G.R.A.S. 41AM User manual

Instruction Manual
Outdoor Microphone System Types 41AM / 41CN
Skovlytoften 33, 2840 Holte, Denmark
www.gras.dk [email protected]
SOUND & VIBRATION
G R A S
...
.

Revision 11 June 2010
LI0002
Outdoor Microphone System
Types 41AM / 41CN
Note: unless individually specied, the term “Outdoor Microphone
System” throughout this document refers to both types, i.e.
Type 41AM and Type 41CN.

Important! any dismantling details given in this document are
addressed solely to qualied service personnel who must ensure
that the unit is rst disconnected from its power supply before pro-
ceeding.

CONTENTS
1. Introduction and Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Available Types and Applications ......................................6
2. Main Components................................................7
2.1 Windscreen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Microphone Assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1 Pressure Equalisation ................................................10
2.2.2 Electrostatic Actuator .................................................10
2.2.3 Microphone ........................................................10
2.2.4 Dismantling the Microphone Assembly ...................................11
3. Access to Electronics Assembly...................................12
4. Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1 Electrostatic Actuator ..............................................14
4.1.1 Adjustment .........................................................14
4.1.2 Calibration Control Box AC0001 ........................................14
4.2 Pistonphone .....................................................15
4.2.1 Setup and adjustment ................................................16
4.3 Factory Calibration ................................................16
5. Maintenance ...................................................17
5.1 Windscreen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2 Dehumidiers ....................................................17
5.3 Multi-frequency Calibration Check (Optional) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.3.1 Output Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Service and Repair ..............................................19
7. Specications ..................................................20
8. Circuit and Component Diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Outdoor Microphone System - Page 5
G.R.A.S. Sound & Vibration
1. Introduction and Description
The G.R.A.S. Outdoor Microphone System (Fig. 1.1) is for outdoor use whenever trouble-free
noise monitoring is required, e.g. around airports or in communities.
It complies with IEC 651 Type 1 and ANSI S1.4 1983 Type 1 requirements and can be used with
any suitable electronic sound or vibration measurement system. It is PTB approved as part of
an IEC 651 Type 1 system.
A ½-inch precision condenser microphone and thick-lm preamplier ensure maximum stability
and performance. Both microphone and casing are made of stainless steel.
Precise in-situ calibration checks at 1000 Hz are enabled any time via a built-in electrostatic
actuator and test oscillator.
Fig. 1.1 Outdoor Microphone System

Outdoor Microphone System - Page 6
G.R.A.S. Sound & Vibration
Fig. 1.2 Outdoor Microphone System. Types, reference
directions and applications
1.1 Available Types and Applications
There are two types of the Outdoor Microphone System, these are Type 41AM and Type 41CN.
Each has its own measurement reference direction and application, i.e.:
 Type 41AM
Measurement reference direction - vertical (along axis of symmetry)
Angle of incidence - 0°
Application - monitoring airport noise
 Type 41CN
Measurement reference direction - horizontal (perpendicular to axis of symmetry)
Angle of incidence - 90°
Application - monitoring community noise
See also Fig. 1.2.
Both types are almost physically identical; differing only in the type of the microphone cartridge
and rain-protection cap used. Type 41AM uses Microphone Cartridge Type 41AS and Type
41CN uses Microphone Cartridge Type 40AS.
Reference Direction
0° incidence (airport noise)
Type 41AM
Reference Direction
90° incidence (community noise)
Type 41CN

Outdoor Microphone System - Page 7
G.R.A.S. Sound & Vibration
2. Main Components
The following is a brief description of the main components of the Outdoor Microphone System
(see also Fig. 2.1).
All external metal parts are made of stainless steel.
 Windscreen and windscreen holder with anti-bird spikes.
 Electrostatic actuator housing mounted on top of the condenser microphone.
 Condenser microphone and preamplier mounted on top of the casing. The microphone
and electrostatic-actuator assembly are galvanically isolated from the system casing.
 The top of the casing has an inspection window for viewing the state of the dehumidier
when the cover screw is removed.
 The cylindrical casing contains the assembly of electronics and desiccator bags for absorb-
ing moisture.
 The electronics include:
A switch-mode power supply for generating:
a) 200 V for microphone polarisation
b) 120 V for the microphone preamplier
And:
c) an A-weighting network
d) a circuit jumper for applying a gain of ± 20 dB to the microphone signal
 A pole adaptor with a standard ISO 228/1 G 1½-in thread. Also used for the tripod adaptor
(Fig. 2.2).
 The Outdoor Microphone System can be mounted on the tripod adaptor using the pole
adaptor. The cable and plug are readily led through the tripod adaptor. The tripod adaptor
is not meant for permanent outdoor installations, but for test setups in labs and short-term
outdoor surveys.
 The connector is a 6-pole LEMO FFA.2S.306, which locks into a socket at the bottom of
the Outdoor Microphone System. It cannot be pulled out by its cable. It can be removed
only by pulling outwards on the knurled outer sleeve.

Outdoor Microphone System - Page 8
G.R.A.S. Sound & Vibration
Fig. 2.1 Main components of the Outdoor Microphone
System
Anti-birds spikes
Windscreen and holder
Electrostatic actuator
½″ microphone
Preamplier
Equalisation vent
Calibration oscillator
Dehumidier inspection window
Lin. or A-weighting
Sensitivity control
0 or ±20 dB gain
Cable driver
DC - DC converter
Dehumidier
Pole adapter
LEMO connectror
12 - 18 V DC
Signal out
Test-oscillator start
Ground
Multi-freq. cal. check (optional)
Tripod adapter

Outdoor Microphone System - Page 9
G.R.A.S. Sound & Vibration
Fig. 2.2 Left to right; tripod and pole adapters
2.1 Windscreen
This comprises a windscreen and a windscreen holder (see Fig. 2.3).
The windscreen itself is made of polyurethane foam. It reduces wind-noise to a minimum and
gives maximum rain protection. It also allows for 0 ° (Type 41AM) and 90 ° (Type 41CN) refer-
ence directions of incidence.
The windscreen holder has four anti-bird spikes to prevent birds from perching on top of the
windscreen. The centre spike prevents smaller birds from perching between the other three
spikes.
Fig. 2.3 Windscreen of the Outdoor Microphone System
2.2 Microphone Assembly
On delivery, the microphone assembly is protected by a transport tube, see Fig. 2.4. A small
hole of 0.5 mm diameter at the top end of the transport tube ensures pressure equalization
during transport. The transport tube is also used when testing for self-noise. In this case, the
hole must be sealed using a piece of tape. Remember to remove the tape after use.
When screwing the windscreen onto the microphone, apply some silicone grease (supplied) to
the thread - see Fig. 2.4). The silicone grease is waterproof and protects against rust.
Anti-birds spikes
Windscreen
Windscreen holder

Outdoor Microphone System - Page 10
G.R.A.S. Sound & Vibration
2.2.1 Pressure Equalisation
The pressure equalisation system, which ensures that the air-pressure within the cartridge of
the condenser microphone is kept equal to the ambient atmospheric pressure, allows sufciently
rapid pressure equalisation while keeping to a minimum the exchange of air between the interior
and exterior. This also extends the working life of the dehumidier.
Pressure equalisation takes place via the 1.5 mm diameter stainless-steel tube visible on the
side of the upper cylindrical section (see Fig. 2.4). This tube is also a conduit for the signal wire
connecting the calibration oscillator with the electrostatic actuator.
2.2.2 Electrostatic Actuator
The position of the actuator relative to the microphone is maintained by a calibration ring posi-
tioned under the Teon sleeve. The calibration ring ensures the correct spacing between the
actuator and the diaphragm of the microphone.
For inspection purposes, access to the signal wire in the pressure-equalisation tube is gained
after removing the rain-protection cap (see section 2.2.4) which is held in place by a left-hand
thread mushroom-head screw. In the case of the Type 41CN, the small black plastic cone
mounted on top of the screw has to be removed rst.
2.2.3 Microphone
If the electrostatic actuator, Teon sleeve and calibration ring are removed, the microphone
can be unscrewed (see section 2.2.4). The microphone is mounted on a galvanically isolated
assembly, together with a thick-lm, high-impedance input preamplier which is miniature,
ceramic-based, and robust.
Fig. 2.4 Showing microphone assembly - note the difference between
the two rain-protection caps
Rain-protection cap and actuator
Teon sleeve
Pressure-equalisation tube
Dehumidier inspection window
Transport tube
Type 41AM
Type 41CN
Add silicone grease to this thread