Paul White describes the engineering and physics of Polar Pickup patterns of microphones and how to use them to your advantage when recording sound.
It is worth remembering that although the printed polar patterns are in two dimensions (as below), the actual pattern is three-dimensional. For example, an omnidirectional microphone pattern drawn on paper looks like a circle, but in reality it is a sphere.
Despite the mics on sale with multiple switchable pickup patterns, there are only two fundamental patterns: the omni and the figure-of-eight. All the other patterns in use today, including the popular cardioid, are created by combining these two in differing proportions.
Omni mics are often referred to as ‘pressure microphones’, because they essentially measure sound pressure at a point in space. A diaphragm is fixed across the mouth of a sealed cavity, so in effect the mic behaves like a very small barometer capable of following audio frequency pressure changes — but it has no means of detecting the direction of the sound waves, hence it has an omnidirectional polar pattern. Because this arrangement only senses pressure, it doesn’t matter what direction the sound approaches from. All that matters is the change in pressure at that point in space, so it is more or less equally sensitive to sounds from all directions — hold this thought, as I’ll come back to it in a moment.
To prevent the microphone behaving too much like a meteorological barometer and responding to changes in the weather, the cavity is engineered with a very small air leak or vent built into it, so that very low-frequency pressure changes due to weather (or altitude) don’t force the diaphragm in or out permanently. At audio frequencies, however, the cavity can be considered to be sealed.
This is a very thorough and technical explanation about microphones that should help a lot of people. I wasn’t taught in film school that the pickup pattern of an omnidirectional mic is spherical rather than circular.