  We predict an existence of an new dissipation channel leading to attenuation
of ultrasound in disordered conductors and superconductors with perfect
electroneutrality. It is due to slow diffusion of thermal energy. We show that
in doped silicon ultrasound attenuation may be enhanced by a factor about 100.
Similar effect is also studied for s-wave and d-wave superconductors. The
latter case is applied to BSCCO family where strong enhancement of ultrasound
attenuation is predicted. For usual s-wave superconductors new dissipation
channel might be important for very low-electron-density materials near the
BCS-BEC crossover.
