We investigate first-order phase transitions arising from hidden sectors, which are in thermal equilibrium with the Standard Model bath in the Early Universe. Focusing on two simplified scenarios, a higgsed U(1) and a two scalar singlet model, we show the impact of friction effects acting on the bubble walls on the gravitational wave spectra and the consequences for present and future interferometer experiments. We further comment on the possibility of disentangling the properties of the underlying theory featuring the first-order phase transition should a stochastic gravitational-wave signal be discovered
