An important part of a human-like robot is robot audition. Previous work presented systems capable of sound localization and source segregation based on microphone arrays of various configurations. However, no theoretical framework for assessing the quality of these array configurations has been presented. In the current paper such a measure is proposed based on the generalized HRTFs that account for microphone positions other than the ears. The measure is analyzed theoretically with respect to beamforming robustness and DOA estimation accuracy. The measure is then used to find the optimal location of a single microphone and a pair of microphones based on the generalized HRTF database obtained by means of BEM simulation. The results are not surprising, showing that the best position of a single microphone is the ear canal. For a pair of microphones, the results generally show that the sensors should be maximally spatially separated.