GRAS 90CA User manual

Instruction Manual
90CA Microphone Calibration System
LI0056 – 13 May 2019

2
Revision History
Any feedback or questions about this document are welcome at [email protected].
Revision Date Description
1 15 October 2009 First publication of this document
2 01 March 2012 Redesign of the manual. Added information about
calibration in general, calibration of microphone sets,
and preamplifiers. New diagrams and drawings about
hardware assembly and electrical connections
3 10 February 2017 File locations on page 64 corrected, new microphone
definition file.
4 13 May 2019 Adapter Cable with Optical Isolation added.
Warning about uninstalling LabView.
Copyright Notice
© 2012-19 G.R.A.S. Sound & Vibration A/S
http://www.gras.dk
Any technical documentation that is made available by G.R.A.S. is the copyrighted work of
G.R.A.S. and is owned by G.R.A.S.
The content in this document is subject to change without notice. G.R.A.S. Sound & Vibration A/S
is not liable or responsible for any errors or inaccuracies that may appear in this document.
Trademarks
Any product names mentioned in this document may be trademarks or registered trademarks of
their respective companies and are hereby acknowledged.

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Contents
Introduction.................................................................................. 5
Using This Manual ..................................................................................... 5
Introduction to Calibration............................................................... 6
What is Acoustic Calibration? ...................................................................... 6
Which type of calibration do I need? ............................................................ 7
How often should I calibrate? ...................................................................... 8
The Concept of the 90CA Microphone Calibration System ..................... 9
Applications ............................................................................................ 10
The Basic Principles of 90CA: Microphone Calibration in Four Easy Steps ....... 11
Functional Block Diagram ......................................................................... 12
Hardware and Software for Level Calibration ............................................... 13
Hardware and Software for Frequency Calibration ........................................ 14
Delivered Components.................................................................. 16
Assembling and Connecting the Hardware ........................................ 17
Workspace for the Assembly...................................................................... 17
PXI: Connections for Test of Microphones (Ext. Pol. and Prepol.).................... 19
PXI: Connections for Test of Externally Polarized Microphone Sets ................. 21
PXI: Connections for Test of Prepolarized Microphone Sets ........................... 23
PCI: Connections for Test of Microphones (Ext. Pol. and Prepol.).................... 25
PCI: Connections for Test of Externally Polarized Microphone Sets ................. 27
PCI: Connections for Test of Prepolarized Microphone Sets ........................... 29
Assembling the 90CA .............................................................................. 30
Settings for the Instruments included in the System..................................... 32
Precautions for certain Types of Microphones .............................................. 32
Powering on the 90CA ............................................................................. 32
Mounting the Microphones........................................................................ 33
System Verification...................................................................... 37
Initial verification of the system setup......................................................... 37
Day-to-day Verification ............................................................................. 37
Introducing the Calibration Software SW0002.................................. 38

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Calibrating Microphones and Microphone Sets................................... 39
Starting the System ................................................................................. 39
Step 1: Registering the Microphone ........................................................... 41
Step 2: Performing a Level Calibration........................................................ 42
Step 3: Performing a Frequency Response Calibration.................................. 45
Step 4: Storing Calibration Results ............................................................ 46
Calibrating a Preamplifier ............................................................. 47
Step 1: Registering Test Conditions............................................................ 48
Step 2: Frequency Response ..................................................................... 51
Step 3: LEMO Reference Level Test........................................................... 52
Step 4: Storing Calibration Results............................................................. 53
High Frequency Calibration............................................................ 54
Working with the System Files and Documentation ............................ 56
Viewing and Editing the Microphone and Preamplifier Definition Files ............ 56
Setup.txt: The Microphone Calibration System Setup File ............................. 59
Editing the Calibration Charts in DIAdem .................................................... 62
The File Structure on the System Controller ................................................ 64
Warranty and Maintenance............................................................ 66
Warranty................................................................................................. 66
Calibration............................................................................................... 66
Cleaning ................................................................................................. 66
Service and Repairs.................................................................................. 67

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Introduction
The G.R.A.S. 90CA Microphone Calibration System provides you with a computer-
controlled level and frequency-response calibration of measurement microphones and
microphone sets. This highly automated process is convenient, yet reliable, with its
ready-to-use software and hardware, and auto-generated customizable documentation.
Additional software can be purchased for the calibration of preamplifiers.
Calibration with the 90CA is in compliance with
• ANSI S1.10 – 1966 (R1976)
• IEC Norm 61094-1
• IEC 61094-6
The 90CA provides accurate calibration hardware, easily changeable test conditions and
a highly reproducible calibration method
Using This Manual
Inside this manual, you will find
A description of the principles of the 90CA calibration system, with an overview of the
hardware and software for the different types of calibration in “” on page 8.
Diagrams and instructions for assembling and connecting the hardware for both the
PXI and the PCI version of 90CA “Assembling and Connecting the Hardware” on page
17.
Illustrations of which components of the 90CA to use for the different types of calibra-
tion for the different sizes of microphones: ”Mounting the Microphones” on page 33.
A description of the special calibration software included with 90CA and where and how
you can customize it: ”Introducing the Calibration Software SW0002” on page 38.
Detailed calibration procedures for “In general, the indicators are green when the
measurement is within the preset limits and red when the the measurement is outside
the limits.” on page 39, and for G.R.A.S. preamplifiers in ”Calibrating a Preamplifier”
on page 47 (if you purchased the preamplifier calibration option), and for ”High
Frequency Calibration” on page 54.
Descriptions of the data files included with 90CA and how to modify them in “Working
with the System Files and Documentation” on page 56.
Additional information about the warranty, cleaning instructions, and service and repairs
are also provided on page 66.

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Introduction to Calibration
For making documented, traceable, and approved measurements, your measurement
equipment must be calibrated.
Basically, calibration is a method that establishes the relationship between a measuring
device and its measurement units. This is done by comparing a device or the output of
an instrument to a standard with known characteristics. As an example, a ruler can be
calibrated by comparing it to a known standard. A meter is defined as the length of the
path travelled by light in vacuum in 1⁄299,792,458th of a second. When the relation-
ship between the ruler and this standard is established, the ruler is calibrated and can be
trusted to provide reliable measurements.
This section introduces the most commonly used terms in relation to acoustic calibration.
What is Acoustic Calibration?
Calibration means the comparison of the value indicated by a measuring instrument with
a nominal value that is provided by a given standard.
The equipment used for the calibration must itself be suitably calibrated to ensure an
unbroken trace back to the standard kept at a national or international institute.
The Calibration Pyramid
National / International Standard
Reference Standard
Working Standard
Measuring Instrument
Secondary
Calibration
Accredited Calibration
Traceable Calibration
Fig. 1. The calibration pyramid
The calibration pyramid is a hierarchy of comparisons of the standards used to calibrate
measurement instruments, ranging from the least accurate standard (many working
standards) at the bottom of the pyramid to the most accurate standard at the top.

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Traceable Calibration
An instrument has a traceable calibration if the measurement instrument used for
performing the calibration is traceable to national or international institutes / standards
through an unbroken chain of comparisons all having stated uncertainties.
Accredited Calibration
An instrument has an accredited calibration if the calibration is performed by an
approved accredited calibration laboratory. Approval for accredited calibration is granted
by a national metrology institute after careful evaluation and inspection of the laboratory
in question.
Secondary Calibration
The secondary calibration has the next highest metrological quality in an area. The cali-
bration value is assigned by comparison with a primary standard of the same quantity.
Primary Standard
The primary calibration (reference standard) has the highest metrological quality avail-
able at a given location or in a given organization. Its value is accepted without reference
to other standards of the same quantity.
In many countries, a body has been established to maintain primary standards of
measurements that will be used to provide traceability to customers’ instruments. The
national metrology institute supports the metrological infrastructure in that country by
establishing an unbroken chain from the top level of standards to an instrument used for
measurement.
National Metrology Institute Country
DANAK Denmark
PTB Germany
NPL UK
NIST USA
Fig. 2. Examples of national metrology institutes
Which type of Calibration do I need?
For performing a quality measurement, you must keep these factors in mind:
1. Resolution: Does the measurement meet the requirements of the application?
2. Accuracy: Does the measurement reflect the true characteristics of what is
measured?
3. Precision: Do you get the same result each time you measure the same quantity?
The accuracy of the calibration needed is directly dependent on these considerations.

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How often should I calibrate?
How often you should calibrate your equipment depends on the type of measurements
as well as on your quality assurance system. Use the following guidelines to determine
when calibration is needed:
• When you receive a new instrument
• When a specified time period has elapsed (often one year)
• When a specified period of usage (operating hours) has elapsed
• When an instrument has been subjected to shock or vibration that may have
affected its performance
• Whenever measurement results appear to be questionable
Fig. 3. Accuracy of Calibration

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The Concept of the 90CA Microphone Calibration System
The G.R.A.S. Microphone Calibration System 90CA provides you with a convenient,
yet reliable and automated level and frequency response calibration of measurement
microphones and microphone sets. The 90CA calibrates in compliance with
• ANSI S1.10 – 1966 (R1976)
• IEC Norm 61094-1
• IEC 61094-6
An optional module for calibration of preamplifiers is available.
The 90CA is delivered with ready-to-use software and hardware, as well as preinstalled
test parameters for G.R.A.S. microphones and preamplifiers. The 90CA is available in
two versions: a PXI-based version with boards for mounting in a NI PXI Express chassis
and a PCI-based version with boards for use in a PC. Functionality and specifications are
identical for the two. See Packing List and Electrical Connections for further information.
Main Features
• A highly automated and computer-controlled calibration process
• Accurate calibration hardware
• Ready-to-use software and hardware
• Preinstalled test parameters for G.R.A.S. microphones and microphone sets
• Optional software for test of preamplifiers
• Test conditions easily changeable
• Highly reproducible calibration method
• Auto-generated, customizable documentation for printing
Fig. 4. Example of a calibration certificate

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Applications
You can use the 90CA Microphone Calibration System for all types of IEC-standardized
1⁄8“, ¼” , ½”, and 1” measurement microphones to perform.
• Level calibration of microphones using a pistonphone and the insert voltage
technique
• Frequency calibration of microphones using an electrostatic actuator
Note: For microphones with a non-removable protection grid, frequency calibra-
tion is not feasible. For G.R.A.S.’ free-field microphones, the system automati-
cally compensates for the free-field correction. For non-G.R.A.S free-field micro-
phones, the free-field correction values can be entered manually.
• Calibration of externally polarized microphone sets
• Calibration of prepolarized microphone sets
• Calibration of preamplifiers
• Calibration up to 92 kHz
Note: The high-frequency calibration above 92 kHz requires a 33210A or
33220A Signal Generator and a 34410A Digital Multimeter.
These applications are described in this manual.
Traceability and Accrediting
The system has been tested by G.R.A.S. Sound & Vibration and is delivered with a test
certificate for the complete system, specific instruments and their serial numbers listed.
The microphones and the pistonphone 42AP that is included are reference traceable to
national standards established at National Physical Laboratory (NPL) in Great Britain.
For daily reference instruments check, use the Reference Microphone 40AG that is
included.
Accreditation for the 90CA Reference Units
An accredited calibration of the 42AP Pistonphone, the 40AG reference microphone,
and the 26AG Insert-Voltage Preamplifier is available from G.R.A.S. Order OP0022
Accredited Calibration of Reference Units.