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9. Adding material to lengthen Shofar: 
If any material was added to lengthen the size of a Shofar in a way that the sound passes through this addition, the Shofar is invalid. This applies even if the Shofar was already Kosher, containing the minimum required measurement, prior to adding the material to lengthen it. It applies whether the added material is made of horn or of other matter. It applies whether the material was added to the narrow end or the wide end. [It applies even if the sound of the Shofar did not change at all due to the addition and even if the material was only added to the thickness of the end of the Shofar in a way that it slightly added to its length.]
From the Rav’s Desk
May/should one blow Shofar with a mask on due to worry of spread of Covid which can be blown into the air at much further distances when blowing.
I am unaware of the medical research on the matter of blowing with versus without a mask, however, certainly blowing with a mask, or any item attached to the Shofar especially by its end or between the mouth and the Shofar is a potential invalidation. If one blows the Shofar while wearing a mask, it is invalid. If one blows it with a mask on top, it is possibly invalid due to the clause that prohibits adding to the end of the Shofar, as well as the clause which invalidates a Shofar whose sound as changed. I am aware of the guidelines issued by some Rabbinical bodies that one should blow the Shofar with a mask on it, although I am unclear as to the basis of their ruling as it is clear from the Poskim that no material may be added to the end of the Shofar, otherwise it is invalid. Thus, in my opinion it should not be done, and as an alternative, the Shofar should be blown outside the Shul. However, in Elul when Shofar blowing is just a custom, certainly one may blow with a mask if such is required due to Covid.
Sources: See Michaber 586:7 regarding attaching non-Shofar material to the Shofar, and if the sound alters; Admur 586:16 regarding the invalidation of a plated Shofar if it is between the mouth of the person and the Shofar, and regarding the invalidation of a plating that changes the sound of the Shofar; Admur 586:11 and 16 and Michaber 586:11 that the Shofar is invalid if any material is added to the end of the Shofar even if the sound does not change.
 The reason: The reason for this is because the sound of the Shofar protrudes through the added material and the Torah validated only the sound which protrudes from a single Shofar and not from two Shofros. [586/10]
 586/17; Admur writes “even if it was added to the wide part by the thickness of the Shofar in a way that it lengthened in size”. Simply this means that one added material to the wide end as opposed to the narrow end. This law was already states in 586/11. Perhaps however it also refers to the top part of the wide end of the Shofar that extends more than then the opposite side of the wide end and hence the novelty is that an extended coating even in that area is invalid.