654 Chapter 22
microphones. This is not necessarily the case. As a class, omnidirectional microphones
exhibit smoother frequency responses than directional microphones. The frequencies of
oscillation triggered by acoustic feedback, the ring frequencies, depend on a number of
factors.
Prominent causative agents are peaks in microphone response and peaks in loudspeaker
response coupled with antinodes in the normal modes of the room. Room modes at
even moderate frequencies can be quite dense. As a consequence, a single peak in either
microphone or loudspeaker response may trigger an entire chorus of slightly different
ring frequencies. This set of facts would tend to favor omnidirectional microphones
over directional ones. The deciding factor is usually not immunity to feedback from the
reverberant fi eld but rather the necessity to reject a nearby source of objectionable sound,
including possible strong discrete refl ections.
A microphone consisting of a separate pressure and pressure gradient element is quite
versatile in that it offers all of the polar response patterns listed in Table 22.2 , assuming
that it contains the appropriate switch selectable passive circuitry necessary to properly
combine the signals from the individual elements. Such a microphone, however,
inherently has a shortcoming in that the centers of the two elements are physically offset.
Sound waves incident on the device in other than the principal plane arrive at the two
elements at slightly different times. The difference in arrival times introduces a phase
difference between the electrical signals generated by the two elements. This phase difference
can be signifi cant at high frequencies and can distort the directional response pattern in the
high-frequency region. Fortunately, it is possible to avoid the offset problem through the
design of a single diaphragm device that also has useful directional characteristics.
Figure 22.4 is a bare bones illustration of a compliantly mounted diaphragm and a
back enclosure that is vented through a porous screen to the external environment. The
diaphragm may be part of either a capacitor or moving coil type of transducer, the details
of which are not shown for simplicity. A sound wave is incident on the left face of the
diaphragm. The direction of the incident wave makes an angle θ with the principal axis of
the system. The principal axis is perpendicular to the plane that contains the diaphragm. The
acoustic pressure on the left face of the diaphragm assuming a spherical wave is given by
pA
r
1 ejtkr()ω. (22.23)