When classifying microphones there are typically three main categories: omnidirectional, unidirectional, and noise cancelling. Omnidirectional or omni microphones pick up sound equally from all angles, basically from 360°. Unidirectional microphones have a preferred direction for sound pickup, sounds from other directions are reduced. Cardioid is the most common type of unidirectional pattern and refers to a heart shaped directional response, where sounds from 180° opposed to the preferred direction are cancelled to the maximum. Noise cancelling microphones pick up very close sounds only and pick up very little from distant sound sources.