  In this paper we consider an in-ear headphone equipped with an external
microphone and aim to minimize the sound pressure at the ear drum by means of a
fixed feedforward ANC controller. Based on measured acoustic paths to predict
the sound pressure generated by external sources and the headphone at the ear
drum, the FIR filter coefficients of the ANC controller are optimized for
different sound fields. Due to the acoustic feedback path between the
loudspeaker and the microphone, a stability constraint based on the Nyquist
stability criterion is introduced. Performance degradations due to reinsertions
of the headphone and intra-subject variations are addressed by simultaneously
optimizing the controller for several measurement repetitions of the acoustic
paths. Simulations show that the controller optimized for an ipsilateral
excitation produces an attenuation of at least -10 dB that extends
approximately to +45{\deg} and -65{\deg} from the ipsilateral DoA. The
controller optimized for a diffuse-field excitation achieves an attenuation of
at least -10 dB over a wider range of DoAs on the ipsilateral side, namely
+90{\deg} to -90{\deg}. Optimizing the controller for several measurement
repetitions is shown to be effective against performance degradations due to
reinsertions and intra-subject variations.
