An intuitive picture of dissipation through pair breaking by macroscopic, vibrating objects in an isotropic superfluid is presented, which predicts a critical velocity for direct quasi-particle emission of upsilon(c) = upsilon-0/(1 + alpha), where upsilon-0 is the Landau velocity and alpha characterizes the superfluid backflow. A new intermediate frequency regime is identified, in which pulsed experiments should reveal a hierarchy of critical velocities and sinusoidally vibrating objects will exhibit a crossover to diffusive behaviour. A model of flow through a channel with a rough surface is analyzed, which yields for the instantaneous dissipation Q approximately (upsilon - upsilon(c))5 and Q approximately upsilon-2, at low and high velocities, respectively.