AuscultifyStethoscope Bell vs Diaphragm: When to Use Each
Use the diaphragm for high-pitched sounds and the bell for low-pitched sounds. In practice that means the diaphragm for S1, S2, most murmurs (especially aortic and mitral regurgitation), lung sounds, bowel sounds and most bruits, and the bell for S3, S4, the rumble of mitral stenosis and some low-pitched bruits. The second half of the rule is pressure: press the diaphragm firmly, and rest the bell lightly.
This page is the deep dive on the chest piece itself. For positioning, environment and the full exam sequence see the stethoscope technique guide.
How the two sides filter sound
The diaphragm is a stiff plastic membrane. When pressed against the chest it vibrates preferentially at higher frequencies and damps the low ones, so it acts as a high-pass filter. The bell is an open cup. It has no membrane of its own; the patient's skin becomes the membrane, and loose skin under light pressure vibrates at low frequencies. The ranges quoted in most textbooks are roughly 20 to 200 Hz for the bell and 100 to 1000 Hz or more for the diaphragm, with overlap in the middle where either side works.
Which chest piece for which sound
| Sound | Chest piece | Pressure | Notes |
|---|---|---|---|
| S1 and S2 | Diaphragm | Firm | Check S2 splitting at the pulmonic area |
| S3 and S4 gallops | Bell | Light | Apex, left lateral decubitus; S4 also at LLSB |
| Aortic stenosis | Diaphragm | Firm | Mid-frequency; the bell also picks it up |
| Mitral regurgitation | Diaphragm | Firm | High-pitched, blowing, apex to axilla |
| Aortic regurgitation | Diaphragm | Firm | Sitting forward, end-expiration; easily missed with the bell |
| Mitral stenosis | Bell | Light | Low rumble at the apex; the opening snap is heard with the diaphragm |
| Tricuspid regurgitation, VSD, HCM | Diaphragm | Firm | Left sternal border |
| Clicks, opening snap, pericardial rub | Diaphragm | Firm | High-pitched, short |
| Breath sounds, crackles, wheezes | Diaphragm | Firm | Bare skin, one full breath per site |
| Bowel sounds | Diaphragm | Light to moderate | Do not press hard enough to stimulate the bowel |
| Carotid, renal and aortic bruits | Bell (or diaphragm) | Light | Pressure on a carotid can create a false bruit |
| Korotkoff sounds (blood pressure) | Either | Light | Bell recommended by the AHA; diaphragm widely used and acceptable |
The pressure rule and the skin trick
Pressing the bell hard stretches the skin under it until it is taut, and taut skin behaves like a diaphragm. The low frequencies disappear and you are effectively listening with the wrong side. This is the most common reason students miss an S3, an S4 or a mitral stenosis rumble that their attending hears at the same spot a moment later.
You can turn this into a diagnostic trick. If you hear a sound with the bell resting lightly and are not sure whether it is low pitched, press harder. If the sound fades or vanishes, it was low frequency (a gallop or a rumble). If it stays the same or gets clearer, it was high frequency (a murmur or a rub). The S3 vs S4 guide covers the gallops in more detail.
Bell or diaphragm for blood pressure?
Korotkoff sounds are low pitched, so the American Heart Association recommends the bell for manual blood pressure. In practice most clinicians use the diaphragm, because its larger flat surface is easier to hold steady under the cuff edge with one hand while the other works the bulb, and the readings are accurate. Both are acceptable. If the sounds are faint (hypotension, a large arm, a noisy ward), switch to the bell, rest it lightly over the brachial artery just below the cuff and not tucked under it, and make sure the patient's arm is supported at heart level.
Tunable diaphragms
Modern stethoscopes such as the Littmann Cardiology and Classic series use a tunable diaphragm on the adult side. Light pressure gives you bell mode (low frequencies); firm pressure gives you diaphragm mode (high frequencies). The convenience comes with a trap: most students press firmly by default and never hear the low-frequency sounds at all. If you own a tunable stethoscope, practice deliberately lightening your grip at the apex until the chest piece is barely resting on the skin. Some dual-head models still carry a traditional open bell on the reverse side, and it remains the most reliable way to hear a faint gallop.
Pediatric and infant sizes
An adult diaphragm spans several rib spaces on an infant and picks up sound from the whole chest, which makes localization impossible. Pediatric and infant stethoscopes have a smaller chest piece that fits within a single intercostal space, and most dual-head pediatric models pair a small diaphragm with a small bell. The same pressure rule applies, and because a child's chest wall is thin, even light pressure transmits well. For infants, the small diaphragm side of a pediatric stethoscope does most of the work; use the bell for suspected gallops or a mitral rumble.
Technique: holding the bell
- Full rim contact. The bell only works with an airtight seal. Rest the whole rim on the skin; on a bony or hairy chest, angle it slightly until the seal forms.
- Hold the stem, not the head. Grip the chest piece between your index and middle fingers at the stem so that the weight of your hand does not push down on the bell.
- Lift and re-seat, never slide. Sliding the bell across skin produces rumbling artifact that mimics a murmur or a rub.
- Skin, not clothing. Fabric between the chest piece and skin scratches like crackles and blocks low frequencies entirely.
- Warm the chest piece. A cold bell makes the patient tense, and muscle tremor is low-frequency noise.
- Keep the tubing still. Tubing rubbing against a sheet or a coat sounds like a pericardial rub.
Hear the difference in Auscultify
Auscultify's library of real clinical recordings covers heart, lung, bowel and vascular sounds, so you can hear what a gallop captured at the apex actually sounds like next to a high-pitched regurgitant murmur. The Spectrogram view (Pro) shows the frequency band of each sound, which makes it obvious why S3 needs the bell and aortic regurgitation needs the diaphragm. Six quiz modes and spaced-repetition weighting of missed sounds keep the low-pitched ones coming back until you stop missing them.
Train your ear free →FAQ
Do you use the bell or diaphragm for heart sounds?
Both. Use the diaphragm with firm pressure for S1, S2, clicks, snaps, rubs and most murmurs, then switch to the bell with light pressure at the apex and lower left sternal border for S3, S4 and the rumble of mitral stenosis. A complete cardiac exam always uses both sides.
Bell or diaphragm for blood pressure?
Either is acceptable. The AHA recommends the bell because Korotkoff sounds are low pitched, but most clinicians use the diaphragm because it is easier to hold in place with one hand and produces accurate readings. If the sounds are faint, switch to the bell and use light pressure over the brachial artery.
Bell or diaphragm for lung sounds?
The diaphragm. Breath sounds, crackles, wheezes and pleural rubs are mostly high frequency, so press the diaphragm firmly against bare skin and listen to a full breath at each site.
Bell or diaphragm for bowel sounds?
The diaphragm, with light to moderate pressure so you do not stimulate the bowel. Bowel sounds are high pitched gurgles and clicks. Use the bell only afterwards if you are listening for a low-pitched abdominal bruit.
When do you use the bell of a stethoscope?
Use the bell, resting lightly on the skin, for low-frequency sounds: S3 and S4 gallops, the diastolic rumble of mitral or tricuspid stenosis, some low-pitched bruits, and Korotkoff sounds if the diaphragm is not picking them up. Pressing hard filters out the low frequencies you are trying to hear.
What is the diaphragm of the stethoscope designed to auscultate?
High-frequency sounds. The stiff membrane transmits higher pitched sounds and attenuates low ones, which makes it the right side for S1, S2, most murmurs, breath sounds, bowel sounds and most bruits.