  We study Tachyacoustic models of cosmology, for which a scale-invariant
perturbation spectrum is generated via superluminal sound speed instead of
accelerated expansion, as in the case of inflation. We derive two bounds on the
size of acoustic horizon which constrain the duration of tachyacoustic
evolution, and therefore generation of primordial perturbations. We show that
existing models cannot solve the horizon problem without violating the
condition that all physical scales -- such as the Hubble parameter, the
pressure, and the length scale at which quantum modes freeze out and become
classical -- be sub-Planckian.
