Crown PZM-10 Application Manual page 13

As Fig. 23 shows, the low-frequency shelf varies with
the angle of the sound source around the boundary. At
90 degrees incidence (sound wave motion parallel to the
boundary), there is no low-frequency shelf.
The depth of the shelf also varies with the distance of the
sound source to the panel. The shelf starts to disappear
when the source is closer than a panel dimension away.
If the source is very close to the PZM mounted on a
panel, there is no low-frequency shelf; the frequency
response if flat.
If the PZM is at the junction of two or more boundaries
at right angles to each other, the response shelves down
6 dB per boundary at the frequency mentioned above.
For example, a two-boundary unit made of 2-foot
square panels shelves down 12 dB at and below 94 Hz.
There are other frequency-response effects in addition
to the low-frequency shelf. For sound sources perpen-
dicular to the boundary, the response rises about 10 dB
above the shelf at the frequency where the wavelength
equals the boundary dimension (see Fig. 23).
For a square panel, F
peak
of sound (1130 feet per second) and D = the boundary
dimension in feet. For a circular panel, F
As example, a 2' square panel has a 10 dB rise above
the shelf at .88C/D = 88 x 1130/2 = 497 Hz.
Note that this response peak is only for the direct sound
of an on-axis source. If the sound field at the panel is
partly reverberant, or if the sound waves strike the panel
at an angle, the effect is much less. The peak is also re-
duced if the mic capsule is placed off-center on the
boundary.
Fig. 23 shows the frequency response of a PZM mounted
on a 2' square panel, at various angles of sound inci-
dence. Note several phenomena shown in the figure:
Fig. 22
Fig. 23
= .88C/D, where C = the speed
= C/D.
peak
• The low-frequency shelf (most visible at 30 and 60
degrees).
• The lack of low-frequency shelving at 90 degrees
(grazing incidence).
• The 10 dB rise in response at 497 Hz.
• Less interference at increasing angles up to 90º.
• Greater rear rejection of high frequencies than low
frequencies.
What are the acoustic causes of
these frequency-response effects?
When sound waves strike a boundary, pressure doubling
occurs at the boundary surface, but does not occur out-
side the boundary. Thus there is a pressure difference at
the edge of the boundary. This pressure difference cre-
ates sound waves.
These sound waves generated at the edge of the bound-
ary travel to the microphone in the center of the bound-
ary. At low frequencies, these edge waves are opposite
in polarity to the incoming sound waves.
Consequently, the edge waves cancel the pressure
doubling effect.
Thus, at low frequencies, pressure doubling does not oc-
cur; but at mid-to-high frequencies, pressure doubling
does occur. The net effect is a mid-to-high frequency
boost, or – looked at another way – a low-frequency loss
or shelf.
Incoming waves having wavelengths about six times
the boundary dimensions are cancelled by edge effects;
waves of wavelength much smaller than the boundary
dimension are not cancelled by edge effects.
Waves having wavelengths on the order of the boundary
dimensions are subject to varying interference vs. fre-
quency; i.e., peaks and dips in the frequency response.
At the frequency where the wavelength equals the
boundary dimension, the edge wave is in phase with the
incoming wave. Consequently, there is a response rise
(about 10 dB above the low-frequency shelf) at that fre-
quency. Above that frequency, there is a series of peaks
and dips that decrease in amplitude with frequency.
The edge-wave interference decreases if the incoming
sound waves approach the boundary at an angle.
Interference also is reduced by placing the mic capsule
off-center. This randomizes the distances from the edges
to the mic capsule, resulting in a smoother response.
Directional Effects
The polar pattern of a PZM on a large surface is hemi-
spherical. The microphone picks up equally well in any
direction above the surface plane, at all frequencies.
By adding boundaries adjacent to this PZM, you can
shape its directional pickup pattern. Boundaries make
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