Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from normal sound in its physical properties, except that humans cannot hear it.
Ultrasound scans are used to evaluate fetal development, and they can detect problems in the liver, heart, kidney, or abdomen. They may also assist in performing certain types of biopsy.
Higher frequencies provide better quality images but are more readily absorbed by the skin and other tissue, so they cannot penetrate as deeply as lower frequencies,lower frequencies penetrate deeper, but the image quality is inferior.