19
Figure 10. Typical Frequency Response - Model QE4146
Microphone
B. Microphone
The Model 1800 is designed to accept either a half-inch or one-inch
microphone of either prepolarized (electret) or condenser type. The
prepolarized (electret) microphone does not require a polarization voltage.
If a condenser type is used, then the 200 volt microphone polarization
voltage must be turned on by a switch located in the battery compartment.
(Sec II. Figure 4., Internal Switches.)
CAUTION: Be careful not to turn on the 200 volt polarization switch if
a prepolarized (electret) microphone is attached. The
microphone may be damaged.
The microphone screws directly onto the preamp which, in turn, connects
directly to the meter. The preamp converts the high output impedance of the
microphone into a low output impedance. This allows the microphone to drive
up to 100 feet of cable for remote operation.
Typical microphones used on the 1800 include the following:
Model QE4146 1/2-inch free-field, prepolarized condenser
(electret), 40 mv/Pa
Model QE4170 1-inch pressure, 200 Volt polarization, 50 mv/Pa
Model QE4160 1/2-inch pressure, 200 Volt polarization, 50 mv/Pa
Model QE4150 1/2-inch free-field, 200 Volt polarization, 50 mv/Pa
Model QE4140 1/2-inch pressure, 200 Volt polarization, 16 mv/Pa
Typical microphone response curves for the Models QE4146 and QE4170 are shown
in Figures 10 and 11 respectively.
20
Figure 11. Typical Frequency Response - Model QE4170 Microphone
Figure 12. Effect of Microphone Output Capacitance on Preamp
Low Frequency Response
C.
Preamplifi
er Input
Characteri
stics
The input
impedance
of the
preamp
affects
both the
low
frequency
response
and the
microphone
attenuatio
n as shown in Figures 12 and 13. The approximate microphone capacitances for
the 1/2 inch and one inch microphones are 18pf and 60pf respectively. The
preamp is removable by turning the black plastic collar below the preamp
housing counter-clockwise when viewed from the top of the meter.
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