The stethoscope is a fundamental diagnostic instrument used in medicine to listen to the internal sounds of the body, a practice known as auscultation. Its chest piece typically contains two distinct listening surfaces: the diaphragm and the bell. Understanding which surface to use and how to apply it is important for accurately interpreting the sounds produced by the heart, lungs, and bowels. Each side is specifically engineered to detect a different range of sound frequencies.
The Diaphragm for High Frequency Sounds
The diaphragm is the larger, flat side of the chest piece, covered by a thin, stiff plastic membrane. This membrane is designed to vibrate in response to higher-pitched sounds. When the diaphragm is pressed firmly against the skin, the tension in the membrane effectively filters out lower-frequency components, emphasizing the sharper, more rapid vibrations.
The diaphragm is used for sounds that are naturally higher in pitch. These include the normal heart sounds, specifically the first sound (S1) and the second sound (S2), which are produced by the closing of the heart valves. The diaphragm is also the standard side for listening to lung sounds, such as normal breath sounds, wheezing, and crackles, due to their higher frequency characteristics. Furthermore, the sounds of peristalsis, or bowel sounds, are typically best heard and evaluated using the diaphragm.
The Bell for Low Frequency Sounds
The bell is the smaller, concave side of the chest piece, characterized by its hollow, cup-shaped design without a taut membrane. This open resonator surface is specifically suited for capturing and amplifying low-frequency sounds that the diaphragm often minimizes. When the bell is placed on the skin, it is the skin itself, acting as a miniature diaphragm, that vibrates with the internal sounds.
The bell is particularly useful for detecting subtle, low-pitched cardiac sounds. These subtle sounds include the third heart sound (S3) and the fourth heart sound (S4), which are difficult to perceive with the diaphragm. The bell is also used to auscultate for soft, low-pitched heart murmurs. Additionally, vascular sounds like bruits, which are the low-frequency sounds of turbulent blood flow in arteries, are best detected using the bell.
Proper Application Technique
The physical application technique determines the functional difference between the diaphragm and the bell. When using the diaphragm to listen for high-frequency sounds, the clinician must apply firm pressure against the patient’s skin. This firm pressure stretches the thin plastic membrane taut, which attenuates low-frequency sounds and optimizes the transmission of high-frequency sounds.
In contrast, the bell requires a very light application. Only enough pressure should be applied to create a seal against the skin without visibly stretching the skin. If too much pressure is applied, the skin itself becomes taut and acts like a makeshift diaphragm, filtering out the low-frequency sounds. Therefore, the correct technique involves maintaining a gentle touch to maximize the bell’s ability to resonate and transmit subtle acoustic waves.