The impact insulation class (IIC) rating of floor/ceiling constructions fails to identify low-frequency footfall noise problems common in contemporary wood-frame multifamily construction. Because the IIC procedure ignores the impact noise spectrum below 100 Hz, the rating is not influenced by the low-frequency impact spectra produced by a typical live-walker, which usually peak in the region of 15 to 30 Hz and are associated with the ``thuds'' and ``thumps'' commonly observed. Data comparing the shape of the typical low-frequency impact noise spectrum produced by a live-walker and that of the standard ISO tapping machine indicate that the tapping machine might potentially be used as the basis for obtaining low-frequency impact noise ratings [W. E. Blazier, Jr. and R. B. DuPree, J. Acoust. Soc. Am. 96, 1521--1532 (1994)]. However, a comparison made between the two sources in the range above 100 Hz shows significant differences in the spectra produced by the tapping machine and a live-walker. This suggests that a more meaningful IIC rating of mid- to high-frequency impact noise might be obtained by cushioning the tips of the hammers in the standard ISO tapping machine, in order to better align the impact spectrum with that of a live-walker.