More on Microphones 
by Michael Williams, (www.williamsmmad.com)
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8.2 - DIRECTIVITY
8.2.2 - BI-DIRECTIVITY OR FIGURE OF EIGHT DIRECTIVITY
In a pressure-gradient acoustic coupled transducer, when the sound wave has unimpeded access to the front and back surfaces of the diaphragm, the directivity is bi-directional, sometimes called ‘figure-of-eight’, as shown in Figure 42. The only difference between the front and back lobes is their polarity – if we consider that the front lobe is of positive polarity, then the back lobe will be negative polarity. In general, the directivity pattern remains remarkably constant throughout the audible frequency range, as the microphone structure is usually sufficiently small, as an acoustic obstacle, not to cause any appreciable shadow effect to sound coming from 90° or 270°.
In the case of a ribbon microphone, the polar diagram will be different according to the orientation of the ribbon. In Figure 42, this is the theoretical directivity response when the ribbon is vertical i.e. the sound arrives perpendicular to the length of the ribbon. If on the other hand the ribbon is horizontal i.e at 90° and 270° the sound arrives in line with the ribbon, then a certain amount of summing will take place along the length of the ribbon when the sound source is off-axis.
Figure 42 – Bi-directional or Figure of Eight Directivity
This will depend on the length of the ribbon with respect to the wavelength, and also the angle of incidence (as shown in Figure 39 - section 8.2.1 - Omnidirectional Directivity Response). The consequence is that the polar diagram in the form of a figure-of-eight will have a tendency to be narrower at high frequencies. The polar diagram of a ribbon microphone does not therefore maintain rotational symmetry about its directivity axis.
In the case of a bi-directional large diaphragm condenser microphone, the polar diagram will again be narrower than the theoretical pressure-gradient directivity function due to pressure-summing across the diaphragm at medium and high frequencies, but as the diaphragm is circular, the polar diagram will this time be symmetrical with rotation about its directivity axis.
