  We examine amplifying behavior of small perturbations about Reynolds-averaged
Navier-Stokes solutions (RANS) of a $M_j = 0.9$ subsonic turbulent jet using
input-output (I/O) analysis. Singular value decomposition (SVD) of the
resolvent of the linearized Navier-Stokes (LNS) equations forms an orthonormal
set of I/O mode pairs, sorted in descending order by the magnitude of the
corresponding singular values. In this study we design a filter to restrict
input forcings to be active only in regions where turbulent kinetic energy
(TKE) of jet turbulence is high. For an output domain, we directly implement
the Ffowcs Williams-Hawkings (FWH) method within I/O analysis framework to
measure far-field sound of observers distributed along an arc. In this way we
find that the resulting TKE-weighted input modes captures coherent structures
in the near-field of turbulent jets. Optimal input modes correspond to
wavepackets represented by asymmetric pseudo-Gaussian envelope functions at a
given forcing frequency. These wavepackets remain similar in shape over a range
of frequencies for $St > 0.5$, and scale as $St^{-0.5}$. While the optimal mode
is a wavepacket, sub-optimal modes represent decoherence of the optimal input
mode. By projecting high-fidelity large eddy simulation (LES) data onto the
basis of input modes, we find that input modes do indeed capture the
acoustically relevant dynamics in the jet. The far-field acoustics predicted by
the LES are recovered using only a few number of I/O modes.
