656 Chapter 22
whereas for a pressure gradient microphone, it is directly proportional to the gradient of
the acoustic pressure.
In the capsule described earlier, the driving force on the diaphragm is proportional
to a linear combination of the pressure and pressure gradient terms. The sizes of the
coeffi cients in the linear combination and consequently the particular directional polar
pattern hinge on the volume of the cavity, the areas occupied by the diaphragm and the
porous screen, the mechanical properties of the diaphragm, and the porosity of the
screen. Such microphones are usually constructed having a dedicated directional pattern.
The majority of the cardioid family of directional microphones is constructed in this
fashion.
Most microphones have cylindrical symmetry and basically circular diaphragms. The
principal axis of such a microphone is centered on the diaphragm, perpendicular to the
plane of the diaphragm, and directed along the cylindrical axis, as illustrated in Figure
22.5. The directional polar pattern in a plane is obtained by varying the angle of incident
sound relative to the principal axis of the microphone.
Figure 22.5 : Illustration of the principal axis of a cylindrically symmetric microphone.Figure 22.6 : Position of the principal axis of a classic ribbon microphone.