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 \gtrsim 0.2 .
In this regime the signal coming from turbulence dominates over that from colliding bubbles .