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The mass source is the right-hand side of the modified Helmholtz equation (e.g., Equation 8-7): https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v192/ans_thry/thy_acou1.html It is not related to mass flow in CFD since, in CFD, we are looking at the net flow of fluid in a domain, but in acoustics in ANSYS Mechanical, there is no net fluid flow – the mass source in acoustics in ANSYS Mechanical is the sinusoidally-varying displacement of fluid causing acoustic waves to be generated. Only for the specific case of planar wave in a pipe, we can treat the acoustic waves as 1D. Since pressure and velocity are related by impedance rho*c, p = rho*c*v and rho*v=Q_surface (for surface), so p = c * Q_surface where Q_surface has units of (kg / m^2 / s). Note that ‘mass source’ has different units, depending on whether it is applied to a point, surface, or volume, and this is described in the section below: https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v192/ans_acous/acous_excit_src.html%23acous_excit_masssrc