The ultrasound unit gets its power from the electricity in the wall outlet. The electricity enters the unit at the standard 60 cycles per second (60 Hertz). The ultrasound machine ramps the frequency up much higher, generally either to one million (one megahertz, or 1 MHz) or 3.5 million (3.5 MHz) cycles per second. The electricity then travels through an insulated cord to the handle. The electricity is passed through a special crystal inside the transducer, the part placed on your body. The electrical current vibrates the crystal to produce sound waves at the same frequency. These sound waves enter your body through the transducer.
The 1 MHz setting sends waves over a deeper and wider area. It is used to reach tissues up to two inches below the skin surface–deeper tissues of the spine, for example. The 3.5 MHz setting creates waves that are packed more closely together. It is used to treat tissues just under the surface of the skin, such as the jaw, tendons of the hands and feet, or bony areas around the elbow.