"אשרי העם יודעי תרועה"
“Fortunate are the people who know the sound of the shofar.”

QUESTION: Instead of “yode’ei” — “who know” — it should have said “toke’ei” — “who blow” — or “shome’ei” — “who hear”?

ANSWER: It is preferable to hear one hundred blasts of the shofar on Rosh Hashanah (see Shulchan Aruch Harav 596:1, Shelah Vol. I p. 220b). Thirty are sounded before Musaf, thirty during the silent prayer of Shemoneh Esreih, thirty during the repetition of the prayer, and ten more during the Kaddish recited after the repetition of the prayer. (The additional thirty shofar blasts which Chabad does after the conclusion of all the prayers, is only a custom in order to confuse the Satan.)

The word “yode’ei” (יודעי) — “who know” — has the numerical value of one hundred.

(הליכות עולם בשם דורשי רשומות)