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This paper describes procedures being used to rank the acoustic performance of all-MDl (diphenylmethane di-isocyanate) foam backed automotive carpet systems. The procedures are based on the airborne noise Insertion Loss of the carpet composite measured under laboratory conditions and well established criteria used to assess human response to noisy environments, such as annoyance and speech perception. Data are presented which demonstrate how the interior noise spectrum might be influenced by the unit mass of the heavy layer, and the thickness of the foam backing layer. It is shown that although low modulus isolating systems are predicted to give superior Articulation Index performance other criteria indicate that for incident airborne noise the best overall performance should be achieved with a system having a resonance frequency within the 1/3 octave bands centred at 260Hz and 315Hz. The influence of system design parameters and foam properties on the resonance frequency is discussed.