The second artificial robot ear was designed as depicted in
Fig. 6 (left). To satisfy the basic assumption guiding our design that
the direction-dependent spectral notch is closely related with the
distance from a meatus entrance to a posterior wall of concha, we
designed a spiral-shaped artificial ear so as to vary the distance
from a microphone to a concha posterior wall according to the
sound direction. In Fig. 6 (left), the contour of the posterior wall
of concha in the YZ-plane is described by a collection of pairs (y, z)
which are expresses as