1. A method comprising: forming a first virtual microphone by generating a first combination of a first microphone signal and a second microphone signal, the first virtual microphone having a first linear response to speech and a first linear response to noise, the first linear response to speech being substantially similar across a plurality of frequencies for a speech source located within a predetermined angle relative to an axis of the microphone array, wherein the first microphone signal is generated by a first physical microphone and the second microphone signal is generated by a second physical microphone; and forming a second virtual microphone by generating a second combination of the first microphone signal and the second microphone signal, the second virtual microphone having a second linear response to speech and a second linear response to noise, the second linear response to noise being substantially similar to the first linear response to noise, one or both of the first linear response to noise and the second linear response to noise being non-zero in a direction toward a source of noise, and the second linear response to speech being substantially dissimilar to the first linear response to speech, wherein the second combination is different from the first combination.