Rosh Hashanah 27b
ראש השנה כ״ז ב — ד"ה נִיקַּב וּסְתָמוֹ
Rosh Hashanah 27b:1
The discussion of the shofar includes cases where a listener hears the sound through an intervening echo or other mediation. While not about amen, it is a classic source for whether mediated sound is considered the original act of hearing.
נִיקַּב וּסְתָמוֹ, אִם מְעַכֵּב אֶת הַתְּקִיעָה — פָּסוּל, וְאִם לָאו — כָּשֵׁר. הַתּוֹקֵעַ לְתוֹךְ הַבּוֹר אוֹ לְתוֹךְ הַדּוּת אוֹ לְתוֹךְ הַפִּיטָס, אִם קוֹל שׁוֹפָר שָׁמַע — יָצָא, וְאִם קוֹל הֲבָרָה שָׁמַע — לָא יָצָא. הַתּוֹקֵעַ לְתוֹךְ הַבּוֹר אוֹ לְתוֹךְ הַדּוּת כּוּ׳. אָמַר רַב הוּנָא: לֹא שָׁנוּ אֶלָּא לְאוֹתָן הָעוֹמְדִים עַל שְׂפַת הַבּוֹר, אֲבָל אוֹתָן הָעוֹמְדִין בַּבּוֹר — יָצְאוּ.
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. § It was taught in the mishna: If one sounds a shofar into a pit or into a cistern, he has not fulfilled his obligation. Rav Huna said: They taught this only with respect to those standing at the edge of the pit, i.e., on the outside, as they can hear only the echo coming from the pit. But those standing in the pit itself have fulfilled their obligation, since they initially hear the sound of the shofar.