Therapeutic ultrasound creates high-frequency sound waves. These waves are not sources of heat themselves, but they can cause a heat reaction in your body. At certain settings, ultrasound waves can cause vibrations in the molecules of your body's tissues. The vibrations cause heat. Also, tissues high in collagen, such as bone, tendons, and ligaments, absorb some of the sound waves. This also creates heat energy.

You can't feel heat below the surface of your skin. However, if there is enough heat energy, or if the heat is created close enough to travel up to your skin, you will feel warmth within the skin during ultrasound.