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In many extensions of the Standard Model, the electroweak phase transition is first order. Such a phase transition proceeds via the formation and collision of bubbles. The bubble collisions can source a stochastic gravitational wave background signal, with characteristic frequency right in the sensitivity band of LISA. We can thus use data from gravitational wave experiments to probe physics beyond the standard model. I will give a brief overview of the different contributions to the gravitational wave signal, and then focus on the contribution from sound waves which get formed in the interactions between the plasma and the bubble walls. I will discuss the parameters that describe the phase transition, and argue that the speed of sound can play a significant role. To quantify the importance of this effect, I will discuss an accurate computation of the sound speed in a representative model. Lastly, I will discuss the velocity with which the bubbles expand. This parameter strongly affects the gravitational wave signal, but is not well-known for most models. Assuming that the plasma is locally in thermal equilibrium, allows for a better estimate of the wall velocity and the corresponding gravitational wave spectrum.