This article is an excerpt from our Sefer
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Placing one’s lips on the Shofar while blowing: 
If one slightly distanced the Shofar from his mouth and blew from it he has not fulfilled his obligation.
 The reason: The reason for this is because one’s mouth must be resting on the Shofar itself. [ibid] This is learned from the law stated earlier in 586/16 that if there is gold interfering between the Shofar and one’s lips then the Shofar is invalid. Hence we see the lips must touch the actual Shofar. [Aruch Hashulchan 586/30]