The following instructions are given for connecting stereo speakers to an amplifier so that they are in phase: “After both speakers are connected, play a monophonic record or program with the bass control turned up and the treble control turned down. While listening to the speakers, turn the balance control so that first one speaker is heard separately, then the two together, and then the other separately. If the bass is stronger when both speakers play together, they are connected properly. If the bass is weaker when both play together than when each plays separately, reverse the connections on one speaker.” Explain why this method works. In particular, explain why a stereo source is not used and why only the bass is compared.