G.R.A.S. 41AM, 41CN Instruction Manual

Instruction Manual

Outdoor Microphone System Types 41AM / 41CN

Skovlytoften 33, 2840 Holte, Denmark
www.gras.dk [e-mail]

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