Crown PZM-10 Application Manual page 12

mentioned in this guide will degrade the frequency re-
sponse and polar patterns of conventional microphones.
Only PZMs can be used effectively in multiple boundaries.
The size, shape and number of boundaries all have pro-
found effects on the performance of a PZM mounted on
those boundaries. Let's discuss these effects in detail.
Sensitivity Effects
Imagine a PZM mic capsule in open space, away from
any boundaries. This microphone has a certain sensitiv-
ity in this condition (Fig. 20).
Now suppose the PZM capsule is placed very near
(within .020" of) a single large boundary, such as a wall.
Incoming sound reflects off the wall. The reflected
sound wave adds to the incoming sound wave in the
"pressure zone" next to the boundary. This coherent
addition of sound waves doubles the sound pressure at
the microphone, effectively increasing the microphone
sensitivity 6 dB.
In short, adding one boundary increases sensitivity
6 dB. This is free gain.
Now suppose the PZM capsule is placed at the junction
of two boundaries at right angles to each other, such as
the floor and a wall. The wall increases sensitivity 6 dB,
and the floor increases sensitivity another 6 dB. Thus,
adding two boundaries at right angles increases sensitiv-
ity 12 dB.
Now let's place the PZM element at the junction of three
boundaries at right angles, such as in the corner of
the floor and two walls. Microphone sensitivity will be
18 dB higher than what it was in open space. This is
increased gain with no increase in noise!
Note that the acoustic sensitivity of the microphone rises
as boundaries are added, but the electronic noise of the
microphone stays constant. Thus, the effective signal-to-
noise ratio of the microphone improves 6 dB every time
a boundary is added at right angles to previous bound-
aries.
If a PZM is in the corner of three boundaries that
are NOT at right angles to each other, the sensitivity
increases less than 6 dB per boundary. For example,
a PZM-2.5 boundary is built with two panels at 135
degrees. This panel assembly is at right angles to a base
Fig. 20
plate. The net gain in sensitivity from these three
boundaries is approximately 16 dB rather than 18 dB.
Direct-to-Reverb Ratio Effects
We mentioned that sensitivity increases 6 dB per
boundary added. That phenomenon applies to the
direct sound reaching the microphone. Reverberant
or random-incidence sound increases only 3 dB per
boundary added. Consequently, the direct-to-reverb
ratio increases 3 dB (6-3dB) whenever a boundary is
added at right angles to previous boundaries.
A high direct-to-reverb ratio sounds close and clear;
a low direct-to-reverb ratio sounds distant or muddy.
Adding boundaries increases the direct-to-reverb ratio,
so the subjective effect is to make the sound source
audibly closer or clearer. That is, "reach" is enhanced
by adding boundaries.
Frequency-Response Effects
The size of the boundary on which the PZM is
mounted affects the PZM's low-frequency response.
The bigger the boundary, the better the bass. Specifi-
cally, the response begins to shelve down 6 dB at the
transition frequency F
F = 750/D
T
D is the boundary dimension in feet. The response is
down 6 dB at the frequency F
F = 188/D
-6
For example, if the boundary is 2 feet square,
F = 750/D = 750/2 = 375 Hz.
T
F = 188/D = 188/2 = 94 Hz.
-6
That is, the microphone starts to shelve down at 376 Hz
and is down 6 dB at and below 94 Hz. (See Fig. 21).
Below 94 Hz, the response is a constant 6 dB below the
upper-mid frequency level. Note that there is a response
shelf, not a rolloff.
If a PZM is mounted on a 4' square boundary,
F = 750/4 = 178 Hz
T
F = 188/4 = 47 Hz.
-6
This result has been loosely called the "4' – 40 Hz" rule.
Fig. 22 shows the PZM response on various sizes of
boundaries.
What if the PZM is on a rectangular boundary? Let's
call the long side "Dmax" and the short side "Dmin."
The response is down 3 dB at 188/Dmax, and is down
another 3 dB at 188/Dmin.
10
, where
T
where
-6
Fig. 21