  The aim of this paper is to substantiate the importance of non-normality of
shear flow linear operators and its consequence -- the non-modal dynamics of
the perturbations -- in the formation of acoustic wave output of
time-developing free shear/mixing layers. Initially, the linear transient
dynamics of spatial Fourier harmonics is considered in a 3D homentropic
parallel unbounded inviscid constant shear flow which can model the
central/body part of the shear layer. The non-modal approach allows to capture
the only linear mechanism of the acoustic wave generation -- \textit{the linear
vortex-wave mode coupling induced by the shear flow non-normality}. We describe
the specific/key features of the generation process that should leave traces on
the further dynamics of the generated waves. Thereafter, the results of direct
numerical simulations of compressible turbulent time-developing mixing layers
for a moderate convective Mach number (specifically, $M_c=0.7$) and simulation
boxes $(L_x,L_y,L_z)$ with fixed streamwise and shearwise lengths ($L_x=100,
L_y=200$) and different streamwise-spanwise aspect ratios ($L_x/L_z=0.5,1,2$)
are presented. The simulations identify the origin of the acoustic wave output:
the dominance of a \emph{linear generation process of acoustic waves in the
shear layer core region, induced by the flow non-normality}, observable in the
near field of acoustic waves emitted by the flow.
