The reason for the process is the compatibility issue between stereophonic and monaural recordings: information which is identical on both the left and right channels of a stereophonic mix sounds too loud when played back on mono AM and FM radio stations and phonographs. When the left and right channels are summed together, any musical parts that are common to both channels combine to be 6 decibels louder than they are in the same mix when played in stereo (a phenomenon known as "center-channel buildup"). Vocals, solo instruments and bass lines are often mixed equally to both stereo channels — these sounds tend to be too loud when heard in mono.