Stethoscope and Physics Behind It

Dr. Cameron Ahmad
3 min readMay 9, 2021

A stethoscope is the first instrument that is used to start the process of diagnosing the disease of a patient. Today, meaning May 9 is the birthday of this primary and essential gadget used in medical practice across the world. The invention of the stethoscope goes back to 1816 in France by Rene Laennec. It was a mono wooden tube that was used to hear the sounds coming from within a human body viz, from the heart. He was not comfortable putting his ear on a woman’s breast and hear her heart’s sounds emanating from within.

Allow me to make readers reminded of the string phone kids play with as per the image below. I too as a child had built one and played with it.

Source: www.alamy.com

The stethoscope functions on similar principles of Physics as we see here children talking. Now the tube in which the girl speaks, meaning the sound waves coming out due to her speaking (kindly refer to my blog PHDM-3) are reflected from its metallic walls and reach the string. Then the waves travel along the string to reach the boy’s end and vibrate the metallic surface which in turn vibrates the air contained in the metallic tube boy is holding Those vibrations reach the boy’s ears and he can listen to what the girl is speaking. The same takes place in vise versa when the boy speaks and the girl listens.

So, how does it give rise to a stethoscope? Well, Rene had inhibition in putting his head on a woman patient’s breast and hear the sounds emanating. So, he used a “hollow wooden tube” to capture the sound (shown in the 2nd image below) which is created by the opening and closing of the heart valves as well as with the blood gushing out or into the chambers of the heart.

It reminds me of the experiment I did in grade 10 of sound and its vibrations taking place in a glass tube whose base was connected to a water reservoir. By changing the length of the water column in the glass tube, we could determine ultimately the frequency of the tuning fork we used in experimenting. By using simple mathematical equations one can determine the frequency of a tuning fork. This was done when a vibrating tuning fork of unknown frequency would be kept at the mouth of the glass tube and its water column is changed till the highest intensity of sound is echoed back from the column of air and water in that tube. Then by using basic equations the frequency of the tuning fork can be calculated, knowing the velocity of the sound and the length of the water column at which the highest sound intensity is heard in the echo.

The early stethoscope is given below.

Source: Science museum London

Here one can see two wooden tubes are coaxial and can be moved to vary the length of the wooden tube. In this manner, an optimum length can be arrived at which gives the highest intensity of sound captured coming out of the heart from a patient’s body.

Today’s stethoscopes are an improvement of the one which Rene invented over 200 years back and look like something given below. Now in the field, one finds there are acoustic, electronic, doppler, recording, etc., types of stethoscopes and literature is now available on the internet about each.

Source: Acoustic stethoscope Wikipedia

Cameron Ahmad

Code PHDM -7

May 9, 2021

Brampton ON

Canada

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Dr. Cameron Ahmad

B. Sc (Hon) in Physics, M. Sc (Biophysics & Electronics). M. Tech (Applied Optics), PhD (Engineering Science), PMP, RDCS, DMS, CET, AScT, CTDP & CECC