Outdoor Microphone System - Page 16
G.R.A.S. Sound & Vibration
4.2.1 Setup and adjustment
1. Screw the support tube (1), with its collar (2) on, onto the thread used by the windscreen.
If the adapter has not been used recently, the collar might stick a little and should be made
free before proceeding.
2. Assemble the two halves of the split coupler (3 and 4) around the microphone assembly
(5). It is very important for the calibration that the two halves fit correctly around the micro
-
phone assembly making sure to accommodate for the thin pressure-equalisation tube.
3. Push the collar up onto the lower part of the split coupler to keep the two halves tightly
together. While doing this, press the two halves of the split coupler against the support
cylinder.
4. Mount the Pistonphone/Calibrator on top of the split coupler and switch on. It is important
for the calibration that the two halves of the split coupler are pressed against the support
cylinder to avoid faulty results.
Do not turn or twist the Pistonphone/Calibrator while
mounting it or removing it because this could damage the thin pressure-equalisa-
tion tube
.
5. With a pistonphone mounted and switched on, adjust P102 (see Fig. 8.3) to give a signal
output level of:
•
5 V for a nominal pistonphone signal of 134 dB.
•
1.6 V for a nominal pistonphone signal of 124 dB.
•
500 mV for a nominal pistonphone signal of 114 dB.
This will ensure that the sensitivity of the Outdoor Microphone System will be 50 mV/Pa.
A Pistonphone calibration will help in tracing any dubious results with the electrostatic-actuator
to one or more of the following:
Actuator
Actuator voltage supply
Microphone preamplifier
Microphone cartridge
Actuator calibration should be carried out at regular intervals. Calibration using a Pistonphone/
Calibrator should be carried out primarily for diagnostic purposes.
4.3
Factory Calibration
The Outdoor Microphone System is a self-contained unit. The microphone polarisation voltage
is supplied internally by the system’s electronics which also supplies the 120 V needed to power
the preamplifier as well as the voltages needed for the built-in actuator. This enables the system
to be calibrated to the nominal sensitivity of 50 mV/Pa as given on the calibration chart.
If a basic verification is desired
in-situ
, this should be carried out using the Pistonphone method
of calibration as described in section 4.2. The frequency-response of the Outdoor Microphone
System is given on the calibration chart. It can be shown that any change in frequency response
of the microphone used in Outdoor Microphone System is highly unlikely without an attendant
change in sensitivity and, hence, in the calibration level. There is no advantage in perform
-
ing annual checks on the Outdoor Microphone System as long as the built-in calibration per
-
forms correctly. Small deviations during warm up and cooling down periods will occur owing to
changes in the static pressure within the microphone unit.
Variations of approximately 0.4 dB will level off within half an hour, depending on the magnitude
of the variations. This is normal and should be taken as a sign of correct system performance.
If required, a timely calibration result can be used to correct the measurement of a short-term
event such as an aircraft flyover.
Multi-frequency calibration check (see section 5.3) is an available option if required. It provides
a check on the frequency response and gain of the electronic circuits. It does not increase
system reliability.
