ARTICLES IN THE BOOK
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A directional microphone usually consists of two or three matched microphones. Some directional microphones consist of three or four microphones, producing an "array". I technical datas they show us the polar diagram in a polar coordinate system.
Commonly seen in hearing aids that form a polar plot, amplification mostly occur at the front. Digital directional microphones create null points at different frequencies, enabling cancellation of various signal around the listener.