What is Ultrasound? Its simply sound that is above the range of human hearing. Humans can hear up to about 20,000 hertz or 20 kilohertz, so anything above that is ultrasound. The lower frequencies in the kilohertz range are more mechanical and used for therapeutic ultrasound with heating. For diagnostic ultrasound we are really talking about frequency ranges in the millions of hertz or megahertz range. The thing with the frequency is that the higher the frequency the better resolution (you’re going to see things better) but it doesn’t penetrate quite as well. Lower frequency probes will penetrate better, but won’t give images that are quite as crisp or clear especially… So we’re talking about basically sound waves that are in the megahertz or millions of hertz range.