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Stethoscopes: An experimental study

11 June 2019

How a stethoscope’s diaphragm is constructed can increase or decrease the instrument’s performance.

The acoustic stethoscope, invented 200 years ago, is ubiquitous in the medical profession. The simple device amplifies the bioacoustic signals at the surface of a patient’s body. On one side of the stethoscope chest piece is a shallow funnel capped with a diaphragm that when pressed against a patient’s body picks up tiny vibrations and generates sound waves that travel up hollow tubes to the examiner’s ears. The other side often has an uncapped funnel; the skin itself acts as the diaphragm. Experiments on the acoustic properties of chest pieces have yielded inconsistent results, and all were made in artificial settings. Łukasz Nowak at the Polish Academy of Sciences and Karolina Nowak of the Centre of Postgraduate Medical Education—both in Warsaw, Poland—have now studied how a diaphragm’s mechanical properties alter the transmitted bioacoustics signals in a realistic setup: with the diaphragm against a volunteer’s body.


That arrangement allowed the researchers to account for the mechanical coupling between the diaphragm and the body. With the volunteer’s heartbeat as the experimental signal, they used a laser vibrometer to measure the vibrations of different positions on the diaphragm surface. They looked at seven common types of diaphragms and also measured the vibrations with no diaphragm present. Contrary to widespread belief, the diaphragms did not significantly filter the sound. But the diaphragm construction can have a significant effect on perceived loudness. Homogeneous flat disks and films performed the worst; diaphragms of nonuniform construction, with a large, stiff center and an elastic suspension ring, performed the best. (L. J. Nowak, K. M. Nowak, Appl. Acoust. 155, 24, 2019.)

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