For the source text click/tap here: Rosh Hashanah 27
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What if someone blows a shofar into an echo-producing chamber, such as a pit, a cistern or a jug.
The mishnah rules that in order to fulfill his obligation he must be sure that he heard the sound of the shofar and not the echo of the sound of the shofar.
There is a geonic commentary on this mishnah, according to which the mishnah refers to a time of persecution when the Romans outlawed the public observance of commandments. They hid their shofar blasts to avoid the authorities.
We present a similar use of the shofar in modern Palestine under the British mandate, where a young Chabad Chalutznik was imprisoned for blowing the shofar at the kotel.