How does a medical stethoscope work?

The internal sounds of the human body such as heartbeat, bowel sounds, wet rales, etc., and even the sound of blood flow are not so large that the reason for "listening" is that the audio is too low or the volume is too small, or is obscured by the noisy environment. The principle of the stethoscope is that the vibration conduction between substances participates in the aluminum film in the stethoscope, and the single non-air changes the frequency and wavelength of the sound, reaching the range of "comfort" of the human ear, while shielding other sounds, "listening" more clear.