The Stethoscope used by a medical practitioner is based on the phenomenon of :

The stethoscope is a medical instrument used for auscultation, primarily based on the phenomenon of multiple reflection of sound waves. Sound waves generated by internal organs, such as the heart or lungs, travel through the stethoscope's chest piece and hollow tube to the physician's ears. This transmission occurs through successive reflections along the inner walls of the tube, which acts as an acoustic waveguide [t1][t8]. By confining the sound waves within the tube, the device prevents the energy from dissipating into three-dimensional space, thereby maintaining the intensity of the sound over the distance from the patient to the doctor [t8]. While sound waves also exhibit properties like refraction and transmission depending on medium impedance [t3][t5], the specific mechanism that allows a stethoscope to conduct body sounds efficiently to the ear is the repeated reflection of sound within its conduit [t1].

Sources

1. [1] https://en.wikipedia.org/wiki/Waveguide