S1 and S2 are the two sounds that make up the familiar “lub-dub” of a normal heartbeat. S1, the first sound, corresponds to the closing of the valves between the heart’s upper and lower chambers at the start of each contraction. S2, the second sound, marks the closing of the valves that guard the exits to the aorta and the pulmonary artery once the contraction finishes. Together they bracket systole, the pumping phase of each cardiac cycle, and give a trained listener a surprising amount of information about what is happening inside the chest.
What Produces S1
The first heart sound happens right at the beginning of ventricular contraction, when the mitral and tricuspid valves snap shut. For a long time, clinicians assumed S1 was simply the noise of those valve leaflets slapping together, but detailed recordings show the picture is more complicated. The valve leaflets themselves close fairly quietly; most of the audible energy comes from vibrations that ripple through the surrounding heart muscle, blood, and valve structures immediately after closure. The mitral component tends to be the louder of the two, partly because the left ventricle generates higher pressures than the right. In a healthy heart, the mitral and tricuspid components are so close together in time that they usually merge into a single “lub.”
What Produces S2
S2 marks the end of the heart’s pumping stroke. As the ventricles relax, blood in the aorta and pulmonary artery briefly flows backward toward the heart, catches the semilunar valve leaflets, and forces them shut. Research using high-fidelity pressure sensors and echocardiography has shown that the valves themselves close almost silently; the audible “dub” arises from the vibrations that follow, rippling through the valve cusps, the walls of the great vessels, and the columns of blood within them.1PubMed Central. Clinical Methods: The History, Physical, and Laboratory Examinations The energy driving those vibrations comes from the sudden deceleration of the retrograde blood column when it hits the tensed leaflets. That abrupt stop sets the whole cardiohemic system ringing, and the result is S2.2PubMed. Echocardiographic observations on the mechanism of production of the second heart sound
How They Sound Different
Even without a phonocardiogram, experienced clinicians can distinguish S1 from S2 by pitch and duration. S1 is typically lower-pitched and slightly longer, while S2 is higher-pitched, crisper, and a bit shorter. Acoustic analysis of normal phonocardiogram recordings puts S1’s average frequency range around 107 Hz and S2’s around 154 Hz, confirming the perceptible pitch difference. The same data show that S1 lasts somewhat longer per beat than S2, though both are brief, each well under 200 milliseconds.3Clinical Case Reports Journal. Analysis of the Four Heart Sounds Statistical Study and Spectro-Temporal Characteristics The frequency content of S2 also fluctuates more from beat to beat than S1’s does, which makes sense given how sensitive S2 is to changes in blood pressure and breathing.
In practical terms, S1 is best identified by timing it against the pulse. Feel the carotid artery in the neck while listening: the sound that comes just as you feel the pulse is S1. The pause between S1 and S2 (systole) is noticeably shorter than the pause between S2 and the next S1 (diastole), at least at resting heart rates, which gives the heartbeat its characteristic galloping rhythm.
Where to Listen on the Chest
Each heart sound is loudest in specific spots on the chest wall, and these spots do not sit directly over the valves that produce the sounds. Instead, they reflect where blood flow carries the vibrations most efficiently. There are five classic auscultation points, each corresponding to a valve area:
- Aortic area: right side of the sternum at the second intercostal space.
- Pulmonic area: left side of the sternum at the second intercostal space.
- Erb’s point: left side of the sternum at the third intercostal space, a useful spot for hearing S2 clearly.
- Tricuspid area: left sternal border at the fourth intercostal space.
- Mitral area (the apex): left side of the chest at the fifth intercostal space, along the midclavicular line, where S1 is typically loudest.
Moving the stethoscope between these five locations lets you compare S1 and S2 intensity. S1 tends to be louder at the apex, where the mitral valve contribution dominates, while S2 is louder at the base of the heart, near the aortic and pulmonic listening areas. If S1 is louder than S2 at the base, or S2 louder than S1 at the apex, something unusual may be going on.
Why S2 Splits When You Breathe In
One of the most clinically useful features of S2 is that it can split into two audible components during normal breathing. The split happens because the aortic valve and the pulmonic valve do not close at exactly the same instant. When you inhale, the pulmonic component (called P2) is delayed slightly relative to the aortic component (A2), widening the gap between them enough that a careful listener can hear two distinct clicks instead of one.
The traditional explanation for this split was that inhalation pulls more blood into the right side of the heart, giving the right ventricle more work to do and delaying pulmonic valve closure. That story is incomplete. Research has shown that the more important factor is an inspiratory drop in the resistance of the pulmonary vascular bed. During inhalation, pulmonary blood vessels dilate, which lowers impedance and changes the timing of the backflow that closes the pulmonic valve.4PubMed. Mechanism of normal splitting of the second heart sound During expiration, the two components move closer together and typically merge back into a single sound. This respiratory variation is normal and expected, and its presence is actually reassuring. S1 can show a split morphology during inspiration as well, though it is subtler and less commonly assessed.5PubMed. Respiratory modulation of heart sound morphology
Abnormal splitting patterns carry diagnostic weight. A split that widens on expiration instead of inspiration (paradoxical splitting) can point to conditions like left bundle branch block or severe aortic stenosis. A split that stays fixed regardless of breathing phase suggests an atrial septal defect. Recognizing these patterns has been a cornerstone of bedside cardiac diagnosis for over a century.
What Changes the Loudness of S1
The loudness of S1 is not fixed. It depends heavily on where the mitral valve leaflets are positioned at the exact moment the ventricle starts contracting, which in turn depends on the timing between the atria and ventricles. When the electrical signal from the atria to the ventricles is fast (a short PR interval on an electrocardiogram), the mitral valve is still wide open when contraction hits, and the valve has to travel a longer distance to close. It slams shut on a steep, rapid part of the pressure rise, producing a loud S1. When the PR interval is long, the leaflets have already drifted mostly closed before the ventricle contracts, and closure is gentle and quiet.6Heart. Mechanism of influence of PR interval on loudness of first heart sound
This relationship has practical consequences. In conditions like first-degree heart block, where the PR interval is prolonged, S1 tends to be soft. In conditions where the PR interval varies from beat to beat, such as complete heart block, S1 varies in loudness too, sometimes dramatically, which can be one of the first bedside clues to the diagnosis. Mitral stenosis can also produce a loud S1, because the stiff valve leaflets are held open longer by the pressure gradient across them and then close abruptly once ventricular pressure overcomes the obstruction.
S2 Loudness and Pulmonary Artery Pressure
A loud P2 component of S2 has long been taught as a sign of pulmonary hypertension, the logic being that higher pressure in the pulmonary artery slams the pulmonic valve shut harder. The clinical reality is less tidy. In one study of patients with interstitial lung disease, neither the overall S2 amplitude nor the P2 amplitude alone showed a statistically significant correlation with pulmonary artery systolic pressure.7PubMed Central. Usefulness of the second heart sound for predicting pulmonary hypertension in patients with interstitial lung disease The problem is that chest-wall thickness, lung hyperinflation, and other acoustic barriers can muffle the sound before it reaches the stethoscope.
A more promising approach looks at the ratio of P2 to A2 rather than the raw loudness of either component. A pediatric study using a digital stethoscope found that the P2-to-A2 intensity ratio, and the P2-to-total-S2 ratio, were both significantly different between children with pulmonary arterial hypertension and those without, and correlated linearly with mean pulmonary artery pressure.8PubMed Central. Time-domain analysis of heart sound intensity in children with and without pulmonary artery hypertension: a pilot study using a digital stethoscope Ratios are more useful than absolute loudness because they cancel out much of the noise introduced by body habitus and recording conditions. This kind of analysis is hard to do by ear, which is one reason digital auscultation is attracting research interest.
Heart Sounds in Older Adults
A common assumption is that older adults typically lose the normal splitting of S2, leaving them with a single second sound. A phonocardiographic study of 103 elderly subjects (ages 60 to 99) without overt heart disease found that about 55% had a normal splitting pattern, while roughly 42% had a single second sound. Those proportions were not significantly different from a comparison group of younger adults.9PubMed Central. The second heart sound in old age A single S2 in an older person, in other words, is not automatically a sign of disease. What was unusual, though, was paradoxical (reversed) splitting, found in only three of those 103 subjects and considered a marker of serious underlying cardiac dysfunction even in the absence of symptoms. The practical takeaway is that the splitting behavior of S2 remains a useful diagnostic clue well into advanced age.
Using Heart Sounds to Measure Cardiac Timing
S1 and S2 serve as acoustic bookends for systole, and the interval between them gives clinicians a non-invasive measure of how long the heart is contracting. Two intervals matter most. The pre-ejection period (PEP) runs from the start of the electrical impulse on an ECG to S1 and reflects how quickly the ventricle builds up enough pressure to open the aortic valve. The left ventricular ejection time (LVET) spans roughly from S1 to S2 and reflects the duration of actual blood ejection.
Measuring these intervals precisely has traditionally required specialized lab equipment, but newer algorithms can extract them from simultaneous ECG and phonocardiogram recordings. One study found that an automated method achieved an average timing error of about 8 milliseconds for PEP and about 11 milliseconds for LVET in healthy subjects, with somewhat larger errors in patients with cardiovascular disease.10Physiological Measurement. Beat-to-beat systolic time-interval measurement from heart sounds and ECG Those numbers are precise enough to be clinically useful for tracking changes in heart function over time without repeated imaging.
Digital Analysis and Automated Segmentation
A growing area of research focuses on teaching algorithms to identify and segment S1 and S2 automatically from audio recordings. This is harder than it sounds. Real-world heart recordings contain breath sounds, bowel noises, ambient hum, and movement artifacts that can obscure the target signals. One recent algorithm based on Shannon energy analysis achieved boundary-marking accuracy within about 0.3 milliseconds for both S1 and S2 on a large public dataset, with a sensitivity above 97% for identifying the correct heart-sound segments.11PubMed Central. A Noise-Robust Heart Sound Segmentation Algorithm Based on Shannon Energy Algorithms like these could eventually power screening tools that flag abnormal heart sounds in primary care or even through smartphone-based recording devices, which would be valuable in settings where specialist cardiology access is limited.
The clinical promise goes beyond sorting normal from abnormal. If an algorithm can reliably locate S1 and S2 boundaries, it can then analyze the interval between them, the relative intensity of each component, and the spectral profile, feeding those measurements into diagnostic models for conditions like valvular disease, heart failure, or pulmonary hypertension. The P2-to-A2 ratio analysis described earlier is one example of a measurement that becomes practical only when a machine does the work.
Mechanical Heart Valves and Altered Sounds
Patients with prosthetic heart valves produce heart sounds that differ from natural ones, and the character of those sounds matters. Mechanical valves generate sharp, high-pitched closing clicks that replace or modify the normal S1 or S2 depending on which valve was replaced. In vitro testing of several common bileaflet mechanical valves showed measurable differences in noise levels and reproducibility across valve designs, with some models producing quieter and more consistent closing sounds than others.12SpringerLink / Journal of Artificial Organs. Bileaflet mechanical heart valve closing sounds: in vitro classification by phonocardiographic analysis For patients living with these valves, the clicking sound can be audible without a stethoscope, sometimes loud enough to affect sleep. For clinicians, changes in the character of the prosthetic valve click over time can signal thrombosis, pannus growth, or leaflet malfunction before imaging catches it, making ongoing auscultation a cheap and useful monitoring tool.
Fetal Heart Sounds
The same S1 and S2 framework applies to the fetal heart, though listening to it through the mother’s abdomen introduces layers of acoustic interference. Fetal phonocardiography, which records heart sounds from sensors placed on the mother’s belly, has been used since the mid-twentieth century to monitor fetal well-being. Recent work has pushed the analysis further, using wavelet transforms to study the individual vibrations that make up fetal S1. A study of over a thousand S1 events from pregnancies between 33 and 40 weeks identified five distinct sub-vibrations within each first heart sound, with characteristics that changed across gestational age groups.13PubMed. Time-frequency characteristics of the vibrations underlying the first fetal heart sound: a preliminary study The goal of this kind of research is to develop non-invasive markers of fetal cardiac maturation and potentially detect structural heart problems before birth, without requiring the specialized equipment and expertise needed for fetal echocardiography.
Two Centuries of Listening
Before 1816, the only way a physician could hear heart sounds was by pressing an ear directly to the patient’s chest. That year, René-Théophile-Hyacinthe Laënnec, examining a young woman with suspected heart disease in Paris, felt that direct contact was inappropriate and improvised by rolling a sheet of paper into a tube. He was startled by how much clearer the heart sounds became, and within a few years he had developed the first true stethoscope, a hollow wooden cylinder.14PubMed Central. The first 200 years of cardiac auscultation and future perspectives The device evolved through monaural and binaural designs across the nineteenth century, with the familiar dual-earpiece configuration becoming standard by the early 1900s.15PubMed Central. The History and Evolution of the Stethoscope
Electronic stethoscopes, which amplify and filter sounds, began appearing in the late twentieth century and are now paired with software that can display spectrograms in real time. Handheld ultrasound devices have also entered the picture, and some researchers have argued they should replace traditional auscultation entirely. Yet the acoustic stethoscope persists, in part because it is cheap, portable, and requires no battery, but also because the skill of listening forces the clinician to think systematically about what is happening inside the heart at each moment of the cardiac cycle. S1 and S2 remain the anchoring landmarks for that thought process, just as they were when Laënnec first pressed a rolled-up paper to a patient’s chest.