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.