So far we have investigated the various methods by which the sound wave is converted into an electric signal. Now it's time to analyze the different construction criteria for microphones, which can be used to obtain different directional characteristics. A whole series of construction techniques have been devised which allow a microphone's sensitivity [Sensitivity ] to be focused towards one or more specific directions. This opens up a whole new world in terms of miking techniques, which we shall explore in the next section.
The sensitivity's rate in relation to the direction of the sound source is illustrated on a graph called polar pattern. In the following diagram we can see the most popular polar patterns and their names. The centre represents the microphone with its diaphragm, whilst around it we have its sensitivity-value for each change in direction. Direction is measured in degrees. 0 degrees is the spot directly in front of the diaphragm, whereas 180 degrees indicates the opposite position, namely behind the microphone. Each concentric ring, starting from the outmost one, indicates a 5 dB loss (for example, on figure b, cardioid diagram, we notice a loss of 5 dB in sounds coming from a direction which have a 45 degree angle in relation to the central direction):
Now let's briefly describe each diagram:
Circular: the microphone is equally sensitive to all directions. Sound is reproduced with the same accuracy regardless of which direction it comes from (in theory anyway, because perfectly circular diagrams are impossible to obtain due to purely physical limitations).
Cardioid: its name comes from the diagram's heart-shaped line. In this case sounds issuing from behind the microphone aren't picked up by it, or rather, as we shall see, they are drastically attenuated.
Figure of 8: in this case the microphone is better at picking up sounds coming both from behind and in front of it, but lacks in sensitivity with sounds arriving from lateral directions.
Super cardioid: the same as the cardioid diagram but with more pronounced directional characteristics. However, to tighten the front-side pattern there is an inevitable insurgence of a little posterior lobe. This entails a slight increase in sensitivity to sounds issuing from behind the microphone.
Hyper cardioid: like the super cardioid but with yet more accentuated directional characteristics. Notice the yet greater presence of the posterior cardioid diagram.
Shotgun: takes its name from the kind of microphone this diagram is associated with, which shall be described in one of the following sections.
In the following diagram, we can see a three-dimensional perspective of the aforementioned polar patterns: