The individualization of head-related transfer functions (HRTFs) is important for binaural reproduction to reduce measurement efforts and localization errors. One common assumption of individualization for frequencies below 6 kHz is that the sound pressure field around a sphere is similar to the one of a human head. To investigate the accuracy of this approximation, this paper compares the frequency-dependent interaural level difference (ILD) from a spherical approximation, a simulation using magnetic resonance imaging and individually measured HRTFs of 23 adults' heads. With this database, it is possible to analyze the influence of the head shape and the pinna on ILD using the boundary element method and the measured HRTFs. Meanwhile the mismatch between the spherical and human ILD below 1.5 kHz in the horizontal plane is small, they differ above. In the frequency range of 1.5 and 3.5 kHz, ILD of one side of the head is dominated by two maxima. The offset of the ear canal entrance towards the back of the head and the depth of the head are the two major influencing factors. In general, it is observed that the maxima of a spherical ILD are much smaller and more widely spaced than in the human ILD. Above 4 kHz the difference between human and spherical ILDs is even stronger.
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