Sound waves sourced by bubble collisions during a cosmological first order phase transition can constitute an observable gravitational wave signal. In the standard picture, the gravitational wave spectrum is described in terms of four parameters: the wall velocity and the strength, duration and temperature of the phase transition. These parameters set the amplitude and peak position of the broken power law that describes the gravitational wave spectrum, which can be measured by LISA. In this talk, I will refine this picture. The sound shell model and hybrid simulations have demonstrated that the gravitational wave spectrum has a more complicated structure than the simple broken power law. I will discuss LISA’s ability to extract this additional information. I will then point out that the gravitational wave spectrum is not just a function of four parameters, but that further details of the new physics model enter via the speed of sound. Lastly, I will demonstrate that density perturbations can enhance and deform the signal.