The second heart sound is produced by the closure of the aortic and pulmonic valves. The sound produced by the closure of the aortic valve is termed A2 and the sound produced by the closure of the pulmonic valve is termed P2. The A2 sound is normally much louder than the P2 due to higher pressures in the left side of the heart, thus A2 radiates to all cardiac listening posts (loudest at the right upper sternal border) and P2 is usually only heard at the left upper sternal border. The A2 sound is thus the main component of S2.

Like the S1 heart sound, the S2 sound is described regarding splitting and intensity. S2 is physiologically split in about 90% of people. The S2 heart sound can exhibit persistent (widened) splitting, fixed splitting, paradoxical (reversed) splitting, or the absence of splitting. The S2 heart sound intensity decreases with worsening aortic valve stenosis due in immobile leaflets. In severe aortic stenosis, the A2 component may not be audible at all.

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