The S1 heart sound is the first of the two sounds you hear in a normal heartbeat, the “lub” in “lub-dub.” It marks the exact moment your heart’s lower chambers (ventricles) begin to contract, produced primarily by the closing of two valves that separate the upper and lower chambers of your heart. When a doctor listens to your chest with a stethoscope, S1 is the louder, lower-pitched sound that kicks off each heartbeat cycle.
What Produces the S1 Sound
Your heart has four chambers. The two upper chambers (atria) collect blood, and the two lower chambers (ventricles) pump it out. Between each upper and lower chamber sits a one-way valve: the mitral valve on the left side and the tricuspid valve on the right. These are called the atrioventricular (AV) valves.
When the ventricles begin to contract, pressure inside them rises sharply. This pressure pushes the mitral and tricuspid valves shut, preventing blood from flowing backward into the atria. The abrupt closure of these valve leaflets, along with the vibrations it sends through the surrounding blood and heart muscle, creates the S1 sound. The mitral valve typically closes a fraction of a second before the tricuspid valve, but the two components happen so close together that they usually blend into a single sound.
The peak frequency of a normal S1 is around 46 Hz, which places it in the low end of what the human ear can detect. It sounds like a brief, somewhat dull thump rather than a sharp click.
Where S1 Falls in the Cardiac Cycle
S1 signals the very beginning of systole, the phase when the ventricles are actively pumping blood. Specifically, it aligns with a brief moment called isovolumic contraction: the ventricles are squeezing, pressure is building, but no blood is moving out yet because the exit valves (aortic and pulmonary) haven’t opened. Think of it as the split second between turning a faucet handle and water actually coming out.
On an EKG tracing, S1 occurs just after the QRS complex, the tall spike that represents the electrical signal triggering ventricular contraction. The second heart sound, S2, follows later when the aortic and pulmonary valves close at the end of systole. The gap between S1 and S2 is shorter than the gap between S2 and the next S1, which is why you hear “lub-DUB… lub-DUB” with a longer pause between heartbeats.
Where to Listen for S1
S1 is loudest at the apex of the heart, which sits near the bottom-left of the chest around the fifth intercostal space (the gap between your fifth and sixth ribs), roughly in line with the middle of your collarbone. This is where the mitral valve sits closest to the chest wall. A doctor will press the stethoscope here and compare S1’s loudness to what they hear at other listening points on the chest.
What Makes S1 Louder or Softer
Three main factors determine how loud S1 sounds: where the valve leaflets are positioned when the ventricles start contracting, how forcefully the ventricles squeeze, and the physical condition of the valves themselves.
Valve position at the moment of contraction matters most. If the AV valve leaflets are still wide open when ventricular contraction begins, they have farther to travel before slamming shut, and they close with more velocity. This produces a louder S1. If the leaflets have already drifted mostly closed before contraction starts, the closure is gentler, and S1 is quieter.
This is directly tied to the PR interval on an EKG, which reflects the time delay between the atria contracting and the ventricles contracting. A short PR interval means the ventricles fire soon after the atria, while the valves are still wide open from atrial blood flow. The result is a loud S1. A long PR interval gives the valve leaflets time to float back toward a closed position before the ventricles contract, producing a softer S1. In an unusual twist, a very long PR interval can sometimes produce a loud S1 if an atrial contraction happens to land right on top of a ventricular contraction, forcing the valves wide open at exactly the wrong moment.
Conditions that weaken the heart muscle reduce the force of ventricular contraction, making S1 quieter. Stiffened or calcified valves, as seen in rheumatic heart disease, can change S1’s character as well, sometimes making it louder and sharper when the thickened leaflets snap shut, or muffled if the valves become too rigid to close properly. Obesity and fluid around the heart (pericardial effusion) can also dampen the sound by placing more tissue between the heart and the stethoscope.
Split S1
Because the mitral and tricuspid valves don’t close at precisely the same instant, S1 can sometimes be heard as two closely spaced sounds rather than one. This is called a split S1. Both components are high-frequency sounds, which helps distinguish a split S1 from other extra sounds that might be confused with it, like a fourth heart sound (S4) followed by a normal S1.
A slightly split S1 can be completely normal. When the split becomes noticeably wide, it may point to a delay in one valve’s closure. Right bundle branch block, a condition where the electrical signal to the right ventricle travels along a slower path, delays tricuspid valve closure and is the classic cause of a wide split S1. The diagnostic value of S1 splitting is considered limited compared to S2 splitting, but it can be a useful clue when other signs point toward a conduction problem.
S1 Versus S2
Telling S1 apart from S2 is one of the first skills learned in cardiac auscultation. S1 is best heard at the apex (lower left chest), while S2 is loudest at the base of the heart (upper chest near the sternum). S1 is generally the louder of the two at the apex and coincides with the pulse you can feel at the wrist or neck. If you place a finger on someone’s carotid artery while listening, the beat you feel lines up with S1.
The interval between S1 and S2 (systole) is shorter than the interval between S2 and the next S1 (diastole), at least at normal resting heart rates. As heart rate increases, diastole shortens more than systole, and the two intervals become harder to tell apart. At very fast heart rates, identifying S1 by feeling the pulse becomes especially helpful.