S1 and S2 are the two normal heart sounds that create the familiar “lub-dub” you hear through a stethoscope. S1 is the “lub,” produced when the valves between your upper and lower heart chambers snap shut at the start of each heartbeat. S2 is the “dub,” produced when the valves leading out of your heart close immediately after. Together, they mark the beginning and end of each pumping cycle.
What Causes S1
S1 happens at the very beginning of ventricular systole, the phase when your heart’s two lower chambers (ventricles) contract to push blood out. As pressure in the ventricles rises sharply, it forces the mitral valve (on the left side) and the tricuspid valve (on the right side) to close. These are the valves that separate the upper chambers from the lower chambers, and their closure prevents blood from flowing backward.
The sound itself comes from vibrations generated by that sudden closure. It actually has two components: the mitral component (M1) hits first because the left ventricle starts contracting a fraction of a second before the right. The tricuspid component follows almost immediately. In most people, the two are so close together they sound like a single beat. S1 is typically the louder of the two heart sounds and is best heard near the bottom left of the chest, around the apex of the heart.
What Causes S2
S2 marks the opposite moment. It occurs at the end of ventricular systole, when the ventricles finish contracting and start to relax. As pressure in the ventricles drops, blood in the aorta and pulmonary artery briefly pushes back against the outflow valves, snapping them shut. The aortic valve closes first (producing a component called A2), followed closely by the pulmonic valve (P2).
S2 is generally a shorter, sharper sound than S1. It tends to be heard most clearly higher on the chest, near the second intercostal space on either side of the breastbone. A particularly useful listening spot is Erb’s point, located on the left side at the third intercostal space, where S2 is often easiest to pick up.
How S1 and S2 Sound Different
Both sounds fall in a frequency range of roughly 50 to 500 Hz, which overlaps with the lower end of normal human hearing. S1 is lower-pitched and slightly longer, often described as a soft “lub.” S2 is higher-pitched and crisper, the “dub.” The pause between S1 and S2 (systole, when blood is being pumped out) is shorter than the pause between S2 and the next S1 (diastole, when the heart fills back up). That timing difference is what gives the heartbeat its characteristic rhythm: lub-DUB… lub-DUB… lub-DUB, with a longer gap after each “dub.”
Learning to tell S1 from S2 is one of the first skills in cardiac auscultation. A practical trick: feel the pulse at the wrist while listening. The beat you feel lines up with S1, because that’s the moment the ventricles are pushing blood into the arteries.
Why S2 Sometimes Splits Into Two Sounds
In many healthy people, S2 briefly splits into two audible components during a deep breath in. This is called physiological splitting, and it’s completely normal. When you inhale, pressure changes in your chest reduce the resistance in the blood vessels of your lungs. That decreased resistance allows the pulmonic valve to stay open slightly longer than the aortic valve, so P2 is delayed just enough to hear it as a separate sound after A2.
The traditional textbook explanation attributed this to increased blood returning to the right side of the heart during inhalation, but research suggests the primary mechanism is actually the drop in pulmonary vascular resistance. Either way, the split disappears when you exhale and the two components merge back together. If the split is present all the time, regardless of breathing, or if the components move in the wrong direction, that can signal structural heart problems.
What Changes in S1 and S2 Can Mean
The loudness, timing, and quality of these sounds give clinicians important diagnostic clues. A louder-than-normal S1 can occur when the mitral valve leaflets are still wide open at the moment the ventricles contract, which happens in certain rhythm abnormalities or when the valve itself is stiff but still mobile. A quieter S1 can suggest the valve isn’t closing properly, or that there’s extra tissue (like chest wall fat or fluid around the heart) muffling the sound.
For S2, a loud aortic component may point to high blood pressure, since the valve is slamming shut against greater pressure. A diminished or absent aortic component can occur when the valve is calcified and no longer moves freely. A fixed split of S2, one that doesn’t change with breathing, is a classic sign of an atrial septal defect, a hole between the heart’s upper chambers.
Extra sounds beyond S1 and S2 also matter. An S3 (a third sound just after S2) can be normal in young, healthy people but in older adults often signals that a ventricle is struggling to handle its blood volume. An S4 (a sound just before S1) suggests the ventricle has become stiff and is resisting filling. Neither S3 nor S4 is as loud as the two main sounds, and hearing them requires careful listening with the right stethoscope technique.
Where to Listen on the Chest
Different listening spots emphasize different valves:
- Aortic area: right side of the chest, second intercostal space, near the sternum. Best for hearing the aortic component of S2.
- Pulmonic area: left side, second intercostal space. Best for the pulmonic component of S2 and for detecting splitting.
- Erb’s point: left side, third intercostal space. A general-purpose spot where both S1 and S2 are clear.
- Tricuspid area: left lower sternal border, around the fourth intercostal space. Emphasizes sounds from the tricuspid valve.
- Mitral (apical) area: left side, fifth intercostal space, in line with the middle of the collarbone. This is where S1 is loudest and where extra sounds like S3 and S4 are most audible.
Moving the stethoscope across all five points, rather than listening at just one spot, gives a far more complete picture of what the heart is doing with each beat.