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LI0151 – 4 July 2017
System Integration
Setting the 41AC-2 for 90 or 0 Degrees of Incidence
The 41AC-2 is designed to be mounted vertically, as described in the previous sections. Mounted
in this way, it can be set up for measurement at either 90 degrees or 0 degrees of incidence.
The correction data contained on the USB flash drive that is part of the delivery must be used if
the 41AC-2 is used for 0 degrees of incidence.
The correction data are measured with a resolution of 1/12 octave.
Polarization Voltage = 200 V
41AC-2 uses an externally polarized microphone with a 26AJ 1/2’’ RemoteCheck Preamplifier
and must be connected to the LEMO input of an analyzer.
Distance to the Ground
When mounted on a pipe or tripod, the distance from the bottom of the microphone to the ground
reflecting surface must be at least 20 cm/7.9” to minimize the influence of ground reflections on
the sound field.
Replacing Microphone and/or its Protective Grid
The 40AF-S2 microphone and its protective grid are designed specifically for the 41AC-2. Only
the 40AF-S2 will ensure the stated measurement results and water protection.
Therefore neither microphone nor its protective grid can be replaced with standard items.
Calibration and RemoteCheck
Introduction
41AC-2 comes with a RemoteCheck facility that makes it possible to check the integrity of the
measurement chain remotely. RemoteCheck allows you to send a test signal from analyzer to
microphone and record the result. It works by applying a precision AC test signal (for example a 1
kHz sine wave) to an RC network in the preamplifier. This signal is applied to pin 1 of the 7-pin
LEMO cable/connector.
This method does not result in a signal proportional to the microphone’s sensitivity, but it does
give you a way to determine whether your system is stable and functional by comparing results
over time. If the newest value differs from previous values, this indicates that the system has
changed and that an error is present in the measurement chain. Note that RemoteCheck cannot
replace a proper acoustic calibration of the microphone.
Implementing RemoteCheck
Prior to implementing your RemoteCheck routine you should perform an acoustic calibration of
the 41AC-2. This will provide you with a reference and can be used to establish the relation
between calibration and subsequent RemoteCheck values. In this way you will have a more
precise knowledge of the stability of your system.