first heart sound
(S1) the heart sound occurring during closure of the mitral and tricuspid valves; it is dull, firm, prolonged, and is heard as a “lubb†sound. It begins with an inaudible, low-frequency vibration (M) occurring at the onset of ventricular systole, followed by two intense, high-frequency vibratory bursts associated with mitral and tricuspid valve closure (M1 and T1, respectively), and ending with several variable low-intensity vibrations. Splitting of M1 and T1 is not usually discernible in normal adults.
Medical dictionary.
2011.