The polar pattern characteristics of microphones
The directional characteristics of microphones can be described in terms of a family of polar patterns.
The polar pattern is a graph showing the sensitivity in a full 360 degree circle around the mic. I say a family of polar patterns but it really is a spectrum with omnidirectional at one extreme and figure-of-eight at the other. Cardioid and hypercardioid are simply convenient way points.
To explain these patterns further, fairly obviously an omnidirectional mic is equally sensitive all round. A cardioid is slightly less obvious.
The cardioid is most sensitive at the front, but is only 6 dB down in response at an angle of 90 degrees. In fact it is only insensitive right at the back. It is not at all correct, as commonly happens, to call this a unidirectional microphone.
The hypercardioid is a more tightly focussed pattern than the cardioid, at the expense of a slight rear sensitivity, known as a lobe in the response.
The figure-of-eight is equally sensitive at front and back, the only difference being that the rear produces an inverted signal, 180 degrees out of phase with the signal from the front.
All of this is nice in theory, but is almost never borne out in practice.
Take a nominally cardioid mic for example. It may be an almost perfect cardioid at mid frequencies, but at low frequencies the pattern will spread out into omni.
At high frequencies the pattern will tighten into hypercardioid.
The significant knock-on effect of this is that the frequency response off-axis – in other words any direction but head on – is never flat.
In fact the off-axis response of most microphones is nothing short of terrible and the best you can hope for is a smooth roll-off of response from LF to HF. Often though it is very lumpy indeed.