As mentioned above, the spectral features in the HRTFs provide
effective sound cues for the vertical angle estimation. To obtain the
HRTF from the measured microphone signal, however, the freefield
pressure at the head center with the head absent should be
known because the HRTF is the ratio of the pressure measured
at the microphone to that measured at the head center. In many
practical applications, it is not possible to measure the pressure
at the head center. In this case, the Interaural Transfer Function
(ITF) fusing the two output signal characteristics is a useful and
practical cue for sound direction estimation because it contains
all information about ITD, ILD, and spectral modifications at two
ear outputs. The ITF is defined as the acoustical transfer function
between two ear output signals, and it can be computed from
the transfer function estimator such as H1 estimator and H2
estimator [27]. The ITF is also equivalent to the ratio of the HRTFs
at two ears when the noise can be ignored, and it can be expressed
as