Ultrasound

Sound waves of frequency below 20 Hz usually cannot be heard by humans, so are called infrasound. Sound waves of frequency above 20 000 Hz usually cannot be heard by humans, so are called ultrasound. Ultrasound waves can cause objects to vibrate, so transfer energy. They do not cause harm to the human body. Ultrasound can be used to image structures inside the body as it reflects from boundaries between tissues. Ultrasound vibrations can clean delicate objects and can help to treat injuries and illnesses in physiotherapy.