When to Use Bell vs Diaphragm of Stethoscope
For blood pressure measurement, either the bell or diaphragm may be used as they perform similarly, though the bell is traditionally preferred for detecting low-frequency Korotkoff sounds; for cardiac auscultation, use the bell for low-pitched sounds (S3, S4 gallops, mitral stenosis) and the diaphragm for high-pitched sounds (most murmurs, S1, S2); for lung sounds, always use the diaphragm. 1
Blood Pressure Measurement
Both bell and diaphragm are acceptable for measuring blood pressure, as multiple major guidelines explicitly state there is little practical difference between them 1:
The American Heart Association guidelines state that "Phase 1 (systolic) and phase 5 (diastolic) Korotkoff sounds are best heard using the bell of the stethoscope over the palpated brachial artery in the antecubital fossa, although some studies have shown that there is little difference when using the bell or the diaphragm" 1
The 2017 ACC/AHA Hypertension Guidelines explicitly state: "Either the stethoscope diaphragm or bell may be used for auscultatory readings" 1
The KDOQI perspective on blood pressure measurement reiterates this same recommendation 1
Practical Considerations for BP Measurement
Place the stethoscope over the palpated brachial artery in the antecubital fossa, 2-3 cm below the cuff edge 1
Research shows the bell placed over the brachial artery pulse may provide slightly clearer Korotkoff sounds and higher systolic readings compared to the diaphragm over the cubital fossa, though this difference is small 2
The quality of the stethoscope matters more than bell vs diaphragm choice—use a high-quality stethoscope with short tubing for accurate auscultatory measurement 1
Avoid artifactual noise by ensuring the cuff does not touch the stethoscope during measurement 1
Cardiac Auscultation
Use the bell and diaphragm for different cardiac sounds based on their frequency characteristics:
Use the Bell for:
- Low-frequency sounds including S3 and S4 heart sounds (gallops) 1
- Mitral stenosis murmur and other low-pitched diastolic murmurs 1
- Apply the bell lightly to the skin to maintain its low-frequency filtering properties
Use the Diaphragm for:
- High-frequency sounds including most systolic murmurs 1
- S1 and S2 heart sounds for optimal clarity 1
- Aortic and pulmonic valve sounds 1
- Press the diaphragm firmly against the skin to filter out low frequencies
Important Caveats
Research shows that acoustic differences between bell and diaphragm modes vary significantly by stethoscope model, with some modern stethoscopes showing minimal differences 3, 4
The Littmann Classic II, for example, shows similar frequency responses between bell and diaphragm modes, unlike older designs 4
In pregnancy, normal physiological changes include a hyperkinetic precordium, louder S1 with prominent splitting, physiologic S3, and soft systolic murmurs—the mammary souffle (continuous murmur over engorged breast) can be obliterated with firm pressure of the diaphragm 1
Lung Auscultation
Always use the diaphragm for respiratory sounds 5:
- Respiratory sounds at the chest wall are predominantly high-frequency 5
- Logic and research support that diaphragms are more appropriate for lung sounds than bells 5
- This is one area where there is clear consensus, unlike blood pressure measurement 5
Special Populations
Elderly Patients with Auscultatory Gap
Be aware that auscultatory gaps (temporary disappearance of Korotkoff sounds between systolic and diastolic pressure) occur more frequently in elderly patients 6
Eliminate auscultatory gaps by elevating the arm overhead for 30 seconds before inflating the cuff, then bringing it back to the usual position 6
Use palpatory method first—inflate the cuff 20-30 mmHg above where the radial pulse disappears to avoid underestimating systolic pressure 6
Consider oscillometric devices as an alternative in elderly patients, as they are not affected by auscultatory gaps 6