Every heartbeat produces a pair of sounds, commonly written as “lub-dub,” that a stethoscope picks up dozens of times per minute. The first sound (“lub”) comes from the left ventricle snapping into action as its valves close, and the second (“dub”) marks the closure of the valves that sit at the exits of the heart’s two pumping chambers. Between, around, and sometimes on top of those two beats, clinicians listen for extras: galloping rhythms, whooshing murmurs, clicks, and rubs. Some of these are perfectly harmless; others are early clues to structural problems that imaging alone might not catch in time.
Where the Lub Comes From
The first heart sound, often labeled S1, coincides with the moment the heart’s lower chambers begin to squeeze. For decades, textbooks attributed it simply to the mitral and tricuspid valves slamming shut. Research has refined that picture considerably. The sound originates primarily in the left ventricle, driven by accelerations and decelerations of the ventricular wall that are “timed” by events at the mitral and aortic valves rather than by raw valve closure alone. Three recognizable components make up S1: the first appears when the ventricular wall and septum reach a certain tension, the second when the aortic valve opens, and the third when the aortic pulse hits its peak.1Japanese Heart Journal. The First Heart Sound in Normal and Pathological Conditions In practical terms, this means the loudness and quality of the lub reflect how forcefully the left ventricle contracts and how quickly pressure builds inside it. A snappy, loud S1 suggests brisk contraction; a muffled one can hint at a sluggish ventricle, a stiff or calcified mitral valve, or simply a thick chest wall dampening the vibration.
Where the Dub Comes From, and Why It Sometimes Splits
The second heart sound, S2, marks the end of each contraction. It happens when the aortic and pulmonic valves close almost simultaneously as blood pressure in the large arteries exceeds the fading pressure inside the ventricles. Because the left and right sides of the heart do not relax at exactly the same instant, the aortic component (A2) and the pulmonic component (P2) can separate slightly, producing what clinicians call “splitting.”2PubMed Central. Second heart sound splitting as an indicator of interventricular mechanical dyssynchrony using a novel splitting detection algorithm
In most people, that split widens during a normal breath in and narrows again during a breath out. The traditional explanation was that breathing in pulls more blood into the right side of the heart, making the right ventricle take longer to empty. The actual mechanism turns out to be more nuanced. A study measuring pressures directly inside the heart found that the single largest contributor to inspiratory splitting is a drop in the resistance of the lung’s blood vessels during inhalation, which delays the pulmonic valve’s closure relative to the aortic valve. Changes in right ventricular filling time played a smaller role than expected.3PubMed. Mechanism of normal splitting of the second heart sound For the person being examined, none of this is audible without a stethoscope. But for clinicians, the way S2 splits, or fails to split, is a fast bedside clue to whether the two ventricles are working in sync.
When splitting is “fixed,” meaning it does not change with breathing, it often points to an atrial septal defect, a hole between the heart’s upper chambers that equalizes filling on both sides. When splitting is “paradoxical,” with the split widening on expiration instead of inspiration, the likely culprit is a delay on the left side, as seen in left bundle branch block or severe aortic stenosis. Learning to hear these patterns is one of the trickiest skills in physical examination, and even experienced clinicians sometimes disagree on what they hear.
The Third Heart Sound and What It Signals
Beyond the normal lub-dub, two extra sounds are worth knowing about. The third heart sound, S3, is a low-pitched thud that occurs shortly after S2, during the phase when the ventricle is filling rapidly with blood. In healthy children and young adults, S3 can be entirely normal, a benign byproduct of a compliant, elastic ventricle filling quickly. In older adults, though, an S3 usually means something is wrong.
A large study of patients with valvular heart disease found that S3 was common in mitral regurgitation, present in about 46 percent of those patients, but uncommon in aortic stenosis, turning up in only about 11 percent. The clinical meaning of S3 depended heavily on the underlying valve problem. In aortic stenosis, hearing an S3 almost always meant the left ventricle’s pumping function had declined significantly and filling pressures were elevated. In mitral regurgitation, S3 was so common that it did not reliably indicate ventricular dysfunction or high filling pressures on its own.4PubMed. Implications of third heart sounds in patients with valvular heart disease A systematic review and meta-analysis confirmed that S3 by itself has lower sensitivity for diagnosing heart failure compared with imaging-based measures of how well the ventricle squeezes, though it can be useful for early detection of pathological changes.5PubMed Central. A systemic review and meta-analysis comparing the ability of diagnostic of the third heart sound and left ventricular ejection fraction in heart failure
The Fourth Heart Sound and a Stiffening Heart
The fourth heart sound, S4, occurs just before S1. It is produced when the atrium contracts forcefully against a ventricle that resists filling, usually because the ventricular wall has become thickened or stiff. You cannot have an S4 if your heart is in atrial fibrillation, because the atria are not contracting in a coordinated way. That quirk makes S4 a marker not just of ventricular stiffness but also of intact atrial function.
A study of patients with transthyretin amyloidosis, a condition where abnormal protein deposits stiffen the heart, illustrated this neatly. Among patients in normal rhythm, about 47 percent had an S4 detected by phonocardiography. Those with an audible S4 actually had better-preserved atrial squeezing function and lower levels of a stress hormone (BNP) released by overstretched heart muscle than those without it. In this population, the absence of S4 in someone whose ventricle was known to be stiff carried a worse short-term prognosis, because it suggested the atrium had lost its ability to generate the forceful kick that produces the sound.6PubMed Central. Importance of fourth heart sound and preserved left atrial function in wild-type transthyretin amyloidosis The broader lesson is that extra heart sounds are not automatically “bad.” Context determines whether they are reassuring, irrelevant, or alarming.
What Creates a Murmur
Murmurs are the whooshing, blowing, or rumbling sounds heard between or during the normal beats. The conventional explanation has long been that turbulent blood flow, analogous to rapids in a river, generates vibrations that travel through the chest wall. Research using direct measurements inside arteries supports turbulence as a primary driver: one study showed a strong linear relationship between turbulent energy and sound energy, with murmurs becoming audible at the chest wall once the turbulent power exceeded a specific threshold.7PubMed. Turbulent blood flow in humans: its primary role in the production of ejection murmurs
An alternative theory proposed that much of the acoustic energy in murmurs comes not from classical turbulence but from periodic fluctuations in the “wake” downstream of any obstacle in the blood’s path, similar to the way wind flowing past a telephone wire creates an audible hum. Under this model, even minor protuberances or irregularities on valve leaflets or vessel walls could produce murmurs, especially when blood velocity increased even modestly.8The American Journal of Medicine. A general theory of the causes of murmurs in the cardiovascular system In practice, both mechanisms probably contribute, and the debate is largely academic for patients. What matters clinically is that murmurs arise when blood flows faster than normal, through a narrowed or leaky valve, across an abnormal structure, or simply through a heart that is pumping hard for any reason.
Murmurs are classified by when they occur in the cardiac cycle. Systolic murmurs happen during contraction, between S1 and S2. Diastolic murmurs happen during the filling phase, between S2 and the next S1. A diastolic murmur is almost always considered pathological and warrants further investigation. A case of mid-ventricular obstructive hypertrophic cardiomyopathy, for instance, produced a distinctive low-pitched diastolic rumble caused by blood being forced through a narrowed segment of the ventricle during filling.9PubMed Central. Diastolic murmur in mid-ventricular obstructive hypertrophic cardiomyopathy: A case report
Innocent Murmurs and Who Gets Them
Not all murmurs mean disease. Innocent murmurs, sometimes called “flow murmurs” or “functional murmurs,” are extremely common in children and are often heard in pregnant women as well. These result from blood moving briskly through a structurally normal heart. In children and pregnant women, the murmurs are associated with lower blood-cell concentration and higher ejection velocity, meaning the heart is pumping a thinner, faster-moving fluid, which creates enough vibration to be heard through a stethoscope.10PubMed. Innocent murmurs: a suspect diagnosis in non-pregnant adults
In non-pregnant adults, however, the label “innocent murmur” deserves more skepticism. The same study noted that the hemodynamic conditions that justify innocent murmurs in children and pregnant women, specifically a lower blood-cell concentration paired with faster flow, are not typically present in other adults. When a new murmur appears in an adult for the first time, the default clinical response is to investigate with echocardiography rather than dismiss it as benign. This is an area where public understanding often lags: many adults assume that a murmur detected during a routine exam is automatically harmless because they remember being told as a child that murmurs are “nothing to worry about.” The pediatric reassurance does not transfer to adulthood without fresh evidence from imaging.
How Doctors Use Heart Sounds at the Bedside
Heart sounds originate as vibrations of the valves and nearby structures, then travel as low-velocity waves across the ventricle walls and great vessels until they reach the chest surface, where they radiate outward like ripples.11PubMed Central. BIOPHYSICS OF HEART SOUNDS AND ITS APPLICATION TO CLINICAL AUSCULTATION Because each valve is closest to a specific spot on the chest, clinicians listen at four traditional areas: the aortic area near the upper right chest, the pulmonic area near the upper left, the tricuspid area at the lower left sternal border, and the mitral area at the apex of the heart near the left nipple. A murmur that is loudest in one area generally points toward the valve nearest that spot, though sound can radiate unpredictably.
Clinicians also use simple bedside maneuvers to sharpen what they hear. The Valsalva maneuver, essentially bearing down as if straining, reduces the amount of blood returning to the heart. Most murmurs get quieter when there is less blood flowing through, but the murmur of hypertrophic obstructive cardiomyopathy gets louder, because the reduced blood volume makes the obstruction worse. This response was described as a bedside diagnostic test specifically useful for distinguishing that condition from other causes of systolic murmurs.12The American Journal of Cardiology. Valsalva’s maneuver and the systolic murmur of hypertrophic subaortic stenosis: A bedside diagnostic test Similarly, having a patient squat (which increases blood return) or stand suddenly (which decreases it) can help sort out what kind of valve problem is producing a murmur, all without any imaging equipment.
Digital Stethoscopes and AI-Assisted Listening
Traditional auscultation demands years of training and a quiet room, and even experienced clinicians miss subtle findings. The electronic stethoscope has opened the door to computer-aided analysis by converting sound into digital signals that can be filtered, amplified, and recorded.13PubMed Central. The electronic stethoscope Several research groups have tested whether machine-learning algorithms can match or exceed human ears.
One deep learning algorithm tested on a large database of digital stethoscope recordings detected murmurs with a sensitivity of about 76 percent and a specificity of about 91 percent overall. When softer, grade-1 murmurs were excluded, sensitivity jumped to 90 percent. For detecting moderate-to-severe aortic stenosis specifically, the algorithm achieved about 93 percent sensitivity and 86 percent specificity, performance described as comparable to expert cardiologists.14PubMed Central. Deep Learning Algorithm for Automated Cardiac Murmur Detection via a Digital Stethoscope Platform A separate study focused on congenital heart disease in children reported even higher numbers: 97 percent sensitivity and 89 percent specificity for detecting abnormal heart sounds.15PubMed Central. Artificial intelligence-assisted auscultation in detecting congenital heart disease A more recent model using a convolutional recurrent neural network architecture reported 90.5 percent accuracy in classifying murmurs from audio recordings.16PubMed Central. AI-Enhanced Detection of Heart Murmurs: Advancing Non-Invasive Cardiovascular Diagnostics
These tools are not replacing cardiologists. Their strongest use case is as a screening layer: a nurse or general practitioner with a digital stethoscope could record a few seconds of heart sound, let the algorithm flag anything suspicious, and refer only those patients for echocardiography. In settings where access to specialists is limited, that workflow could catch valve disease years earlier than it would otherwise be found. The technology is still maturing, and real-world accuracy in noisy clinic environments tends to be lower than in curated research databases, but the trajectory is clear.
When You Can Hear Your Own Heart
Most people never notice their heartbeat acoustically unless they are lying in a very quiet room with one ear pressed into a pillow. But some conditions make the heart’s sounds audible to the person themselves. Pulsatile tinnitus, a rhythmic whooshing or thumping in one or both ears that matches the heartbeat, is often vascular in origin. It can arise from arterial turbulence near the ear, carotid artery narrowing, abnormal connections between arteries and veins, valvular heart disease, or any condition that increases cardiac output and blood flow velocity.17PubMed Central. A case report of self-audible pericardial rub secondary to acute pericarditis from post-pericardiotomy syndrome (pericardial rub tinnitus) In one unusual case described in the literature, a patient could hear a scratchy, rhythmic sound caused by inflammation of the sac around the heart (pericarditis) following surgery, a phenomenon the authors labeled “pericardial rub tinnitus.” If you notice a new, persistent sound in your ears that pulses with your heartbeat, it is worth mentioning to a doctor. It is often benign, but occasionally it is the first clue to a treatable vascular or cardiac problem.
Prosthetic Valve Sounds and Living With a Clicking Heart
People with mechanical heart valve replacements live with an extra set of sounds. Mechanical valves, made of carbon or metal, produce distinct clicking sounds as their leaflets open and shut. These clicks can be loud enough that the person hears them constantly, and in quiet environments, other people nearby may hear them too. Acoustic analysis of prosthetic valve closing sounds has shown that it is possible to detect minor mechanical changes to a valve by examining the sound’s characteristics, potentially serving as a non-invasive way to monitor valve function over time.18PubMed. Acoustic analysis of the closing sounds of implanted prosthetic heart valves
For patients, the clicking is sometimes more than a curiosity. Some find that the constant audible reminder of their mechanical valve disrupts sleep, especially in the early months after surgery. Research has explored non-drug interventions, such as sound-masking techniques, relaxation training, and environmental modifications, to improve sleep quality in these patients, with positive results.19PubMed Central. Cardiac Valve Noise Reduction by Non-Drug Interventions Improves the Sleep Quality of Patients after Mechanical Cardiac Valve Implantation Bioprosthetic valves, made from treated animal tissue, are quieter because they flex rather than snap, which is one reason some patients and surgeons prefer them despite their shorter lifespan. For anyone weighing valve replacement options, the acoustic profile of the prosthesis is a quality-of-life factor that is easy to overlook in preoperative discussions but hard to ignore at three in the morning.