Some microphones are designed to hear equally in all directions (omnidirectional), and some are designed to be more sensitive in certain directions. Directional microphones can be a big advantage when such a microphone is used in a hearing aid or in an Assistive Listening Device (ALD), because it can be used to enhance some sound that you need to hear (such as speech from someone you are talking with) and to diminish background noise.
A single microphone can be made somewhat directional by having ports to receive the sound on both sides of the sensor which detects the sounds. Sounds coming from both sides (most likely background noise) will cancel each other, while sounds coming only from one direction (in front or back of the sensor) will not cancel itself.
A single microphone with multiple ports may also use time delay processing on one of the ports to enhance the cancellation effect of sounds from certain directions.
Multiple microphones can work together to perform even more sophisticated directionality. With multiple microphones, the directionality can be improved because the distance between the microphones makes it possible to be more selective of which sounds are cancelled and which are not.
Some hearing aids, now have 2 or even 3 microphones and offer an effective noise reduction capability. Most aids with multiple microphone directionality are BTE aids. A few ITE aids do offer the feature, but smaller aids do not.
Some assistive listening devices also have directional microphones including: