An isohyet or an isohyetal line is a line that joins places of same rainfall on a map within a given time period. A map of isohyets is also known as an isohyetal map.
Ultrasound refers to a sound wave with a frequency greater than the upper limit of human hearing, which is generally over 20 kHz. In general, humans can hear sounds with a frequency between 20 Hz and 20 kHz.