Hertz Health Dictionary

Sound with a frequency that is greater than the human ear’s upper limit of perception: that is, higher than 20,000 hertz (cycles per second). Ultrasound used in medicine for diagnosis or treatment is typically in the range of 1–15 million hertz (see ultrasound scanning; ultrasound treatment).... ultrasound

n. a unit of loudness of sound. The intensity of a sound to be measured is compared by the human ear to a reference tone of 2 × 10?5 pascal sound pressure and 1000 hertz frequency. The intensity of the reference tone is increased until it appears to be equal in loudness to the sound being measured; the loudness of this sound in phons is then equal to the number of decibels by which the reference tone has had to be increased.... phon