Abstract: Lighthill's theory of aerodynamic noise is re-examined, and an equivalent of his U8-law derived in terms of pressures. The total acoustic power of jet noise is expressed as
W = 8KD^2 \frac(P _1 - P_0)^4\rho 0c0P_1^2
K being the Lighthill's constant, D the diameter of the nozzle, P₁ the plenum pressure and P₀ the atmospheric pressure. This formula applies to cold-air jet at low pressures. Further development, the formula is generalized to high pressures, working material of molecular weight M at temperature T. Sound pressure level in the direction penpendicular to the jet is seen more exact to describe the field, and this at 1 meter from the nozzle is found as
L = 80 + 10\log \frac(R - 1)^4R^2 - R + 0.5+ 20\log \fracTM_0T_0M + 20\log d
where R=P₁/ P₀ and d=diameter in mm. The general formula reduces to the empirical formulae upon neglecting some small terms.