The prediction of the reduction of impact sound pressure level ?L according to annex C of the standard ISO 12354-2 gives an acceptable estimation of the floating floors performance for thin resilient layers. However, the performance is often largely overestimated for thick resilient layers or for resilient layers combined with thermal layers. One reason for this is that the simplified model doesnt account for the thickness resonances in the underlays which can greatly affect ?L. This is confirmed by comparing finite element and transfer matrix method simulations with experimental results. This paper establishes the mechanisms leading to the development of these resonance waves and provides some guidelines to estimate their negative effects on the ?L.