The stethoscope is a universal symbol of medical practice used for auscultation, the technical term for listening to internal body sounds. The ability to distinguish between different sounds, such as the rhythmic closure of heart valves or the rustle of air in the lungs, is a powerful diagnostic tool. The chest piece, placed directly on the patient’s body, is designed to capture a wide spectrum of sound frequencies.
Anatomy of the Chest Piece
The chest piece of a traditional acoustic stethoscope has two distinct sides to capture different sound waves. The larger, flat side is the Diaphragm, covered by a thin, taut membrane. The opposite side is the Bell, a smaller, concave, open cup shape. A rotating mechanism in the stem allows the practitioner to switch between them, activating one side’s acoustic pathway while sealing the other.
Using the Diaphragm for High-Frequency Sounds
The Diaphragm is engineered to filter out lower-frequency sounds, allowing the listener to focus on sounds with a higher pitch. The taut, flat membrane vibrates readily in response to faster, shorter sound waves. This side is most frequently used for a general examination because many common body sounds fall into this higher frequency range.
The primary sounds auscultated with the Diaphragm include the normal heart sounds (S1 and S2), which represent the closing of the heart’s valves. Most breath sounds (vesicular, bronchial, and bronchovesicular) are also best heard with the Diaphragm. Bowel sounds are another common high-frequency sound picked up by this side. Using the Diaphragm is the starting point for a comprehensive physical assessment.
Using the Bell for Low-Frequency Sounds
In contrast to the Diaphragm, the Bell transmits low-frequency sounds that the taut membrane would otherwise suppress. The Bell’s open, shallow-cup design allows the skin underneath to act as the vibrating membrane. This design is more effective at resonating with slower, longer sound waves, making the Bell indispensable for detecting subtle, deeper sounds.
Specific Sounds Heard with the Bell
The Bell is useful for identifying abnormal heart sounds, such as the third heart sound (S3) and the fourth heart sound (S4), often called gallops. These low-pitched sounds are difficult to hear with the Diaphragm and provide clues about issues like heart failure or ventricular stiffness. The Bell is also the preferred method for listening to certain heart murmurs, specifically the low-pitched diastolic rumbles associated with conditions like mitral stenosis. It is also used to detect vascular sounds called bruits, which are turbulent flow noises in arteries, such as the carotid arteries in the neck.
Proper Technique and Listening Environment
Effective auscultation requires not only selecting the correct side of the chest piece but also mastering the technique of application. The pressure applied to the chest piece determines the function of the Bell. When using the Bell, the practitioner must apply only very light pressure, just enough to create a seal against the skin without stretching it. Applying too much pressure converts the skin into a taut membrane, essentially making the Bell function like a Diaphragm and filtering out the very low-frequency sounds it is meant to capture.
Conversely, when using the Diaphragm for high-frequency sounds, firm pressure must be applied to stretch its membrane fully against the skin. This tautness is what enables the Diaphragm to filter out the low-frequency noise and accentuate the higher-pitched sounds. To achieve the best possible sound clarity, the stethoscope must always be placed directly onto bare skin, as clothing creates significant friction and ambient noise that can mask subtle internal sounds. The listening environment is equally important, requiring minimal ambient noise to ensure the faint, low-volume sounds from the body are not obscured.