  A stochastic background of gravitational waves can be generated during a
cosmological first order phase transition, at least by two distinct mechanisms:
collisions of true vacuum bubbles and turbulence in the cosmic fluid. I compare
these two contributions, analyzing their relative importance for a generic
phase transition. In particular, a first order electroweak phase transition is
expected to generate a gravitational wave signal peaked at a frequency which
today falls just within the band of the planned space interferometer LISA. For
this transition, I find constraints for the relevant parameters in order to
produce a signal within the reach of the sensitivity of LISA. The result is
that the transition must be strongly first order, alpha > 0.2. In this regime
the signal coming from turbulence dominates over that from colliding bubbles.
