This study examined inter-subject differences in the transfer functions from the free field to the human ear canal, which are commonly know as head-related transfer functions. The directional components of such transfer functions are referred here to as directional transfer functions (DTFs). The DTFs of 45 subjects varied systematically among subjects in regard to the frequencies of spectral features such as peaks and notches. Inter-subject spectral differences in DTFs were quantified between 3.7 and 12.9 kHz for sound-source directions throughout the coordinate sphere. For each pair of subjects, an optimal frequency scale factor aligned spectral features between subjects and, thus, minimized inter-subject spectral differences. Frequency scaling of DTFs reduced spectral differences by a median value of 15.5% across all pairs of subjects and by more than half in 9.5% of subject pairs. Optimal scale factors showed a median value of 1.061 and a maximum of 1.38. The optimal scale factor between any pair of subjects correlated highly with the ratios of subjects' maximum interaural delays, sizes of their external ears, and widths of their heads.