We investigate the effect of extra singlets on the electroweak phase
transition (EWPT) strength and the spectrum of the corresponding gravitational
waves (GWs). We consider here the standard model (SM) extended with a singlet
scalar with multiplicity N coupled to the SM Higgs doublet. After imposing all
the theoretical and experimental constraints and defining the region where the
EWPT is strongly first order, we obtain the region in which the GWs spectrum
can be reached by different future experiments such as LISA and DECIGO.Comment: 11 pages, 4 figures, published version matche