  The necessity of a newly proposed (PRD 70 (2004) 64004) non-Riemannian
acoustic spacetime structure called acoustic torsion of sound wave equation in
fluids with vorticity are discussed. It is shown that this structure, although
not always necessary is present in fluids with vorticity even when the
perturbation is rotational. This can be done by solving the Bergliaffa et al
(Physica D (2004)) gauge invariant equations for sound, superposed to a general
background flow, needs to support a non-Riemannian acoustic geometry in
effective spacetime. Bergliaffa et al have previously shown that a Riemannian
structure cannot be associated to this gauge invariant general system.
