  The recent identification of a modulation of acoustic waves that is driven by
spatial velocity gradients, using acoustic black and white hole analogues
[Schenke et al., J. Acoust. Soc. Am. 154 (2023), 781-791], has shed new light
on the complex interplay of acoustic waves and non-uniform flows. According to
the virtual acoustic black hole hypothesis, these findings should be applicable
to acoustic waves propagating in non-uniform flows of arbitrary velocity. In
this study, the propagation of acoustic waves in non-uniform flows is
investigated by incorporating a leading-order model of the acoustic pressure
modulation into a Lagrangian wave tracking algorithm. Using this numerical
method, the acoustic pressure modulation is recovered accurately in non-uniform
subsonic flows. This suggests that spatial velocity gradients drive acoustic
pressure modulations in any non-uniform flow, which can, as shown here, be
readily quantified.
