Crown PZM-10 Application Manual page 14

the PZM reject sounds coming from behind the bound-
aries. In addition, making the PZM directional increases
its gain-before-feedback in live reinforcement applica-
tions. Directional PZMs also pick up a higher ratio of
direct sound to reverberant sound, so the resulting au-
dio sounds "closer" and "clearer."
In general, sound pickup is fairly constant for sound
sources at any angle in front of the boundaries, and
drops off rapidly when the source moves behind the
boundaries.
For sounds approaching the rear of the panel, low
frequencies are rejected least and high frequencies
are rejected most.
A small boundary makes the PZM directional only
at high frequencies. Low frequencies diffract or bend
around a small boundary as if it isn't there. The bigger
you can make the boundary assembly, the more direc-
tional the microphone will be across the audible band.
The bigger the boundary, the lower the frequency at
which the PZM becomes directional. A PZM on a square
panel is omnidirectional at very low frequencies, and
starts to become directional above the frequency F,
where F = 188/D and D is the boundary dimension in
feet. Sound familiar? That's the same equation used to
predict the 6-dB-down point in the frequency response.
Boundaries create different polar patterns at different
frequencies. For example, a 2' square panel is omnidirec-
tional at and below 94 Hz. At mid-frequencies, the polar
pattern becomes supercardioid. At high frequencies, the
polar pattern approaches a hemisphere (as in Fig. 24).
Two boundaries are more directional than one, and
three are more directional than two.
Fig. 24 – Polar response of 2–foot square boundary
With multiple boundaries, the shape of the pickup pat-
tern approximates the shape of the boundary assembly.
For example, a V-shaped boundary produces a polar
pattern with a lobe whose sides are defined by the sides
of the "V." Note, however, that the polar pattern varies
with frequency.
This "V"-shaped boundary works like a horn loud-
speaker in reverse. Speaker horn theory applies to mi-
crophone horns. For instance, if you want a constant
directivity boundary horn, the horn must flare out
exponentially like a well-designed loudspeaker horn.
Disadvantages of Boundaries
Boundaries must be large to be effective. Their size
and weight makes them cumbersome to mount or hang.
Large boundaries are also visually conspicuous, but this
problem is reduced by using clear plastic.
Many users claim that the sound quality and flexibility
if multiple-boundary PZMs outweigh the disadvantages.
For those users who need a directional PZM but prefer
not to use boundaries, Crown makes the PCC®-160 and
PCC-170, which are supercardioid surface-mounted mi-
crophones. They use a directional mic capsule, rather
than boundaries, to make the microphone directional.
The PCC-130 is cardioid.
Summary
• Microphone sensitivity increases 6 dB for every
boundary added at right angles to previous bound-
aries (less than 6 dB if not at right angles).
• For a flat panel, the frequency response begins to
shelve down at the transition frequency F
where D = boundary dimension in feet. The response
shelves down 6 dB at and below the frequency
F = 188/D. This shelf disappears if the sound source
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is at the side of the panel, or if the source is very close
to the microphone (less than a panel dimension away).
• For a square panel, the frequency response rises about
10 dB above the low-frequency shelf (when the source
is perpendicular to the boundary) at the frequency
F = .88C/D, where C = the speed of sound (1130 fps)
and D = the boundary dimension in feet.
• The PZM/panel assembly is omnidirectional at and be-
low the frequency F = 188/D, where D = the boundary
dimension in feet. The panel becomes increasingly
directional as frequency increases.
• Use the biggest boundaries that are not visually
conspicuous. Big boundaries provide flatter response,
better bass, and more directionality than small
boundaries.
• The flattest response for a single panel occurs for
angles of incidence between 30º and 90º to the axis
perpendicular to the boundary.
• For the flattest response, place the PZM 1⁄3 of the way
off-center (say, 4" off-center for a 2' panel). For flattest
response on multiple boundaries, place the tip of the
PZM cantilever touching against the boundaries (leav-
ing the usual gap under the mic capsule).
• To increase directionality and reach, increase bound-
ary size or add more boundaries.
Construction Tips
You can obtain clear acrylic plastic (plexiglass) from
a hardware store, plastic supplier or a fabrication com-
pany. Plastic ⁄ " thick is recommended for good sound
1
4
12
= 720/D,
T