Polarity is vitally important when you interconnect electronic equipment. This is particularly true for speakers—if one cone is moving in while the other moves out, the sound quality will be adversely affected.
While it seems obvious, it’s always important to double check to make certain that all your speakers are connected with the positive outputs from the amplifier connected to the positive inputs for your speakers. The same, obviously, is true for the negative connection points.
If for any reason you doubt that the polarity is correct, it can be checked in a safe, simple operation: with the speaker wire disconnected at the amplifier or receiver, connect the loose end of the cable to a 9-volt battery [positive to positive / negative to negative]. The woofer will move forward if the polarity is correct. If it moves backward, the polarity is reversed, meaning that it was connected positive to negative or negative to positive. Commercially available polarity testers can also be used to ensure that the speakers are properly wired. To prevent this from happening in the first place, the simplest solution is to use color coded speaker wire.