September 25, 2018

Are All Microphones Created Equal? Part 3

While not to be confused with the actions of big white bears in the Arctic, microphones have polar patterns of their own. There are a number of polar patterns that various microphones employ, and in this week’s segment I’m going to cover three of the six main types. A mic’s polar pattern defines where it is most sensitive, or it’s directionality. We refer to these points of sensitivity as their axis or axes if or when a mic has multiple points of sensitivity. In other words, particular microphones are going to be more sensitive to sounds that are arriving “on axis/axes” to the microphone's diaphragm.

The cardioid (Greek for heart) pattern has a very distinct look. As one might imagine, the cardioid polar pattern is in the shape of a heart and is most sensitive at 0° (0 degrees); this is also its on-axis point. As the signal is refocused on various other positions, off axis from the diaphragm, it starts to reject and once past 90° and 270° one can really start to notice the rejection as evident in the decrease in amplitude. Theoretically, a cardioid mic should reject all signal at 180° but there will still be some signal as sound diffracts or bends around objects.

Cardioid polar pattern of a Shure SM 57 dynamic microphone

Next in the queue is the bipolar/bidirectional (figure 8) pattern. Care to guess what this looks like? Most ribbon microphones utilize this pattern, which can also lead to some cool phase tricks, but that's for a later discussion. This pattern has two main axes, one at 0° and the other at 180°. This pattern is most sensitive or directional at the front and back of the diaphragm, but the sensitivity at the rear can lead to unwanted room tone or other reflections that might cause issues. One should also notice that maximum rejection happens at 90° and 270°.

Bidrectional pickup pattern of a Coles 4038 ribbon microphone-rotated 90° counter-clockwise to better illustrate the axes of the microphone

An omnidirectional microphone has no axes, or one could say it is directionally agnostic. The diaphragm is going to respond equally no matter where the sound is coming from. Depending on the microphone and the size of the diaphragm, omnidirectional microphones tend to be flatter in their frequency response, but isolation from other sound sources can prove to be difficult.