LLSB: Normally Split First Sound
A Split of the first sound occurs when both components that make up the sound are separately distinguishable. In a normal split first sound, the mitral and tricuspid components are .02 seconds apart.
A split of the first sound is heard in about 85% of normal subjects, and is heard best at the LLSB with the diaphragm firmly pressed.
The sound you heard is a normally split first sound. The split is taken away and then added back .