Ultrasound
Definition
Sound waves too high-pitched for human ears — above about 20,000 Hz — used in medical imaging and industrial testing.
Unit: Hz (typically MHz for imaging)
Technical definition
Acoustic waves with frequency above the human hearing range (>20 kHz). Short wavelengths allow fine resolution imaging; reflection at tissue boundaries creates the echoes used in sonography.
Example
Medical ultrasound uses 2–15 MHz waves — millions of cycles per second — to image fetuses and organs safely, without ionizing radiation.
Common confusion
Ultrasound is just ordinary sound at high frequency — not radiation. It is fundamentally different from X-rays: no ionizing radiation is involved, which is why it is considered safe for prenatal imaging.