The term ultrasound refers to acoustic vibrations that are imperceptible to the human ear. These vibrations occur when crystals in the sound applicator are exposed to high voltage electricity. The crystals rapidly expand and contract forming sound waves. As the ultrasound waves penetrate human tissue they cause the tissue molecules to vibrate at the same high frequency. This vibration is sometimes referred to as "micromassage" at the cellular level. This vibration also causes a heating effect which increases blood flow, relaxes muscle spasms, temporarily blocks nerve impulses which cause pain, and disrupts fibrous adhesions.