In theoretical acoustics, it is often desirable to work with the acoustic wave equation of the velocity potential Φ instead of pressure p and/or particle velocity u. ∇ ∇ 2 Φ Φ − − 1 c 2 ∂ ∂ 2 Φ Φ ∂ ∂ t 2 = 0 Solving the wave equation for either p field or u field does not necessarily provide a simple answer for the other field. On the other hand, when Φ is solved for, not only is u found as given above, but p is also easily found—from the (linearised) Bernoulli equation for irrotational and unsteady flow —as p = − − ρ ρ ∂ ∂ Φ Φ ∂ ∂ t .