Inter-aural time di erences (ITDs) constitute an important localization cue for azimuth estimation, particularly below 1.5 kHz. As a first approximation, it is commonly assumed that ITDs do not depend on frequency. Nevertheless, Kuhn (JASA, 1977) shows theoretically and experimentally that due to di ffraction e ffects around the head, ITDs depend on frequency. Low frequency ITDs should thus theoretically be 1.5 times greater than high frequency ones. To study this point, di erent classical tools are adapted to compute the ITD variations with frequency: onset time di fferences, maximum of the cross correlation, and phase di erences. The reliability of each tool regarding ITD computation is assessed on the basis of head-related transfer functions (HRTFs) coming from a spherical head model. The eff ective frequency dependence of ITDs is finally shown by analyzing real animal HRTFs.