The sound of a heartbeat comes from valves snapping shut inside your heart. Each “lub-dub” you hear through a stethoscope (or feel thumping in your chest) is produced by two pairs of valves closing in quick succession as blood moves through the heart’s four chambers. It’s not the heart muscle contracting or blood flowing that makes the noise. It’s the valves.
What Creates the “Lub-Dub”
Your heart has four valves that act as one-way doors, keeping blood moving in the right direction. Two of them sit between the upper chambers (atria) and lower chambers (ventricles), and two guard the exits where blood leaves the ventricles and heads to the lungs or the rest of the body.
The first sound, the “lub,” happens at the very start of each heartbeat when the two valves between the upper and lower chambers slam shut. This occurs because pressure in the ventricles rises above the pressure in the atria, forcing the valve flaps closed. That closure sends vibrations through the surrounding blood, heart muscle, and chest wall, producing a low, slightly longer sound that doctors call S1.
The second sound, the “dub,” is shorter and sharper. It happens when the two exit valves close after the ventricles finish squeezing blood out. Once the ventricles relax, pressure inside them drops below the pressure in the large arteries, and the exit valves snap back into place. That’s S2. The brief silence between S2 and the next S1 is the pause while the ventricles refill with blood.
Why Valves Make Sound
When a valve closes, the leaflets (thin flaps of tissue) come together and suddenly stop the column of blood that was trying to flow backward. That abrupt stop creates vibrations, much the same way slamming a door sends a pressure wave through the air. The vibrations travel through the blood itself, through the muscular walls of the heart, and outward through the chest wall, where a stethoscope or a hand can pick them up.
The pitch and loudness of each sound depend on how fast the valve closes, how much pressure difference exists across it, and how stiff or flexible the surrounding tissue is. A healthy valve in a healthy heart produces a crisp, well-defined sound. A damaged or thickened valve may produce a duller closure sound or allow blood to leak, creating additional turbulence that shows up as a murmur.
The Cardiac Cycle Behind the Sounds
The two sounds bookend a phase called systole, which is the contraction phase. Right after S1, the ventricles are sealed chambers: the inlet valves just closed, and the exit valves haven’t opened yet. For a brief moment (called isovolumic contraction), the muscle squeezes without moving any blood at all, just building pressure. Once the pressure exceeds what’s in the arteries, the exit valves pop open and blood rushes out.
After the ventricles empty, pressure drops, the exit valves close (S2), and another sealed-chamber moment occurs in reverse (isovolumic relaxation). Then the inlet valves open silently as blood drains from the atria back into the ventricles, refilling them for the next beat. This filling phase, called diastole, is normally quiet. The whole cycle repeats roughly once per second at a resting heart rate of 60 to 100 beats per minute.
Extra Heart Sounds and What They Mean
Sometimes the heart produces a third or fourth sound, turning the normal “lub-dub” into a three-beat pattern sometimes called a gallop.
A third heart sound (S3) occurs early in the filling phase, right after the valves open and blood rushes into the ventricles. In children and young adults, this can be completely normal because their heart walls are elastic and compliant. In older adults, an S3 often signals that the ventricle is overfilled or under higher-than-normal pressure, which can happen during heart failure.
A fourth heart sound (S4) shows up just before S1, during the final push of filling when the atria contract. If the ventricle wall has become stiff (from thickening of the heart muscle, for example), the atria have to squeeze harder to force blood in. That extra effort creates turbulent flow and a low-pitched thud that a stethoscope can pick up. An S4 is almost always abnormal in adults and points to a stiffened ventricle.
Murmurs: When Blood Flow Gets Noisy
Blood normally moves through the heart in smooth, orderly layers. When something disrupts that flow, the blood tumbles and swirls, producing a whooshing or swishing noise called a murmur. Common causes include a valve that doesn’t open fully (forcing blood through a narrower gap at higher speed), a valve that doesn’t close completely (letting blood leak backward), or an abnormal connection between chambers.
Not all murmurs are dangerous. Many children and some adults have “innocent” murmurs caused by blood flowing a little faster than usual through a structurally normal heart. These tend to be soft, occur during the contraction phase, and disappear with changes in position or as a child grows. Murmurs that are loud, harsh, or occur during the filling phase are more likely to reflect a structural problem worth investigating.
Where Heart Sounds Are Loudest
Each valve’s sound carries best to a specific spot on the chest, and those spots don’t sit directly over the valves themselves. Instead, they follow the direction blood flows after passing through each valve.
- Mitral valve: best heard at the lower left side of the chest, near the nipple line.
- Tricuspid valve: heard between the fourth and fifth ribs, close to the left edge of the breastbone.
- Aortic valve: heard between the second and third ribs on the right side of the breastbone.
- Pulmonary valve: heard in the same area as the aortic spot, but on the left side.
When a doctor moves the stethoscope to several positions across your chest, they’re isolating each valve’s sound to check for abnormalities at each location.
How Stethoscopes Capture the Sound
A standard stethoscope has two sides: a flat, wide diaphragm and a smaller, cup-shaped bell. The diaphragm picks up higher-pitched sounds, like the normal S1 and S2 valve closures. The bell is better at capturing low-pitched sounds, like the S3 and S4 gallops. By switching between the two and pressing with different amounts of force, a clinician can selectively tune in to different frequency ranges.
Traditional stethoscope listening has limitations in sensitivity and accuracy, which is why digital stethoscopes and phonocardiograms have been developed. A phonocardiogram uses a microphone or pressure sensor placed on the chest to convert heart sounds into a visual waveform. This makes it possible to measure the exact timing, frequency, and intensity of each sound, and increasingly, to run those recordings through software that can flag abnormal patterns automatically.