Physics of Heart Sounds & Murmurs
Now that you understand what normal heart sounds are and when they occur, let's dive into the why. Understanding the physics behind heart sounds will transform you from someone who memorizes findings to someone who can reason through any cardiac exam.
Here's a counterintuitive question to consider: if a patient has severe aortic stenosis, would you expect their murmur to be louder or softer than someone with moderate stenosis?
Counterintuitively, the murmur may be softer in severe stenosis. As the valve becomes critically narrowed, cardiac output drops—there's simply less blood flowing across the valve. Less flow means less turbulence, despite the high velocity. This is the "orifice size paradox": moderate disease is often louder than severe disease. Never judge severity by murmur intensity alone.
What Is a Heart Sound?
At its most basic, a heart sound is vibration. When valves snap shut, when blood decelerates suddenly, or when turbulent flow occurs, these events create vibrations in the cardiac structures and blood column. These vibrations transmit through the chest wall to your stethoscope.
The Frequency Spectrum
Heart sounds and murmurs occur across a range of frequencies. Low-frequency sounds (20-100 Hz) include S3, S4, and the rumbles of mitral and tricuspid stenosis. Medium-frequency sounds (100-500 Hz) encompass S1, S2, and most murmurs. High-frequency sounds (above 500 Hz) include aortic regurgitation, mitral regurgitation, and clicks.
Your ear can detect roughly 20-20,000 Hz, but your stethoscope (and the chest wall) filters out many frequencies. This is why technique matters—the diaphragm filters out low frequencies, while the bell (when applied lightly) preserves them.
Laminar vs. Turbulent Flow
Normal blood flow is laminar—smooth, parallel layers sliding past each other like cars in separate lanes on a highway. Laminar flow creates minimal vibration and is essentially silent. Your blood is always flowing through your heart, yet you only hear sounds at specific moments (valve closure, turbulent flow). The flow itself, when laminar, doesn't create sound.
Murmurs occur when blood flow becomes turbulent—chaotic, swirling, with eddies and vortices. This creates vibrations across a range of frequencies that you can hear. Turbulence is promoted by high velocity, narrow orifices, sudden changes in diameter, low viscosity (anemia), and irregular surfaces.
Clinical Application: Anemia
Anemic patients often have systolic flow murmurs not because they have structural heart disease, but because their blood has lower viscosity. This creates turbulent flow across normal valves. Treat the anemia, and the murmur often disappears.
The Orifice Size Paradox
This is counterintuitive and clinically crucial. In mild-to-moderate stenosis, the valve is narrowed enough to create high-velocity jets, but cardiac output is still adequate—result: LOUD murmur. In severe stenosis, the valve is so narrow that cardiac output drops—result: softer murmur despite worse disease.
Critical Clinical Pearl
Loud ≠ Severe. A loud murmur does NOT mean severe disease, and a soft murmur does NOT mean mild disease. Always correlate with hemodynamics, symptoms, and imaging.
Pitch and Your Stethoscope
High-frequency murmurs (regurgitant lesions like AR, MR, TR) are best heard with the diaphragm. Low-frequency murmurs (stenotic AV valves, S3, S4) require the bell applied lightly. If you're using the diaphragm and miss a rumble that the bell picks up, you've just found mitral stenosis.
Sound Transmission
Sound must travel from the heart through tissue to your stethoscope. Dense tissues (bone, muscle) conduct well—this is why AS radiates to the carotids. Air conducts poorly—COPD muffles heart sounds. Fat dampens sound—obesity makes everything quieter. Fluid can muffle high frequencies—pericardial effusion creates distant heart sounds.
Understanding these physics principles transforms you from a passive listener into an active diagnostician. This is the foundation for everything that follows.