If a person blows [a shofar] into a pit or into a cistern or into a cask: if one 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.  Similarly, if one were passing behind a synagogue, or if his house was close to a synagogue, and he heard the sound of the shofar or the sound [of the reading] of the Megillah: if he concentrated, he has fulfilled his obligation; but if not, he has not fulfilled his obligation —- even though both he and someone else listened, this one concentrated but the other did not concentrate.

And it came to pass when Moses raised his hand that Israel prevailed, etc.  Was it Moses’ hands that won the battle or lost the battle? Rather, [the Torah] teaches you: As long as Israel looked heavenward and subjected their heart to their Father in Heaven, they would prevail; but when they did not, they would fall.  In a similar matter you learn: Make a fiery serpent and set it upon a pole, and it shall come to pass that whoever is bitten will look at it, and live.  Was it the serpent that killed or the serpent that gave life? Rather, when Israel looked heavenward and subjected their heart to their Father in Heaven, they were healed; but when they did not, they perished.  A deaf person, an imbecile, and a minor cannot discharge the obligation of the many. This is the general rule: Whoever is not obligated [himself] in a matter cannot discharge the obligation of the many.