Omnidirectional and Unidirectional Microphone

Imagining that a microphone is set in the center of a sphere, and the sound is generated on the surface of the sphere. Because the sound waves are diffused and diffractive, the SPL which the omnidirectional microphone receives from different angles (in the same distances) should be the same in theory.

Depending on the different designs, a unidirectional microphone can receive the sound from some particular angles, but none of the sound is from the back of the sound receiving holes, the sensitivity might be at least 12dB difference between 0 degree (the front) and 180 degree (the back). For example, the wide-angle unidirectional microphone can be used in a conference to receive the human voice; and the narrow-angle one may be used outdoors to get the sound of wild animals.

Besides, there is another type of microphone, noise canceling microphone, the low frequency response is the characteristic of it which related to the distance. The low frequency will be reduced at the further distance; it is usually noted in the graph with two frequency response curves at 1 cm and 50 cm in the specification. The frequencies of the noise are usually low, such as the movement of cars or the sound of the wind.