How Does a Stethoscope Make Sounds Louder?


As a doctor, you are aware that the stethoscope is an indispensable tool in medicine. It amplifies sounds produced inside the chest and abdomen, and helps you diagnose abnormalities such as heart murmurs. However, do you know how a stethoscope does this? How does a stethoscope make sounds louder?

The chest piece of a stethoscope is the part that is placed on the patient’s chest to listen to sounds. This chest piece has two sides– the diaphragm and the bell. The diaphragm is the large, flat circular end of the chest piece whereas, the bell is the smaller, circular end of the chest piece. The diaphragm-end of the stethoscope being the larger surface helps pick up the high frequency sounds that are strong enough to vibrate the diaphragm in the body.

To begin with, let’s go back to the stethoscope’s anatomy. We find that its chest piece has two sides – the bell and the diaphragm. This chest piece is connected to the earpiece by means of acoustic tubes. A short stem connects the chest piece to the acoustic tubing.

When we place the stethoscope to the chest of the patient, we hear the sounds that are made by structures inside the chest including the heart, lungs, and the diaphragm. When placed on the abdomen, we can hear the sounds produced by the various processes occurring inside the stomach and intestines. But, why do we need a stethoscope? Well, we need it because the sounds produced inside the body are very low frequency sounds and we cannot hear them clearly with our ears. So, a stethoscope is needed to amplify it.

Now, let’s understand how a stethoscope does this? When we place the diaphragm-end of the stethoscope to a patient’s chest, it forms a sealed surface due to the outer ring present on it. This diaphragm now moves like a sealed membrane just like our ear drums. So, when the diaphragm moves, the column of air inside the tube also moves up and down, which is then transmitted to the ear and the air in the ear canal moves too. Thus, sound is heard. But, the diaphragm-end of the stethoscope has a greater surface area than the column of air it moves inside the acoustic tubes of the stethoscope, which results in the air inside these tubes must travel more than the diaphragm. This causes a magnification of air pressure waves in the ear and these larger pressure waves make louder sounds, thereby amplifying sound.

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