disqualification discussed here is a change in the nature of the sound, but not a reduction in the volume. Indeed, as mentioned, the Gemara states explicitly that there is no required quality of sound, and even a low, muffled sound fulfills the mitzva. Necessarily, then, it is only when the foreign material transforms the sound that it disqualifies the blowing, and not when it has merely a muffling effect.
Moreover, Rav Weiss adds, the Ritva writes that a shofar with a foreign substance on its exterior must be presumed invalid because we deal with a Biblical obligation, regarding which we must apply strict standards of certainty. This year, when the first day of Rosh Hashanah falls on Shabbat, the shofar is sounded only on the second day, when blowing the shofar is required only by force of rabbinic enactment. Certainly, then, there is room to allow sounding the shofar this year with material covering the shofar’s opening if one feels this is necessary as a safety precaution.