The stethoscope is a fundamental diagnostic instrument used by healthcare professionals to listen to internal body sounds, a practice known as auscultation. This device amplifies low-level acoustic vibrations produced by the heart, lungs, and abdomen. The component placed directly on the patient, the chest piece, is typically designed with two functional sides, each engineered to pick up different sound frequencies. Understanding the unique purpose of these two sides is paramount for accurate diagnosis, as selecting the incorrect side can cause important sounds to be missed entirely.
The Diaphragm: Capturing High-Frequency Sounds
The diaphragm is the larger, flat side of the stethoscope chest piece, covered by a taut plastic or fiberglass membrane. This membrane is designed to vibrate primarily in response to higher-pitched, or high-frequency, sounds. The tension of the diaphragm effectively filters out the lower-frequency sounds. When using the diaphragm, the practitioner applies firm pressure against the patient’s skin to ensure optimal sound transmission. This side is the workhorse for general screening because many common body sounds fall within the high-frequency range.
It is the preferred choice for listening to normal lung sounds, which include the rustling sounds of air moving through the airways. The diaphragm is also used to hear the first and second normal heart sounds (S1 and S2), which are the familiar “lub-dub.” Furthermore, the sounds used to determine blood pressure, known as Korotkoff sounds, are typically best detected using this side.
The Bell: Isolating Low-Frequency Sounds
In contrast to the flat diaphragm, the bell is the smaller, concave, cup-shaped side of the chest piece, lacking a taut membrane. This open, non-tensioned design allows the bell to pick up subtle, low-frequency sounds that the diaphragm often muffles or filters out. The bell works by creating a small air cavity between the skin and the chest piece, and its effectiveness relies on the skin acting as the primary vibrating membrane. To function correctly, the bell must be placed very lightly on the patient’s skin, creating an air seal without stretching the underlying tissue. Applying too much pressure will stretch the skin tautly, causing the bell to inadvertently function like a diaphragm and filter out the very low-frequency sounds it is intended to capture.
Specific, low-pitched abnormal heart sounds, such as the third (S3) and fourth (S4) heart sounds, sometimes called gallops, require the sensitivity of the bell for detection. The bell is also useful for listening to specific vascular sounds, such as bruits, which are murmurs produced by turbulent blood flow in arteries.
Practical Applications: Choosing the Right Side
Most dual-headed stethoscopes require the user to rotate the chest piece 180 degrees to switch between the active diaphragm and the active bell. This physical rotation ensures that only one side is acoustically connected to the tubing at any given time. A practical approach for general examination involves starting with the diaphragm to screen for the most common, higher-pitched sounds, such as lung sounds and the primary heart sounds. If the clinician suspects a subtle, low-pitched abnormality, such as a specific heart murmur or an S3 gallop, they will then switch to the bell.
Many modern stethoscopes feature a single-sided chest piece with a tunable diaphragm that performs the function of both sides. With these models, light pressure activates the low-frequency “bell” mode, while firmer pressure tensions the diaphragm to activate the high-frequency “diaphragm” mode. Whether using a traditional dual-sided head or a tunable single-sided one, the underlying principle remains the same: the selected side must be acoustically tuned to the specific frequency range of the body sound being assessed.