  This paper uses asymptotic analysis within the generalized acoustic analogy
formulation (Goldstein. J. Fluid Mech 488, pp. 315-333, 2003) to develop a
noise prediction model for the peak sound of axisymmetric round jets at
subsonic acoustic Mach numbers ($Ma$). The analogy shows that the exact formula
for the acoustic pressure is given by a convolution product of a propagator
tensor (determined by the vector Green's function of the adjoint linearized
Euler equations for a given jet mean flow) and a generalized source term
representing the jet turbulence field.
  Using a low frequency/small spread rate asymptotic expansion of the
propagator, mean flow non-parallelism enters the lowest order Green's function
solution via the streamwise component of the mean flow advection vector in a
hyperbolic partial differential equation (PDE). We then address the predictive
capability of the solution to this PDE when used in the analogy through
first-of-its-kind numerical calculations when an experimentally-verified model
of the turbulence source structure is used together with Reynolds-averaged
Navier Stokes solutions for the jet mean flow. Our noise predictions show a
reasonable level of accuracy in the peak noise direction at $Ma=0.9$, for
Strouhal number up to about $0.6$, and at $Ma=0.5$ using modified source
coefficients. Possible reasons for this are discussed. Moreover, the prediction
range can be extended beyond unity Strouhal number by using an approximate
composite asymptotic formula for the vector Green's function that reduces to
the locally parallel flow limit at high frequencies.
