Rosh Hashanah 27B

ניקב וסתמו, אם מעכב את התקיעה ⁠— פסול, ואם לאו ⁠— כשר. התוקע לתוך הבור או לתוך הדות או לתוך הפיטס, אם קול שופר שמע ⁠— יצא, ואם קול הברה שמע ⁠— לא יצא.

If the shofar was punctured and the puncture was sealed, if it impedes the blowing, the shofar is unfit, but if not, it is fit. If one sounds a shofar into a pit, or into a cistern, or into a large jug, if he clearly heard the sound of the shofar, he has fulfilled his obligation; but if he heard the sound of an echo, he has not fulfilled his obligation.

וכן מי שהיה עובר אחורי בית הכנסת, או שהיה ביתו סמוך לבית הכנסת, ושמע קול שופר או קול מגילה, אם כוון לבו ⁠— יצא, ואם לאו ⁠— לא יצא. אף על פי שזה שמע וזה שמע, זה כוון לבו, וזה לא כוון לבו.

And similarly, if one was passing behind a synagogue, or his house was adjacent to the synagogue, and he heard the sound of the shofar or the sound of the Scroll of Esther being read, if he focused his heart, i.e. his intent, to fulfill his obligation, he has fulfilled his obligation; but if not, he has not fulfilled his obligation. It is therefore possible for two people to hear the shofar blasts, but only one of them fulfills his obligation. Even though this one heard and also the other one heard, nevertheless, this one focused his heart to fulfill his obligation and has therefore indeed fulfilled it, but the other one did not focus his heart, and so he has not fulfilled his obligation.

גמ׳ תנו רבנן: ארוך וקצרו ⁠— כשר. גרדו והעמידו על גלדו ⁠— כשר. ציפהו זהב במקום הנחת פה ⁠— פסול. שלא במקום הנחת פה ⁠— כשר. ציפהו זהב מבפנים ⁠— פסול. מבחוץ, אם נשתנה קולו מכמות שהיה ⁠— פסול, ואם לאו ⁠— כשר.

Talmud: A baraita states: If a shofar was long and it was shortened, it is still fit. If it was scraped out, so that only its outer layer remains, it is also fit. If the shofar was plated with gold at the place where one puts his mouth, it is unfit; if it was plated not at the place where he puts his mouth, it is fit. If it was plated with gold on the inside, it is unfit, as one does not hear the sound of a shofar but the sound of a golden instrument. If, however, it was plated with gold on the outside, the following distinction applies: If its sound changed from what it was before the plating, it is unfit, but if not, the gold plating is mere ornamentation and it is therefore fit.