Apex: Normal First Sound (S1)
The major audible components of the First Sound are related to mitral and tricuspid valve closure.
Mitral (M1) closure is the 1st audible component of the 1st sound. It normally occurs before tricuspid (T1) closure and is of slightly higher intensity and frequency than T1. The mitral component is heard best at the apex, although it is discernible at any of the sites of auscultation. It is heard best with the diaphragm firmly pressed.
Tricuspid closure (T1) is the 2nd component of the first sound. It has less intensity, and may only be audible at the left lateral sternal border or llsb. Its high frequency sound is heard best with the diaphragm firmly pressed.
The normal first sound you heard consists of pairs of sounds. Each pair, lub-dub, lub-dub begins with the 1st sound and ends with the 2nd.