An investigation of low-frequency footfall noise in multifamily, wood-frame residential construction has led to the conclusion that, at present, there is no economically practical method of avoiding the perception of ‘‘thuds’’ and ‘‘thumps’’ in rooms beneath the walking surface. The IIC rating of a floor system is meaningless with respect to the perception of these low-frequency components of footfall noise, because the methodology ignores the frequency spectrum below 100 Hz; the peak energy in a footfall spectrum occurs at the fundamental natural frequency of the floor/ceiling system, which with typical light-weight structural framing is usually between 15 and 30 Hz. Although the construction of floated floors, or the addition of carpeting, is effective in attenuating mid- to high-frequency components of footfall noise, the data indicate that the amplitude of floor response at the natural frequency is actually increased. It is believed that this occurs because walking on a more resilient system results in a longer rise-time of the footfall waveform, which permits more low-frequency energy to be coupled into the system. The principal factor controlling the perception of low-frequency footfall noise is the point-stiffness of the structural floor system. In normal light-weight residential construction, the stiffness is usually much less than that required to avoid an audible disturbance. However, such problems are relatively rare in concrete structural floor systems, due to their substantially increased stiffness.