A battery operated sound calibrator complying with the
IEC 60942 (2003-01) Class 1
producing 114 dB @ 1000 Hz.
The sound calibrator shuts off
automatically when the microphone is removed from the coupler.
user documentation
nor1251
class 1 sound calibrator
im1251_1Ed1R3En
user documentation
nor1251
class 1 sound calibrator
Nor-1251 – September 2004 Edition
im1251_1Ed1R3En
Norsonic is a registered trademark of Norsonic AS.
All other brand or product names are trademarks or
registered trademarks of their respective companies.
Every effort has been made to supply complete and
accurate information. However, Norsonic AS
assumes no responsibility for the use of – nor for the
consequential damages of the use of – this information and/or the instrumentation described herein.
Furthermore Norsonic AS assumes no responsibility
for any infringement of the intellectual property rights
of third parties, wherever applicable, which would
result from such use.
Norsonic AS reserves the right to amend any of the
information given in this manual in order to take
account of new developments.
If you wish to communicate with us, please feel
welcome. Our address is:
Norsonic AS, P.O. Box 24, N-3421 Lierskogen,
Norway
www.norsonic.com
Tel: +47 3285 8900,
Fax: +47 3285 2208
e-mail: info@norsonic.com
Copyright © Norsonic AS 1994–2004
All rights reserved
Finding the Information You Need
hank you for choosing Norsonic! The Sound Calibrator Nor1251 has been designed to provide you with many years of
T
safe, reliable operation.
Your approach to the Nor-1251 documentation depends on
what you want to do and how much you already know.
The manual has been divided into two parts – one dealing
with the use of the calibrator while the other discusses the
operating principles and presents the technical specifications.
Depending on your requirements and your familiarisation
with technical acoustics as such, you may find that you use
some parts of this manual often and others not at all.
Our main objective with this manual was to address your
goals and needs. Please let us know how well we succeeded!
5
What Is a Calibrator for?
hen you are going to make a measurement of any
kind, you need to ascertain that what you measure is
W
cedure of making a measuring device measure correctly, is called
calibration.
ferred to as sound measuring instruments, calibration is no less
than paramount – since sometimes legal action will be taken
based on the sound and noise levels measured!
easier to design a stable calibrator than a stable sound level
meter. Today, sound measuring instruments generally are as
stable as calibrators.
signed to fulfil all specifications required. Hence they are vulnerable and easily subject to damage unless great care is taken.
indicated correctly by your measuring device. The pro-
For sound level meters and -analysers, hereinafter all re-
The use of calibrators date back to those days when it was
However, measuring microphones are delicate devices de-
Internal Reference Calibration
Some sound measuring instruments offer the ability to
calibrate by means of an internal reference oscillator. Although it may sound convenient, this method is not at all
recommended, neither is it regarded as sufficient when
measuring according to applicable standards.
The most vulnerable part of a sound measuring instrument is always its microphone. If dropped on the floor, for
instance, damage is likely. This will, however, not be detected when calibrating by means of an internal oscillator.
The oscillator signal will pass through virtually every part
of the measuring chain, except the microphone cartridge.
Hence, the most vulnerable part will not be exposed to
the calibration, which in turn is why the method should be
avoided.
One may therefore say that a calibrator is just as much a
device for verification of proper operation as it is a device for
readjustment of the sensitivity of sound measuring instruments.
Whenever you are going to make measurements in accordance with applicable standards, calibration is always required
before you start measuring. Some standards require calibration
to take place after the measurement as well! The former is to
ensure that correct values are acquired, while the latter serves
to confirm that nothing has changed during the measurement
session.
The Calibration Process
To calibrate a sound measuring instrument we use a sound calibrator. A sound calibrator is designed to produce a known sound
pressure level when used correctly together with the measuring
microphone of the sound measuring instrument.
All sound measuring instruments of the quality levels considered here, feature some way of sensitivity adjustment. Some
have a small potentiometer that can be adjusted with a screwdriver, while others do it in software.
If the level indicated by your sound measuring instrument
deviates from the output level of the calibrator, the sound measuring instrument sensitivity is adjusted until the reading coincides with the calibrator’s output level. The sound measuring
instrument is then said to be calibrated.
In case the level deviates significantly from earlier or nominal values, a thorough check of the instrumentation will be
needed.
Tip: As a general rule, use a calibrator of the same class
or higher (i.e. same, or lower class/type number) than
your sound measuring instrument. Otherwise, the accuracy of your acquired measurement data will be affected.
6
Accuracy and Tolerances
Above, we talked about the need to measure correctly. As we
shall see, this is strictly speaking not absolutely true, since a
measuring device can only estimate the real value.
An uncertainty will always be present. The aim of the calibration is then to bring this uncertainty to within given limits or
tolerances. For a sound measuring instrument, the width of the
interval of acceptable estimates will depend on which class it
belongs to.
Instrument Classes
According to the international standard called IEC61672-1,
sound level meters should be divided into two classes, depending on their accuracy: Types 1 and 2 with type 1 as the best, i.e.
with the most narrow tolerances, and type 2 as the least good.
Table showing the permissive tolerances as defined by IEC61672
(Sound Level Meters) and IEC 60942 (Sound Calibrators) for
frequencies around 1000 Hz and close to reference conditions.
Note that the numbers includes the uncertainty of measurement.
You may experience that national and/or international standards may impose restrictions on which instrument types are
considered usable for a given measurement task.
Our “problems” do not end with the uncertainties of the
sound measuring instruments. Even sound calibrators are “infected” with level uncertainties! Hence, these have also been
divided into types (called classes) depending on their level accuracy and level stability. This is to ensure that measurements
made with your high-quality sound measuring instrument are
not ruined by inaccurate calibration.
The Sound Calibrator Nor-1251
Sound Measuring Instruments
Type 1 2
Tolerances ±1,1 dB ±1,4 dB
Sound Calibrators
Class 1 2
Tolerances ±0,4 dB ±0,75 dB
7
Using the Sound Calibrator Nor-1251
he Sound Calibrator Nor-1251 is a small, battery operated
sound source for the calibration of microphones and sound
measuring instruments.
T
The microphone is placed in an acoustic coupler where the
calibrator produces a regulated sinusoidal sound pressure signal. The calibrator is supplied with an output of 114dB SPL @
1000Hz.
Due to the principle of operation
virtually independent of ambient conditions like temperature,
atmospheric pressure and humidity within the specified range
of operation. The calibrator complies with IEC60942 Class1 and
has been designed to serve one-inch and smaller microphones
and sound level meters equipped with such microphones.
One-inch microphones fit directly in the calibrator coupler,
while half-inch and smaller microphones are served by application of suitable adaptor rings. Adaptor for half-inch microphones
are included, while adaptors for other sizes must be ordered
separately.
If you Have a Free-field Microphone
Most ½” free-field microphones (as defined by
IEC 61672-1) have a pressure response which gets lower
as the frequency of the input sound signal is increased.
The reason for this is that free-field microphones have
been designed to compensate for their own presence in
the sound field.
For example; when using the Norsonic microphone cartridge 1225, at 1000 Hz the sensitivity of your sound measuring instrument should be adjusted to a value 0.15dB
lower than the nominal calibrator output level. This difference corresponds to the difference between the pressure and the free-field response of the microphone – cf.
the table starting at page 15.
1
, the calibration level is
To activate the sound calibrator:
• Press the ON button to turn on the calibrator.
The calibrator shuts off by itself after about five minutes. How-
ever, if a microphone has not been inserted or if it is removed,
the unit will switch off after a few seconds.
Your sound measuring device may or may not use a spectral
weighting network (such as the A-weighting network). For sound
calibrators with an output frequency of 1000Hz, no compensation needs to be made for this.
Operation
Using the Sound Calibrator Nor-1251 is easy and uncomplicated.
To calibrate with the Nor-1251, do as follows:
1. Insert the microphone to be calibrated in the coupler of the
calibrator. Use a suitable adaptor, if necessary.
2. If the background noise is high, watch the noise level and
press the ON button to switch on the calibration signal.
Verify that the calibration signal is at least 20dB above the
background noise floor.
3. Adjust the sensitivity of the microphone or measuring instru-
ment to display an excitation sound pressure level correspond-
ing to the level indicated on the calibrator itself.
When the ON button has been operated, and a microphone
inserted, the sound calibrator will stay on for approximately five
minutes. The on-time will be reduced as the battery-voltage de-
creases.
Proper operation of the calibrator is indicated by the green
LED indicator – see Fig. to the right for details.
If the battery-voltage is too low to guarantee operation within
the specifications, the calibrator will switch off as soon as the
1)
Patent DE 4294875 CE
8
Vital parts of the Sound Calibrator Nor-1251…
Adaptor to facilitate the calibration of half-inch cartridges in addition to full-
inch is included, while adaptors for other sizes must be ordered separately
A green LED illuminates when the sound
pressure level inside the coupler cavity
is correct.
Miniature loudspeaker generating the sound pressure level
inside the coupler.
The power button. Depress to start generating the sound pressure level. The
sound calibrator switches itself off automatically if the required sound pressure
level cannot be established, for example
if the microphone to be checked has not
been inserted.
The unit also switches itself off after approximately five minutes.
Frequency adjustment potentiometer
(do not adjust unless
you know what you
are doing!)
Level adjustment
potentiometer (do
not adjust unless
you know what you
are doing!)
Silicone microphone monitoring
the sound pressure level inside
the coupler. One side of the microphone diaphragm is exposed to the sound pressure
inside the coupler while the rear
side of the diaphragm is located
in a separate vented cavity to
prevent pick-up of extaneous
noise signals.
The cord is attached to the battery connector and the bottom
lid. Remove the lid and pull the
cord to get the battery out.
Potentiometer for frequency adjustment
Potentiometer for level adjustment
ON button is released. However, you may still use the type
1251 for calibration by keeping the ON button depressed while
you calibrate, provided that the LED is illuminated.
Background Noise Considerations
The high sound pressure level generated in the coupler makes
the calibrator virtually insensitive to external noise. Nevertheless, care should always be taken when calibration is made in
places with high extraneous noise levels.
It is always good practice to verify that the background noise
level measured with the microphone mounted in the calibrator—but prior to switching it on—is sufficiently below the calibration level (20dB or more below).
Due to the power-cords, you will not be able to mount the
rear cover while at the same time having the calibrator connected to an external power source.
Without the rear cover, the stated calibrator output level and
the frequency will be altered slightly. The sound from the rear
side of the loudspeaker will be less attenuated and the calibration process will also be more sensitive to extraneous noise. We
therefore recommend that you always cover the rear side or
compensate for the variation in level and frequency whenever
the rear cover is removed
Battery Replacement
Remove the battery as soon as it is discharged or if the Sound
Calibrator is stored for a prolonged period of time. Leakage from
the battery may otherwise destroy the electronic components.
To change the battery:
1. Remove the rear cover by pulling it straight backwards
2. Pull out the battery from the battery compartment and unclip
the battery connector
3. Replace the battery with a fully charged battery of the type
IEC type 6LR61 (nine-volt alkaline)
4. Alternatively, a nine-volt lithium battery of identical physical
dimensions may be used
5. Install the battery and the rear cover
See also the Fig. on the previous page for details.
Consider Locking the On Button
If the application requires excitation signal for longer periods of
time, you may apply adhesive tape to lock the ON button in its
on position. In this case no indication of proper power supply
voltage will be given as the battery comparator will not be able to
switch the calibrator off.
External Supply of Power
For some applications it may be required to power the calibrator
from an external voltage in the range 8–15V to the battery connector.
Due to the power-cords, you will not be able to mount the
rear cover while at the same time having the calibrator connected to an external power source. See also Noise Considerations
on the previous page spread for more on this.
The calibrator’s aluminium casing is connected to a voltage
between the positive and negative battery terminals. An external supply must therefore be electrically floating relative to the
casing of calibrator.
Recalibration
At regular intervals – at least once per year – proper operation
of the sound calibrator should be verified, preferably by an accredited acoustic calibration laboratory or the Norsonic factory.
The calibration should be carried out at room temperature and
should cover:
• Frequency accuracy
• Level accuracy
• Harmonic distortion
• Level stability
10
If the level or frequency needs to be re-adjusted, this can be
done by adjusting the corresponding potentiometer. The location of the adjustment potentiometers are shown in the Fig.
on page 9. Note that the access to the potentiometers may
have been blocked by a sealing mark.
The level should be measured with a working standard
microphone type WS2 (according to IEC61094–4) where the
pressure sensitivity at the working frequency of your calibrator is known with a sufficiently high accuracy.
Calibrator sound pressure level change [dB] as a function of
the microphone load volume for ½” microphones
0,1
[dB]
0,0
Microphone Effective Front Volume Corrections
Various types and makes of microphones may have different
effective front volume. Due to its working principle, the Nor1251 has a large effective coupler volume. The variations in
sound pressure level due to variations in effective front volume among microphones is therefore for most applications
insignificant.
The calibrator is adjusted with the ½” adaptor for an equivalent microphone volume of 250 mm³, which corresponds to
most ½” measuring microphones with protecting grid mounted
like the Norsonic types 1220, 1225 and 1230. If the actual
microphone has a volume deviating from this reference volume, the accuracy may be improved by applying a volume
correction. The typical sensitivity for the level change is
+0.0003 dB/mm
an effective front volume of 150 mm³ is used, the level in the
coupler will typically decrease by 0.03dB because of the
100mm³ volume reduction.
If no adaptor rings are used, the nominal effective front
volume for no correction is 1333 mm³. This corresponds to most
1” measuring microphones with the protection grid mounted.
3
at 1000 Hz. Hence, if a ½” microphone with
-0,1
0
100
200 300 400 500 mm
3
Removing the rear cover will alter both the level and the frequency!
Note that the level and frequency will be altered slightly when the sound calibrator is operated without its rear cover.
Whenever adjusting the level and/or the frequency, this must be taken into account.
11
Operating principles and Characteristics
The Sound Calibrator Nor-1251 principles of operation is shown in
the below block diagram and is as follows:
• The microphone to be calibrated is placed in the coupler of the
calibrator where the sound pressure signal is generated by a
miniature loudspeaker.
• The electric signal—driving the loudspeaker—is generated by
an electronic oscillator.
• The sound pressure generated is measured with a pressure
sensitive silicon sensor
1
. This signal is used to adjust the level of
the oscillator signal. The rear side of the silicon sensor is located
in a separate vented cavity to prevent pick-up of extraneous
noise signals. Noise pick-up is also limited by a bandpass filter
in the feedback path.
• Because of the high stability of the silicon sensor and the electronic controller, the acoustic signal generated, is virtually independent of battery voltage and ambient conditions such as temperature, humidity and atmospheric pressure. The feedback
principle automatically compensates for variation in the equivalent volume of the microphones. Hence, it creates an effective
coupler volume many times the volume given by the mechanical dimensions of the coupler. The system also compensates for
drift in the loudspeaker.
A simplified block diagram of the Nor-1251
• The acoustic coupler is vented to the inside of the calibrator,
which in turn is vented to the outside for equalization to the
atmospheric pressure. A separate channel vents the rear side of
the reference transducer to the outside of the calibrator.
• A Light Emitting Diode (LED) is illuminated whenever the level
control is in balance. When no microphone is placed in the
coupler, the loudspeaker will generally not be able to generate
and maintain the correct pressure in the coupler. This situation
will be indicated by extinguished LED.
• An electronic circuit inside the calibrator will switch off the power
at approximately five minutes after the ON button was operated. If you need that the calibrator stays on for a longer time,
keep the ON button depressed—either by depressing it manually, or by use of adhesive tape.
• If the calibrator fails to establish the correct sound pressure level
(an extinguished LED), or if the battery voltage is too low, the
automatic switch-off circuitry will be activated. Hence the calibrator will switch off a few seconds after the microphone has
been removed.
Oscillator
Level
controller
LED
Microphone
Loudspeaker
Coupler
Vented
cavity
Pressure
sensor
Amplifier
1)
Patent DE 4294875 CE
DetectorBP filter
Influence of Ambient Conditions
The applied operating principles and the
careful selection of critical components
ensures that the Sound Calibrator Nor1251 exhibits a low sensitivity to variations in the ambient conditions.
The diagrams to the right show typical
sensitivity to variations in the ambient (atmospheric) pressure, the ambient temperature and the relative humidity. The
requirements as given by the international
standard IEC60942 (2003-1) are also
shown together with the headroom for
uncertainty of measurement.
Nor-1251 is checked at three temperatures (–10°C, +23°C and +45°C) to ensure that the output level is well within
the requirements set by IEC 60942 and
ANSI S 1.40 over the specified temperature range.
Calibrator sound pressure level change [dB] as a function of temperature
23°C
0.4dB
0dB
–0.4dB
0 10 20 30 40 50°C–10
Calibrator sound pressure level change [dB] as a function of the ambient pres-
101.325 kPa
0.4dB
0dB
[°C]
sure [kPa]
–0.4dB
Calibrator sound pressure level change [dB] as a function of the relative humidity
0.4dB
0dB
–0.4dB
70 75 80 85 90 100 105 110kPa95
65
50%RH
10 20 30 40 50 60 70 80 90 %RH
[%RH]
Specifications
OUTPUT SIGNAL
Sound pressure level: 114dB (re. 20µPa) @ 1000Hz
SPL accuracy at reference conditions: ±0.2dB
Frequency accuracy: ±0.2%.
Accuracy: Complies with IEC60942 (2003-01) Class 1 and ANSI S 1.40. Typical change in SPL per year: <0.01dB/year
REFERENCE CONDITIONS
Temperature: 23°C
Ambient pressure: 101.325kPa
Humidity: 50% RH
Effective load volume: 250mm
Reference direction for EMC test: Microphone coupler facing the source.
GENERAL
Sensitivity to change in load volume: 0.0003dB/mm
Time for level to stabilise: Max 2 sec.
Microphone size: 1”, ½” (Adaptor Nor-1443 – included) and ¼” (Adaptor Nor-1444 – available separately) according to
IEC61094-4
Harmonic distortion: Max. 1%
Controls: Power-on push button. Automatic shut-off
CE classification: EMC: EN 61000-6-1, EN 61000-6-3. Safety: EN 61010-1, 1993 for portable equipment, pollution category
2
AMBIENT REQUIREMENTS FOR SPECIFIED
OPERATION
Temperature range: –10 to +50°C Class 1
Ambient pressure range: 65–108kPa
Humidity range: 10–90% RH
POWERING
Battery type: 9V 6LR61
Battery lifetime: Approx. 20 hours. Use of a 6F22G battery will yield a shorter battery lifetime, while a 9V lithium battery
will yield an extended operating time
External supply voltage: 7.5–15V. Automatic shut-off when V
OVERALL WEIGHT AND DIMENSIONS
Weight: 185g with battery
L: 109.5mm; D: 40mm
3
with adaptor Nor-1443
3
(typical) @ 1000Hz
< 7.5V.
Batt
14
Microphone Cartridge Calibration Values
Microphone cartridges should be calibrated in accordance with their size and working principle. The table lists some
commonly used microphone cartridges and their corresponding calibration values applicable with the protection grid mounted.
Manufacturer Microphone Type Calibration Level
Norsonic 1220 113,85dB
1221 113,85dB
1223 113,85dB
1225 113,85dB
1226 113,85dB
1227 113,85dB
1228 113,85dB
1230 114,00dB
1233 114,00dB
1234 114,00dB
1235 114,00dB
1236 114,00dB
1237 114,00dB
1245 114,00dB
1246 114,00dB
Microtech MK102 113,70dB
MK103 113,70dB
MK202 113,85dB
MK221 113,85dB
MK222 113,85dB
MK231 114,00dB
MK250 113,85dB
G.R.A.S. 40AC 113,85dB
40AD 114,00dB
40AE 113,85dB
Manufacturer Microphone Type Calibration Level
G.R.A.S. cont... 40AF 113,85dB
40AG 114,00dB
40AI 114,00dB
40AN 113,85dB
40AP 114,00dB
40AQ 114,00dB
40AR 114,00dB
40BD 114,00dB
40BE 114,00dB
40BF 114,00dB
40BP 114,00dB
Rion UC-53 113,85dB
B&K 4144 114,00dB
4145 113,70dB
4179 113,70dB
4133 113,85dB
4134 114,00dB
4147 114,00dB
4149 113,85dB
4155 113,85dB
4165 113,85dB
4166 114,00dB
4180 113,94dB
4188 114,84dB
4936 114,84dB
1
1)
without protection grid
15
Declaration of Conformity
We, Norsonic AS, Gunnersbråtan 2, Tranby, Norway, declare under our sole responsibility that the
product:
Sound Calibrator type 1251
to which this declaration relates, is in conformity with the following standards or other normative
documents:
Product Standard IEC 60942 (2003-01) Class 1
ANSI S1.40–1997
Safety: EN61010–1:1993 for portable equipment and pollution category 2.
EMC: EN 61000-6-1
EN 61000-6-3
following the provisions of the LVD- and EMC-Directive.
This product has been manufactured in compliance with the provisions of the relevant internal Norsonic production standards. All our products are tested individually before they leave the factory. Calibrated equipment—traceable to national and
international standards—has been used to carry out these tests.
This Declaration of Conformity does not affect our warranty obligations.
Tranby,
September 2004
The declaration of conformity is given according to EN 45014 and ISO/IEC Guide 22.
Norsonic AS, P.O. Box 24, N-3421 Lierskogen, Norway
Dagfinn Jahr
Quality Manager
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