Abstract The natural frequencies of vibration of rigid polyurethane foam cored panels are theoretically investigated. Particular attention is paid to those modes which will transmit sound and it is shown that an undesirable resonance can be associated with a dilatational mode of vibration depending primarily upon the core stiffness and the skin masses. Three panels, with different core stiffnesses, have been tested for sound transmission and the observed resonant frequencies agree well with those predicted. The core stiffness can be optimized so that both the flexural critical frequency and the minimum dilatational frequency are above 4000 c/s. Then, provided that the low order flexural modes occur below loo c/s, the transmission loss of the construction will follow the theoretical mass law throughout the frequency range of interest.