The stethoscope is a fundamental tool used by medical professionals worldwide. It allows for auscultation, the act of listening to internal body sounds to assess organ function and detect abnormalities. Sound waves travel to the chestpiece, the part placed directly on the skin. The chestpiece has two distinct sides designed to capture the full range of acoustic frequencies produced by the heart, lungs, and blood flow.
The Small Side: The Bell’s Design and Purpose
The smaller, cup-shaped side of the chestpiece is known as the bell. Its unique structure is engineered to detect low-frequency sounds. Unlike the diaphragm, the bell does not have a tight, stretched membrane covering its opening. This lack of a barrier enables the transmission of deeper, quieter vibrations.
When the bell is placed lightly on the patient’s skin, the skin acts as the vibrating membrane. The shallow cup creates an air cavity that allows low-frequency sound waves to resonate effectively. These sounds are typically fainter and more difficult for the human ear to perceive directly. Using the bell requires a gentle touch, as pressing too firmly stretches the skin and filters out the low sounds the bell is intended to detect.
Clinical Applications of Low-Frequency Auscultation
The bell amplifies low-frequency sounds, making it useful for detecting specific medical conditions. In cardiac assessment, it is primarily used to listen for extra heart sounds that occur late in the cardiac cycle. These include the third heart sound (S3) and the fourth heart sound (S4). These low-pitched sounds are associated with issues like heart failure or reduced ventricular compliance.
The bell is also preferred for hearing certain low-pitched heart murmurs, such as the rumbling mid-diastolic murmur of mitral stenosis. Beyond the heart, the bell is used for vascular auscultation to listen for bruits. A bruit is a turbulent, low-pitched sound over an artery, often indicating narrowed blood flow due to conditions like atherosclerosis. The bell’s smaller diameter also makes it practical for focusing on small areas, such as when assessing pediatric patients.
How the Diaphragm Functions Differently
The larger, flat side of the chestpiece is called the diaphragm, and its purpose is to transmit higher-frequency sounds. It features a thin, tight membrane stretched across its opening. When body sounds cause this membrane to vibrate, the taut material filters out low-frequency waves while efficiently transmitting the higher-frequency waves. This side is used for the majority of routine auscultation.
Primary applications include listening to sharp, high-pitched breath sounds in the lungs, such as wheezes or crackles. The diaphragm is also used for listening to the normal first (S1) and second (S2) heart sounds. Additionally, it is used to listen to bowel sounds and to hear the Korotkoff sounds when measuring blood pressure. Unlike the bell, the diaphragm must be pressed firmly against the skin to create a good seal and properly engage the membrane.