The Bell Chest Piece

1. What is the relation between the tautness (stiffness) of a membrane that collects sound from the chest wall and the ability of the membrane to transmit high or low frequencies?

ANS: The tauter the membrane, the higher is its natural frequency of oscillation and the more efficient it is at higher frequencies.

Note: The terms frequency and pitch are usually used interchangeably. However, frequency refers to the number of oscillations per second made by a sound-producing structure, while pitch is what you hear when those vibrations act on your hearing apparatus.

2. What is the ideal membrane to apply to a chest wall in order to bring out low frequencies?

ANS: A membrane that is as loose and flabby as possible.

3. How does the use of a bell chest piece fit into these acoustical laws?

ANS: The bell allows you to use the skin as a flabby diaphragm. The skin can be turned into a taut diaphragm if enough pressure is applied to the skin to produce pain.

4. Which chest piece diameter picks up the most sound, a very small one or a very large one?

ANS: A very large one. The ability of a chest piece to collect sound is proportional to its diameter.

5. Which chest piece diameter picks up low frequencies better, a small one or a large one?

ANS: A large one.

6. How much pressure should be applied with a bell chest piece?

ANS: Just enough to prevent room-noise leak. Any more pressure will tighten the skin and tend to damp out the low frequencies.

Note: An exception to this occurs when you are listening for an S4. Firm pressure with the bell will sometimes bring out an S4.

7. What is the relationship between the internal volume of a stethoscope (air space enclosed by the chest piece and tubing) and the loudness of the transmitted sound?

ANS: There is an inverse relationship—i.e., the smaller the internal volume, the greater is the loudness of the sound.

8. What bell design will give the smallest internal volume and the largest diameter?

ANS: A shallow shell rather than a deep cone.

Note: It is believed by some cardiologists, with no explanation or testing other than their own ears, that a third chest piece consisting of a large-diameter corrugated diaphragm applied with light pressure (actually only the weight of the three-headed stethoscope) is sometimes best for hearing low frequencies. They still advise having a bell handy, however, both because a small bell is needed for auscultation in small places, such as the supra-clavicular fossa or between the ribs on a bony chest, and because the large bell is occasionally superior for certain low frequencies.

9. What kind of murmurs and sounds are best heard with the bell?

ANS: Murmurs: diastolic murmurs through atrioventricular valves (mitral and tricuspid). Sounds: the diastolic sounds known as the S3 and S4.

Note: The kettledrum (tympany) is bell-shaped and is also used to bring out the low-frequency, booming tones.

Adults can hear frequencies of up to 14,000 cycles per second (cps). However, since cardiac sound does not extend much above 1,000 cps, loss of ability to hear frequencies above 3,000 cps, which is the usual type of hearing loss in older physicians, should not interfere with the hearing of any cardiac sounds or murmurs.

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Blood Pressure Health

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