Answer:
When a doctor or nurse places a stethoscope diaphragm on a patient's chest, sound waves traveling through the patient's body cause the flat surface of the diaphragm to vibrate. Those vibrations would travel outward if the the diaphragm were a standalone device, but because the vibrating object is attached to a tube, the sound waves are channeled in a specific direction.
Each wave bounces, or reflects, off the inside walls of the rubber tube, a process called multiple reflection. In this way, each wave, in succession, reaches the eartips, or rubber nubs on the ends of the device, and finally the listener's eardrums.
The waves of high-pitch sounds, like breath and heartbeats, are traveling at higher frequencies, meaning they cause a greater number of pressure fluctuations in a given time period. Higher-pitch sounds will directly vibrate the surface area of the large, flat disc (and the plastic disc inside). This basically means the sound waves caused by the opening and closing of an artery, for instance, are the same ones that travel through the stethoscope tubing to the listener's ears.
