Stethoscopes have a special metal-made listening device that captures sounds and transmits them via hollow rubber tubes to the ears of a doctor. Usually this listening device has two sides - a flat one, that transmits higher-frequency vibrations from inside of a body, and the cup-shaped side, that passes low-frequency skin vibrations. Purely mechanical stethoscopes produce very low volume sounds, therefore electronic devices were invented. They can amplify the volume level and make it easier to inspect the sounds. Some of these electronic gadgets can even record the sounds for later analysis. There are also doppler stethoscopes, which are a subdivision of the previous wife. They are based on doppler effect and measure the internal ultrasound levels. Such gadgets are typically used for heart analysis. A separate type of such device is called fetal, it’s used if you want to hear the sounds made by the fetus inside the abdomen.