Adventitious heart sounds are any audible cardiac events beyond the two normal heart sounds you expect to hear through a stethoscope. They include extra sounds like third and fourth heart sounds, murmurs of varying pitch and timing, clicks, snaps, and friction rubs from the pericardium. Some are harmless, others signal serious disease, and telling them apart remains one of the trickier skills in clinical medicine.
What Counts as Normal and What Does Not
A healthy heart produces two clearly defined sounds per cycle. The first (S1) corresponds to the closure of the mitral and tricuspid valves at the start of ventricular contraction, and the second (S2) corresponds to the closure of the aortic and pulmonary valves when ventricular contraction ends. Research using simultaneous echocardiography and phonocardiography has confirmed that these sounds result from the sudden deceleration or acceleration of blood as valves snap shut or open, setting the entire heart-and-blood system vibrating.1The American Journal of Cardiology. First heart sound and ejection sounds: Echocardiographic and phonocardiographic correlation with valvular events Everything else you hear on top of those two sounds is, by definition, adventitious. That extra category is broad and encompasses sounds with very different causes, timing, and clinical significance.
Third and Fourth Heart Sounds
The third heart sound (S3) occurs in early diastole, just after the ventricles begin to fill. It happens when blood rushing in from the atria suddenly decelerates against a ventricle that has already partially filled. Doppler studies show that an S3 is linked to an unusually forceful early filling phase followed by an abrupt halt of inflow.2European Heart Journal. Pulsed Doppler evaluation of left ventricular filling in subjects with pathologic and physiologic third heart sound In younger people and pregnant women, an S3 can be entirely normal because the heart fills vigorously. In adults over about 40, an S3 more often points to a ventricle that is overstretched or poorly compliant. In severe cases, the sound reflects critically impaired blood flow, as demonstrated in a case where phonocardiography traced an S3 directly to massive mitral valve leakage and low cardiac output.3PubMed Central. Ultimate Third Heart Sound
The fourth heart sound (S4) shows up just before S1, during the final phase of diastole when the atria contract to push their last bit of blood into the ventricles. It is a low-pitched thud that occurs when the ventricle has become stiff and resists that late filling. The S4 is almost always pathologic in adults and is strongly associated with conditions that thicken or stiffen the ventricular wall, such as long-standing high blood pressure or hypertrophic cardiomyopathy. A case report documented an S4 that actually “wandered” in timing, shifting its position relative to the atrial contraction beat by beat, illustrating how closely the sound is tied to the dynamic interplay between atrial squeeze and ventricular compliance.4PubMed Central. Wandering fourth heart sound Neither the S3 nor the S4 is loud. Both sit in a very low frequency range that makes them easy to miss, especially in a noisy room or a patient with a thick chest wall.
Murmurs and What Creates Them
Murmurs are the most commonly encountered adventitious heart sounds, and they arise primarily from turbulent blood flow. When blood moves smoothly through the heart’s chambers and vessels, it is silent. When it speeds up, squeezes through a narrowed opening, or leaks backward through a faulty valve, the resulting turbulence produces vibrations that a stethoscope can pick up. Research measuring sound energy inside arteries alongside the turbulence of the flowing blood found a strikingly close relationship between the two: as turbulent power increased, so did the loudness of the murmur, and clinicians’ grading of murmur intensity tracked that power reliably.5Circulation Research. Turbulent blood flow in humans: its primary role in the production of ejection murmurs In practical terms, a louder murmur generally reflects more disrupted flow, though the relationship is not perfect because chest-wall thickness and lung tissue can dampen the sound before it reaches the stethoscope.
Systolic Murmurs
Systolic murmurs happen between S1 and S2, while the ventricles are contracting. They fall into two broad camps. Ejection murmurs grow louder in mid-systole and then taper off; they are the classic sound of aortic or pulmonary valve narrowing, where blood is forced through a tightened opening. Regurgitant murmurs tend to occupy the full span of systole and result from blood leaking backward through a valve that should be sealed shut, as happens with mitral or tricuspid regurgitation. Many systolic murmurs in otherwise healthy people, particularly children and young adults, are innocent flow murmurs that produce no long-term harm. Distinguishing an innocent murmur from one caused by structural heart disease is a central challenge of auscultation.
Diastolic Murmurs
Diastolic murmurs occur between S2 and the next S1, while the ventricles are filling. They are almost always pathologic. The classic textbook description of a diastolic murmur from aortic valve leakage is a simple, fading (decrescendo) blowing sound. The reality is messier. Phonocardiographic analysis showed that the murmur of aortic regurgitation frequently takes on a complex shape, with an early peak followed by multiple waves of waxing and waning intensity driven by pressure reflections in the aorta and even the contraction of the atria pushing against the aortic root.6The American Journal of Cardiology. Complex shape and variability of the diastolic murmur of aortic regurgitation Mitral stenosis, where a scarred mitral valve obstructs filling, produces a different diastolic murmur: a low-pitched rumble best heard with the bell of the stethoscope pressed lightly over the cardiac apex. Because diastolic murmurs are softer and lower in frequency than most systolic murmurs, they are among the easiest adventitious sounds to miss entirely.
Continuous Murmurs
A continuous murmur runs through both systole and diastole without stopping, because the pressure difference driving flow persists across the entire cardiac cycle. The best-known example in children is the venous hum, a benign sound produced by blood flowing through the jugular veins in the neck. It disappears when the child lies down or turns their head. Pathologic continuous murmurs, though, can signal abnormal connections between arteries and veins, a patent ductus arteriosus (a fetal blood vessel that fails to close after birth), or coronary artery fistulas. A review of the literature describes continuous murmurs as relatively rare but warns that they often hide complex cardiovascular disease underneath.7PubMed Central. Continuous murmur–the auscultatory expression of a variety of pathological conditions When a clinician hears one, further imaging is almost always warranted.
Clicks, Snaps, and Pericardial Friction Rubs
Not all adventitious heart sounds are murmurs. Some are brief, sharp events. An ejection click is a high-pitched sound that occurs immediately after S1, usually caused by a stiffened but mobile aortic or pulmonary valve popping open. A mid-systolic click, often followed by a late systolic murmur, is the hallmark of mitral valve prolapse, where one or both leaflets of the mitral valve billow back into the atrium during contraction. An opening snap is heard in early diastole when a stenotic mitral valve, stiffened by rheumatic disease, suddenly opens against resistance.
Pericardial friction rubs sit in their own category. They are scratchy, grating sounds produced when inflamed layers of the pericardium (the sac surrounding the heart) rub against each other. A classic friction rub has up to three components per cardiac cycle: one tied to atrial contraction in presystole, one during ventricular systole, and one during the rapid filling phase in early to mid-diastole.8The American Journal of Cardiology. Auscultatory findings in diseases of the pericardium If a patient has atrial fibrillation and therefore no organized atrial contraction, the rub drops down to two components. Rubs are notoriously transient; they may appear one hour and vanish the next as fluid accumulates in the pericardial space and separates the inflamed surfaces.
Innocent Murmurs in Children
Many children are referred to a cardiologist after a murmur is heard during a routine check-up, and most of the time the news is reassuring. Innocent (also called functional or benign) murmurs are extremely common in pediatric patients. The most recognized type is Still’s murmur, a musical or vibratory sound heard over the lower left part of the breastbone. Others include the venous hum and pulmonary flow murmurs. A deep-learning study that trained an algorithm to classify pediatric heart sounds was able to distinguish normal heart sounds, innocent murmurs, and pathologic murmurs with strong performance, achieving an area under the curve of about 0.83 for innocent murmurs and 0.88 for pathologic ones.9Artificial Intelligence in Medicine. Identifying pediatric heart murmurs and distinguishing innocent from pathologic using deep learning The fact that a machine-learning model can separate these categories suggests there are consistent acoustic features that differentiate them, even when human ears struggle. For parents, the key takeaway is that a murmur in a child is not synonymous with heart disease. It often reflects normal turbulence in a small, fast-beating heart.
How Body Size Affects What Clinicians Hear
The chest wall is not just a passive surface. It acts as an acoustic filter, and the thicker the tissue between the heart and the stethoscope, the more sound gets absorbed. Experimental data bear this out clearly: as body mass index rises from about 24 to 38, the peak amplitude of heart sounds drops by roughly 60%.10Ultrasound. Transmission of body sounds: an overview Because many adventitious heart sounds, especially the S3, S4, and low-pitched diastolic murmurs, already sit at the edge of human hearing, obesity can push them below the threshold of detection entirely. This is a genuine clinical problem. A pathologic S3 that would prompt urgent treatment in a thin patient may simply be inaudible in someone with a higher BMI. The effect is so significant that clinicians sometimes rely more heavily on echocardiography in larger patients rather than trusting a “normal” auscultation exam.
Bedside Maneuvers That Change What You Hear
One of the most useful features of adventitious heart sounds is that many of them change predictably with simple physical maneuvers, which helps pin down the diagnosis. Squatting, for instance, increases blood return to the heart and raises the resistance the heart pumps against. In hypertrophic cardiomyopathy, where a thickened heart muscle creates a dynamic obstruction during contraction, the murmur characteristically gets quieter or even disappears when the patient squats, because the extra blood volume opens up the outflow tract. A case report documented exactly this: two patients with obstructive hypertrophic cardiomyopathy had systolic ejection murmurs that increased with the Valsalva maneuver and upon standing, but during a prompt squat, the murmur vanished in one and dropped to barely audible in the other.11PubMed Central. Squat in Obstructive Hypertrophic Cardiomyopathy
Other maneuvers include having the patient lean forward and exhale (which brings the heart closer to the chest wall and makes aortic regurgitation murmurs louder), rolling onto the left side (which brings the cardiac apex closer to the stethoscope, amplifying a mitral murmur or S3), and sustained handgrip (which raises blood pressure and makes regurgitant murmurs louder while quieting stenotic ones). None of these maneuvers requires any equipment, and each exploits a different aspect of how blood flow and heart loading change with posture and effort.
Why Auscultation Skills Are Eroding and What Helps
There is a well-documented decline in auscultation proficiency among clinicians, driven partly by the ready availability of echocardiography. When an ultrasound machine can show you the valve in real time, the incentive to train your ears fades. But echocardiography is expensive, not always immediately available, and completely impractical for screening large populations. Auscultation remains the first filter that determines whether further testing happens.
Simulation-based training has proven to be one of the most effective remedies. A study found that third-year medical students who went through a structured simulation program achieved significantly higher accuracy at identifying heart sounds than untrained fourth-year students, both with simulated sounds (about 94% versus 74%) and with real patients (about 82% versus 75%).12PubMed Central. Simulation-based mastery learning improves cardiac auscultation skills in medical students A meta-analysis pooling results from 13 studies that compared simulation training to no intervention confirmed large positive effects on both knowledge and hands-on skill.13PubMed Central. Simulation-based training for cardiac auscultation skills: systematic review and meta-analysis A broader systematic review reached the same conclusion, adding that simulation also reduced learner anxiety and improved confidence.14PubMed Central. Utilization of Simulation to Teach Cardiac Auscultation: A Systematic Review Hands-on repetition with a simulator, rather than just listening to recordings, appears to be the key ingredient.
Digital Stethoscopes and AI-Assisted Listening
Digital stethoscopes convert acoustic signals into electronic data that can be amplified, filtered, recorded, and analyzed by software. This opens the door to artificial intelligence tools that could flag abnormal sounds automatically, potentially catching murmurs or gallops that a busy clinician’s ear might miss. The pediatric deep-learning model mentioned earlier is one example of this approach in action.
A practical obstacle, however, is that different digital stethoscope models pick up sound differently. A technical comparison of several commercial devices found significant variability in their frequency responses, meaning the same heart sound recorded by two different stethoscopes can look quite different to an algorithm.15PubMed Central. Technical characterisation of digital stethoscopes: towards scalable artificial intelligence-based auscultation Even two units of the same model showed moderate variation. For AI-based auscultation to scale, the field needs standardized ways to normalize recordings across devices so that an algorithm trained on one stethoscope’s data works reliably with another.
A separate challenge is perceptual. Much of the heart sound spectrum sits in very low frequencies where human hearing is naturally weak. A real-time frequency-shifting system was developed to address this, pushing heart sound energy into a higher range where the ear is more sensitive.16Electronics. Real-Time Implementation of a Frequency Shifter for Enhancement of Heart Sounds Perception on VLIW DSP Platform Whether through frequency shifting, amplification, or AI classification, the trend is toward augmenting the human ear rather than replacing it.
Sounds From Prosthetic Heart Valves
Patients who have had a mechanical heart valve implanted carry a distinctive set of adventitious sounds that are entirely expected. Mechanical valves produce audible clicks when their leaflets snap shut, and the acoustic profile of those clicks varies by valve design, implant position, and how well the valve is functioning. An in vitro study using a pulse duplicator measured the closing sounds of five commercial bileaflet mechanical valves and found that each had a characteristic spectral signature. Different working conditions changed the sound profile, and the reproducibility and intensity of the closing sounds allowed the researchers to rank how “noisy” each valve was.17PubMed. Bileaflet mechanical heart valve closing sounds: in vitro classification by phonocardiographic analysis For patients and clinicians, this matters because a change in the character of the click over time, particularly if it becomes muffled or irregular, can be an early warning that a leaflet is stuck or that tissue is growing over the valve. Some patients learn to recognize the rhythm of their own valve click and notice changes before a stethoscope does.
Auscultation Beyond Humans
Adventitious heart sounds are not uniquely a human concern. Veterinary medicine relies heavily on cardiac auscultation, especially in large animals where echocardiography can be logistically difficult. Horses, for example, have a high prevalence of cardiac murmurs and arrhythmias, many of which are physiologic and performance-compatible. Training veterinary students to auscultate equine hearts presents its own challenges: heart rates are slower, the anatomy is different, and normal variants that would be alarming in a human (like certain irregular rhythms) are expected in horses. A study assessing a teaching module for veterinary students found that fourth-year students rated themselves as less competent at identifying equine murmurs and arrhythmias compared to fifth-year students, but after exposure to the structured learning resource, pooled student abilities to identify these sounds improved substantially.18PubMed Central. Assessment of a Teaching Module for Cardiac Auscultation of Horses by Veterinary Students The parallel to human medical education is clear: structured, repetitive exposure to the sounds is what builds competence, regardless of the species being examined.