The propagation of sound waves in air can be modeled by an equation of motion and an equation of continuity. With some simplifications, they can be given as follows:
where is the acoustic pressure and is the acoustic fluid velocity vector, is the vector of spatial coordinates , is the time, is the static density of air and is the speed of sound in air.
is the acoustic pressure and 
is the acoustic fluid velocity vector, 
is the vector of spatial coordinates 
, 
is the time, 
is the static density of air and 
is the speed of sound in air.