Manual for 42AG - G.R.A.S. Sound &

Instruction Manual
G.R.A.S. 42AG Multifunction Sound Calibrator
LI0196 – 19 July 2017
www.gras.dk
Revision History
Any feedback or questions about this document are welcome at gras@gras.dk.
Revision
Date
Description
1
23 May 2017
First edition
2
19 July 2017
Caution about the speaker diaphragm added
Copyright Notice
© 2017 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.
LI0196 – 19 July 2017
2
Contents
Introduction................................................................................... 4
Main Features............................................................................................ 4
About this Manual....................................................................................... 4
Principles of Operation................................................................................. 5
Description................................................................................................. 6
Using the 42AG............................................................................. 7
Microphone Calibration................................................................................ 7
Calibration Considerations................................................................ 9
Accuracy.................................................................................................... 9
Background Noise....................................................................................... 9
Corrections................................................................................................. 9
Maintenance and Calibration.......................................................... 11
Replacing Batteries................................................................................... 11
Calibrating the 42AG Sound Calibrator........................................................ 11
Adjusting the 42AG's Levels....................................................................... 11
Delivered Items............................................................................ 13
Technical Specifications................................................................. 13
Warranty, Service and Repair.......................................................... 14
LI0196 – 19 July 2017
3
Introduction
Main Features
The 42AG Multifunction Calibrator is a portable, battery operated precision instrument for calibrating microphones and sound level meters. It produces an output level of either 94 dB or 114
dB at 250 Hz or 1 kHz.
It is designed for use in the field and comes with a protective casing. It provides display of
ambient calibration conditions, with pressure, temperature and humidity. It is delivered with an
individual calibration certificate. Its main features are:
• Specifications according to IEC Standard 60942 (2003), Class 1 sound calibrator.
• Calibration at two different frequencies:
• 250 Hz or 1 kHz.
• Calibration at two different levels:
• 94 dB or 114 dB.
• 1” microphones fit directly in the calibrator coupler. Adapters for ½", ¼" microphones and
⅛” microphones are included in the delivery.
• Green LED lights when calibration level is OK.
• Automatic compensation for ambient air pressure, temperature and humidity.
• Measurement of ambient air pressure, temperature (in ˚C or ˚F) and humidity.
• Automatic power off after 10 to 20 seconds.
• Automatic shut off when batteries are too low.
About this Manual
This manual contains the main sections listed below. When referring to 42AG's buttons, these
symbols are used:
. See "Description" on page 6.
dB
Hz
Met
Introduction
This section outlines how the 42AG works. Also, the user interface, the battery compartment and
the location of the calibration button are described.
Using the 42AG
This section describes how 42AG is operated and discusses the factors that must be considered
to achieve a correct calibration result.
Maintenance and Calibration
This section explains how to change batteries and how to calibrate the 42AG.
Delivered Items and Specifications
These two sections list the items delivered with 42AG and its technical specifications.
LI0196 – 19 July 2017
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Principles of Operation
The 42AG consists of a sound source – a small loudspeaker – a reference microphone, and an
electronic control circuit. The special reference microphone is extremely stable and independent of
variations in barometric pressure, temperature, and humidity. Together, the reference microphone
and control circuit maintain a constant sound pressure level inside the calibration coupler, making
it possible to calibrate without
separately accounting
for the ambient calibration conditions and
Venting
Coupler
changes to the load volume.
42AG can produce a sinusoidal signal of 250 Hz or 1 kHz at 94 dB or 114 dB. The environmenAmpli fier
Sinus gen erator
tal conditions: air pressure, temperature and humidity
can be displayed.
Microphon e
to b e
calibratorcalibrated
complies
The
with IEC 60942 (2003), Class 1 and has been designed to serve 1" and
smaller microphones and sound level meters equipped with such microphones.
42AG is designed for use in the field. The accuracy that can be achieved is within ±0.2 dB which
is more than adequate for field checks.
Feed-back an d
For proper laboratory calibration, we recommend the use of a pistonphone,
Control Circuitor sending the microphone to a calibration laboratory.
Venting
Microphon e
to b e
calibrated
Coupler
Ampli fier
Sinus Gen erator
Feed-back an d
Control Circuit
Fig. 1. Simplified block diagram illustrating 42AG's principles of operation.
Caution. The loudspeaker inside the calibration coupler is not protected. Therefore, do not touch
the diaphragm and also take care to prevent any hard objects from entering the coupler as this
may deform or damage the diaphragm.
LI0196 – 19 July 2017
5
Description
Display
G.R.A.S. 42AG
On/Off
Status indicator - red/green
Level selection
94 dB or 114 dB
dB
Hz
Met
Frequency selection
1 kHz or 250 Hz
Display of ambient conditions
– temperature
– ambient pressure
– relative humidity
Calibration coupler
Fig. 2. 42AG’s user interface.
Switches 42AG on and off. Automatic power off after 20 seconds.
Displays temperature (˚C or ˚F), ambient pressure and relative humidity. The temperature may
be displayed in either degree Celsius (˚C) or degree Fahrenheit (˚F). To change the unit, keep the
button
depressed during power on.
Met
Met
Hz
Toggles between 1 kHz and 250 Hz. Default is 1 kHz.
dB
Toggles between 94 dB and 114 dB. Default is 94 dB.
Status Indicator. Lights green when the pressure in the coupler has stabilized.
Multifunction Sound Calibrator
G.R.A.S. 42AG
IEC 60942
Multifunction
Sound Calibrator
Batteries: S1.40
2xAAA
(LR03)
1.5V Class1
ANSI/ASA
- 2006
(R2011)
IEC 60942
Batteries: 2xAAA (LR03) 1.5V
Battery
compartment
ANSI/ASA S1.40
- 2006 (R2011) Class1
G.R.A.S.
42AG
Display. Displays the actual values for calibration and ambient conditions.
S e r iSael rNi aol. N o .
888888888
888888888
Calibration button
(sealed)
Fig. 3. Battery compartment and Calibration button.
Battery Compartment. Holds two LR03/AAA batteries.
The Calibration Button. This button is used when 42AG must be calibrated.
LI0196 – 19 July 2017
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Using the 42AG
Microphone Calibration
Adapter Selection
From the factory, the 42AG has a 1” calibration coupler mounted with an adapter for the 1/2”
microphone typically used with Sound Level Meters.
If you are going to calibrate a 1” microphone, remove the 1/2” adapter. For 1/4” and 1/8”
microphones, mount the appropriate adapter, RA0049 or RA0069. These are included in the
delivery.
Calibration
The microphone must be placed in the acoustic coupler of the calibrator before the calibrator is
switched ON. If you are going to calibrate a sound level meter, turn it on and let it stabilize before
calibration. At least three minutes are required.
1. Place the microphone to be calibrated in the coupler of the calibrator. Use a suitable adapter, if
necessary.
Use a slow movement, since a fast movement may create an excessive pressure which
temporarily may change the sensitivity of the microphone. Wait a few seconds for the pressure
equalization to stabilize.
. Wait for the LED above the
2. Press
dB
button to change from red to green.
The level and frequency is indicated in the display of the calibrator. After powering the sound
calibrator, the first combination of frequency level is 94 dB re. 20µPa and 1 kHz. Select the
or
, respectively.
appropriate level and frequency for your calibration by pressing
dB
Hz
Normally the background level should be more than 20 dB lower than the level from the
calibrator in order to not influence the reading. Therefore it is considered good practice to
notice the indicated sound level before the sound calibrator is switched on.
3. Press
Hz
4. Press the
to switch frequency. Press
Met
dB
to switch level.
to display the environmental conditions: air pressure, temperature and humidity.
The temperature and humidity is measured in the acoustic coupler. Note that due to heat
generated by the calibrator’s electronics the temperature may rise above the ambient condition
after longer time of operation (typically one degree ˚C) affecting both the temperature and the
humidity indication.
It is therefore considered good practice to measure the conditions shortly after powering the
unit. The temperature may be displayed in centigrade (Celsius) or in degree Fahrenheit. The
rising temperature does not affect 42AG's output.
5. Press
Met
again for returning to the sound calibrator mode.
LI0196 – 19 July 2017
7
6. Press
for switching the calibrator off.
The calibrator shuts off by itself after ten minutes. However, if a microphone has not been
inserted or the level has not otherwise reached stability, the unit will switch off after approximately 10 to 20 seconds. The automatic power off may be deactivated by keeping the
button depressed for more than 3 seconds when the unit is switched on.
Calculation of Microphone Sensitivity
The sensitivity of a microphone under test can be calculated via a measurement of the microphone’s output voltage. If the measured output voltage is Vo, and the sound pressure level in the
Pistonphone’s coupler is L C (decibels), the microphone sensitivity S is given by:
S=
V o
20 μPa ⋅ 10
L c
20
The value 20 μPa is the standard reference sound pressure. The result here includes the loading effect of the preamplifier’s input impedance as well as the gain or attenuation within the
preamplifier.
To measure the “Open Circuit Sensitivity” of the microphone (i.e. when the microphone’s output is
not affected by the load of a preamplifier), a special preamplifier, for example the G.R.A.S. 26AG
1/2'' Insert Voltage Preamplifier with Integrated Connector, should be used.
Such measurements can be performed using 42AG, but would normally be considered outside
the intended use for an instrument primarily designed for field use.
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Calibration Considerations
Accuracy
42AG is a precision instrument with an accuracy better than ±0.2 dB. Calibration of the sound
calibrator at regular intervals will ensure that drift does not occur and that it performs within
specifications.
If an accuracy better than ±0.2 dB is desired, we recommend using a G.R.A.S. 42AP Intelligent
Pistonphone, Class 0 where an accuracy better than 0.09 dB can be obtained.
Background Noise
It is always good practice to verify that the background noise level is sufficiently below the calibration level to not influence the calibration. It must be at least 20 dB below the calibration level.
Corrections
Ambient Calibration Conditions
The 42AG Sound Calibrator automatically adjusts the calibration level to the nominal level.
Nevertheless, it is considered good practice to note the ambient calibration conditions as part of
each calibration routine.
Temperature
The temperature may be displayed in either degree Celsius (˚C) or degree Fahrenheit (˚F). ln order
depressed during power on.
to change the unit, keep the button
Met
Load Volume
Various types and makes of microphones may have different effective load volume. Due to its
working principle, the 42AG has a large effective coupler volume. The variations in sound pressure level due to variations in effective load volume among microphones are therefore for most
applications insignificant.
An equivalent microphone volume of 250 mm3 is used as reference load volume for 1/2” microphones. This corresponds to most 1/2” measuring microphones with protecting grid mounted. lf
no adaptor is used, the nominal reference front volume is 1333 mm3. This corresponds to most
1” measuring microphones with the protection grid mounted.
LI0196 – 19 July 2017
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Weighting Network
The stated levels for calibrator 42AG are all unweighted levels for the two calibration frequencies,
250 Hz and 1 kHz.
All sound level meters shall display at least the A-weighted sound pressure level. The
A-weighting is a standardized, frequency dependent weighting which originally was developed to
mimic our auditory organ and is specified in the standards for a sound level meter. The nominal
attenuation of an A-weighting network is 0 dB at 1 kHz (reference frequency) and 8,6 dB at 250
Hz.
The nominal A-weighted sound pressure levels for the calibrator will therefore at 250 Hz be 85,4
dB and 105,4 dB, respectively. The attenuations of the C-weighting are for both frequencies 0,0
dB and C-weighted levels therefore correspond to the unweighted levels.
Free-field Microphones
A free-field microphone is designed to compensate for the pressure build-up that takes place
when it is used in a free-field. When calibrated with a sound calibrator at 1 kHz, the output
from the microphone will be lower compared to the output at free field conditions because the
calibrator performs a pressure calibration of the microphone. Therefore, the result of a calibration
of a free-field microphone must be corrected, for a 1/2" microphone (type WS2F) the pressure
increase at free field conditions will typically be 0.15 dB at 1 kHz.
Most sound level meters are designed to show the correct free field sound pressure level. This
means that the indication shall correspond to the level in a free, progressive field before the
sound level meter was placed in the field. The size of the microphone but also the sound level
meter body will modify the pressure just in front of the microphone - especially at higher frequencies. Your sound level meter should therefore be adjusted to 93,8 dB for a 94 dB, 1 kHz calibration signal.
LI0196 – 19 July 2017
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Maintenance and Calibration
Replacing Batteries
42AG uses two 1.5 V AAA size cells. Remove the batteries as soon as they are discharged or
if the 42AG is stored for a prolonged period of time. Leakage from the batteries may otherwise
destroy the electronic components.
To change the batteries:
1. Remove the rear cover by pulling it straight backwards.
2. Pull out the batteries from the battery compartment.
3. Replace them with two fully charged batteries.
4. Install the rear cover.
Calibrating the 42AG Sound Calibrator
At regular intervals proper operation of the sound calibrator should be verified, preferably by an
accredited acoustic calibration laboratory or the G.R.A.S. factory. A calibration at least once a year
is recommended. The calibration should be performed as specified for periodic verification in the
international standard for sound calibrators 60942 (2003), Class 1.
The level should preferably be measured with a laboratory standard microphone type LS2, according to IEC 61094-1, where the pressure sensitivities at the different frequencies are known with
a sufficiently high accuracy
Adjusting the 42AG's Levels
Caution. It is possible to adjust 42AG's levels: the SPL, the displayed temperature, air pressure
and relative humidity. However, if you do this, you will invalidate the factory calibration and break
the link to previous measurements. For this reason, we have sealed the access to this function
which should only be used by authorized personnel at a calibration facility. If you nevertheless
choose to enter the calibration mode, do not do this without access to reliable references.
Adjusting the Sound Pressure Level
1. Place the reference microphone in the coupler and select the appropriate frequency.
2. Press the Calibrator button (See Fig. 3 on page 6 – use an end of a paper clip or similar)
for about one second until the display starts flashing and the Status indicator turns red. The
Calibration button is accessible through a small hole in the left-hand side. Use a paper clip or
similar
to increase the level or
3. While the display is flashing, press
level. The level is changed in steps of about 0.02 dB.
Hz
dB
to reduce the sound pressure
When the correct level is obtained, press the calibrator button for returning to normal
operation.
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Adjusting the Displayed Temperature, Air Pressure and Relative Humidity
1. Press the calibration button for about one second until the display starts flashing and the LED
turns red.
The calibration button is accessible through a small hole in the left-hand side of the cabinet.
See Fig. 3 on page 6. Use an end of a paper clip or similar.
2. Press
Met
and the temperature value starts flashing.
3. While the display is flashing, press
Hz
to increase the reading or
4. When the correct value is displayed, press
Met
7. While the display is flashing, press
Hz
Met
dB
to reduce the reading.
and the value for humidity start to flash.
to increase the reading or
8. When the correct value for humidity is displayed, press
42AG and return to normal operation mode.
LI0196 – 19 July 2017
to reduce the reading.
and the value for air pressure starts to flash.
5. While the display is flashing, press Hz to increase the reading or
6. When the correct value is displayed, press
dB
Hz
dB
to reduce the reading.
to terminate calibration of the
12
Delivered Items
42AG
Sound Calibrator
42AG
1/2” Adapter
RA0297
1/4” Adapter
RA0049
1/8” Adapter
RA0069
LR6-AA alkaline cells
EL0001
Technical Specifications
42AG
Standards
IEC 60942 (2003)
ANSI/ASA S1.40 (2006)
Frequency
250 Hz (251.19 ±0.30)
1000 Hz (± 1 Hz)
Level
94 dB ±0.2 dB
114 dB ±0.2 dB
Distortion
< 2.0%
Sensitivity to environmental conditions
IEC 60942 Class 1
Display of temperature, air pressure and humidity
Temperature
-10˚C to +50˚C; accuracy ±2˚C, resolution 0.1˚C
Atmospheric pressure
65 kPa to 108 kPa; accuracy ±0.4 kPa,
resolution 0.1 kPa
Relative humidity
25% to 90%, accuracy ±6%, resolution 1%
Microphone sizes
1", 1/2", 1/4", 1/8"
Power
Two 1.5V LR03/AAA Size alkaline cells
Display
Monochrome OLED with 128 x 64 resolution
Weight
124 g
LI0196 – 19 July 2017
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Warranty, Service and Repair
Calibration
Before leaving the factory, all G.R.A.S. products are calibrated in a controlled laboratory environment using traceable calibration equipment.
We recommend a yearly recalibration, depending on the use, measurement environment, and
internal quality control programs.
We recommend calibration prior to each use to ensure the accuracy of your measurements.
Warranty
G.R.A.S. products are made of components from our proven standard portfolio and are all manufactured of high-quality material and branded parts that were chosen and processed to ensure
life-long stability and robustness. The warranty for the calibrator is 2 years.
The warranty does not cover products that are damaged due to negligent use, an incorrect power
supply, or an incorrect connection to the equipment.
Service and Repairs
All repairs are made at G.R.A.S. International Service Center located in Denmark. Our Service
Center is equipped with the newest test equipment and staffed with dedicated and highly skilled
engineers. Upon request, we make cost estimates based on fixed repair categories. If a product
covered by warranty is sent for service, it is repaired free of charge, unless the damage is the
result of negligent use or other violations of the warranty. All repairs are delivered with a service
report, as well as an updated calibration chart.
Manufactured to conform with:
CE marking directive:
93/68/EEC
WEEE directive:
2002/96/EC
RoHS directive:
2002/95/EC
G.R.A.S. Sound & Vibration continually strives to improve the quality of our products for our
customers; therefore, the specifications and accessories are subject to change.
LI0196 – 19 July 2017
14
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