A major approach investigated recently for stimulated Brillouin scattering (SBS) suppression in fiber amplifiers for high power single frequency fiber lasers is to explore designs of acoustic waveguide in optical fibers. This acoustic waveguide can be implemented to some extent independent of the optical waveguide by using a combination of dopants which modify the host glass by varying levels in acoustic and optical properties. Although this approach provides some SBS suppressions, the new analysis described in this work, considering the often omitted leaky acoustic modes, demonstrates its limit. A complex acoustic mode solver, reported in details elsewhere, was recently developed to find solutions for simultaneous longitudinal and shear acoustic wave equations which satisfy rigorous boundary conditions in an arbitrary circular acoustic waveguide. By taking advantage of this new tool, it is possible to find the leaky acoustic modes for acoustic waveguides designed for SBS suppressions. In these acoustic waveguides, the guided acoustic modes often have a reduced overlap with the optical modes by design and, sometimes, do not exist at all, while the leaky acoustic mode can have a significant stronger overlap with the optical modes and, consequently, can play a dominant role in the SBS gain spectrum. It is, therefore, critical to consider the leaky acoustic modes in these cases. Two prominent transverse acoustic waveguide designs proposed recently for SBS suppression in optical fibers are studied here. This work reports, for the first time, the limits of these designs for SBS suppression. With consideration of the leakage acoustic modes, some new insights are provided on the underlying physics as well as on how to optimize these designs for optimal SBS suppression.