The stethoscope is the primary instrument used by healthcare professionals for auscultation, the practice of listening to internal body sounds. Modern stethoscopes have a chest piece designed with two distinct sides, the diaphragm and the bell, each engineered to detect different sound characteristics. Understanding the functional difference between these two components is fundamental to accurately interpreting the subtle acoustic cues generated by the cardiovascular system.
The Diaphragm and High-Frequency Sounds
The diaphragm is the flat, rigid side of the stethoscope’s chest piece, covered by a tight, thin plastic membrane. This structure is specifically designed to detect sounds with higher frequencies (high pitch) by filtering out the lower-pitched sounds. When the clinician presses the diaphragm firmly against the patient’s skin, the tight membrane vibrates primarily in response to high-frequency sound waves. The firm pressure applied is necessary to maintain the tension on the diaphragm’s membrane, which is the key to its filtering action.
The diaphragm is the tool of choice for listening to most normal, higher-pitched heart sounds. The first heart sound (S1) and the second heart sound (S2) are typically high-frequency sounds. Certain pathological sounds, like the high-pitched early diastolic murmur of aortic regurgitation, are also best heard with this component.
The Bell and Low-Frequency Sounds
In contrast to the diaphragm, the bell is the smaller, cup-shaped side of the chest piece with no membrane covering the opening. This design is optimized for picking up low frequencies (low pitch). When the bell is placed lightly on the skin, the patient’s skin itself acts as the vibrating membrane, which is loose and compliant. This loose membrane responds readily to the slower, low-frequency sound waves.
The bell is employed to detect specific, lower-pitched sounds that the diaphragm filters out, such as the third heart sound (S3) and the fourth heart sound (S4). The S3 sound, sometimes called a ventricular gallop, is a low-frequency sound of rapid ventricular filling heard in early diastole. Similarly, the S4 sound, or atrial gallop, is produced by atrial contraction forcing blood into a stiff ventricle in late diastole.
It is crucial to apply only a very light touch with the bell. Excessive pressure stretches the skin too tightly, which effectively converts the bell into a pseudo-diaphragm and causes the low-frequency sounds to be lost.
Clinical Application to Specific Heart Sounds
A thorough cardiac examination requires the use of both the diaphragm and the bell to capture the complete spectrum of heart sounds. The clinician systematically switches between the two sides to listen for different diagnostic clues at various points on the chest.
The diaphragm is used to listen for high-pitched systolic clicks or the blowing sound of regurgitation murmurs, like aortic insufficiency. Conversely, the subtle, low-frequency diastolic rumble of mitral stenosis, a narrowing of the mitral valve, often requires the bell at the apex of the heart for accurate detection.
If the bell is not used, this specific, quiet murmur could be completely missed, leading to a delayed diagnosis. By combining the high-pass filter of the diaphragm with the low-frequency sensitivity of the bell, the clinician can isolate sounds based on their pitch and timing within the cardiac cycle, ensuring a comprehensive assessment.