S1: The first heart sound occurs due to the sudden closure of the tricuspid and mitral valves during systole. It coincides with the peak of the R wave of the ECG and is a long, soft, low-pitched sound. It resembles the spoken word “LUB” of “lub-dub”.

S2: The second heart sound occurs due to the sudden closure of the pulmonic and aortic valves during the onset of diastole. It coincides with the T wave of the ECG and is a short, sharp, and high-pitched sound. It resembles the spoken word “DUB” of “lub-dub”.