Microphones 525floor and a wall, the wall increases sensitivity 6 dB, and
the floor increases sensitivity another 6 dB. Adding two
boundaries at right angles increases sensitivity 12 dB.
With 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.
Note that the acoustic sensitivity of the microphone
rises as boundaries are added, but the electronic noise of
the microphone stays constant, so the effective SNR of
the microphone improves 6 dB every time a boundary is
added at right angles to previous boundaries.16.6.1.2.2 Direct-to-Reverberant Ratio EffectsDirect sound sensitivity increases 6 dB per boundary
added, while reverberant or random-incidence sound
increases only 3 dB per boundary added. Consequently,
the direct-to-reverberant ratio increases 3 dB
(6 dBdir3dBrev) whenever a boundary is added at
right angles to previous boundaries.
16.6.1.2.3 Frequency-Response Effects
The low-frequency response of the PZM or PCC
depends on the size of the surface it is mounted on. The
larger the surface, the more the low-frequency response
is extended. The low-frequency response shelves down
to a level 6 dB below the mid-frequency level at the
frequency where the wavelength is about 6 times the
boundary dimension. For example, the frequency
response of a PZM on a 2 ft × 2 ft (0.6 m × 0.6 m) panel
shelves down 6 dB below 94 Hz. On a 5 inch × 5 inch
(12 cm × 12 cm) plate, the response shelves down 6 dB
below 376 Hz.
For best bass and flattest frequency response, the
PZM or PCC must be placed on a large hard boundary
such as a floor, wall, table, or baffle at least 4 ft × 4 ft
(1.2 m × 1.2 m).
To reduce bass response, the PZM or PCC can be
mounted on a small plate well away from other
reflecting surfaces. This plate can be made of thin
plywood, Masonite, clear plastic, or any other hard,
smooth material. When used on a carpeted floor the
PZM or PCC should be placed on a hard-surfaced panel
at least 1 ft × 1 ft (0.3 m × 0.3 m) for flattest high-
frequency response.
To determine the frequency f6 dB where the response
shelves down 6 dB, use
(16-20)where,
D is the boundary dimension in feet or meters.For example, if the boundary is 2 ft (0.6 m) square,
the 6 dB down point isBelow 94 Hz, the response is a constant 6 dB below
the upper mid-frequency level. Note that there is a
response shelf, not a continuous roll-off.
When the PZM is on a rectangular boundary, two
shelves appear. The long side of the boundary is Dmax
and the short side Dmin. The response is down 3 dB at(47-21)and is down another 3 dB at(47-22)The low-frequency shelf varies with the angle of the
sound source around the boundary. At 90° 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 dimen-
sion away. If the source is very close to the PZM
mounted on a panel, there is no low-frequency shelf; the
frequency response is flat.
If the PZM is at the junction of two or more bound-
aries at right angles to each other, the response shelves
down 6 dB per boundary at the above frequency. For
example, a two-boundary unit made of 2 ft (0.6 m)
square panels shelves down 12 dB below 94 Hz.
There are other frequency-response effects in addi-
tion to the low-frequency shelf. For sound sources
on-axis to the boundary, the response rises about 10 dBf–6 dB 188*
D------------=*57.3 for SI unitsf– 6 dB^188
D---------=188
2---------==94 Hzf– 3 dB 188*
Dmax= ------------*57.3 for SI unitsf– 3 dB 188*
Dmin------------=*57.3 for SI units