| acoustic energy flow rate in the direction of propagation per unit area normal to the direction of propagation
Note – Instantaneous intensity is the product of instantaneous acoustic pressure and instantaneous particle velocity. It is difficult to measure intensity in the ultrasound frequency range. For measurement purposes and if it is reasonable to assume that the instantaneous acoustic pressure and instantaneous particle velocity are substantially in phase along the beam(-alignment) axis for a given frequency, than the instantaneous intensity, I(t), is approximated as
p(t) is the instantaneous acoustic pressure;
ρ is the density of the medium;
c is the velocity of sound in the medium.