System Description

Calibration SystemNor-1504A

The Sound Level Meter Calibration System Nor-1504A is now in its third generation. The system is

based on more than 15 years of experi-ence with computer controlled calibra-tion of sound measuring equipment.

The system is capable of calibrating virtually any type of sound measuring instruments, sound calibrators and microphones commercially available, provided that they have been designed in accordance with applicable interna-tional standards. Hence, the system is by no means limited to Norsonic prod-ucts alone.

The process of verification and cali-bration of sound level meters in accord-ance with international standards is in general divided into an electrical and an acoustical part. The standards mainly used are the IEC60651 for sound level

meters (SLMs) and the IEC 60804 for inte-grating-averaging sound level meters. National standards, like the ANSI S1.4, which in general is similar to the IEC 60651, come in addition to this.

Special national standards have been designed to cover special needs. A typi-cal example of this could be the German DIN 45 657 which describes the testing of time constant F down to 0.25ms; test-ing of statistical calculations and the Taktmaximal 3 and 5.

The IEC 60 651 and the 60 804 both describe tests that partly have been designed for type approval of an instru-ment design only. They do, however, also include tests meant to be made on a regular basis. The Organisation Inter-nationale de Métrologie Légale (the Inter-national Organisation for Metrology) has made a recommendation for test-

Uses:

• Acoustical and electrical calibration of sound measuring instruments, mea-suring microphones and sound calibra-tors in accordance with the applicable national and international standards

Features:

• Test signals comply with IEC 60 651, 60 804, and DIN 45 657 as well as the proposed IEC standard for sound mea-suring instrument tests, designed to replace the IEC 60 651 and 60 804 stan-dards

• Fast and accurate calibration of sound measuring instruments using three dif-ferent interface modes; manual; semi-automated and fully automated

• Frequency response calibration of microphones using electrostatic or acoustical method

• Sensitivity calibration of microphones using the insert voltage method ensures a high degree of accuracy

• Fast and accurate calibration of sound calibrators, pistonphones and associ-ated barometers in accordance with IEC 60942

• Sound level meter calibration in accordance with IEC 60 651, 60 804, DIN 45 657, ANSI S1.4, ANSI S1.43 and the OIML recommendation R58 & 88

• Test of fractional octave filters in accor-dance with IEC 61 260

• Full test report generated• Built-in self test features

PD 1504A Ed. 2 Rev. 0 ENGLISH 01.00

ing instruments in accordance with IEC 60 651 and 60 804. This recommendation describes the tests applicable to type evaluation (pattern approval) as well as the tests applicable on a more regular basis (periodic verification).

FunctionalityThe Norsonic Calibration System for sound measuring instruments – our type Nor-1504A – is mainly designed for calibration, but by using anechoic and climatic chambers etc., it is possible to apply the system to type approvals as well, since all the test signals needed are generated.

The system is also prepared to meet the upcoming proposed IEC standard which will replace the current IEC 60 651 and 60 804 due to its ability to generate the new proposed test signals.

Calibration of Calibrators• Measurements according to IEC

60942• Observed sound level in dB re.

20µPa• Sound level corrected for pressure,

temperature and humidity• Short term stability expressed in

dB (option)• Output frequency in Hz• Total distortion in % (option).

Calibration of Microphones• Measurements made in accord-

ance with IEC 61 094• Sensitivity of the microphone as

mV/Pa and dB rel.1V/Pa• Frequency response using an elec-

trostatic actuator in the frequency range 250–20 000Hz

• Frequency response using a mini anechoic chamber in the frequency range 1–20kHz (option)

• Low frequency response using sound calibrator type 1504A/7 down to 0.03 Hz (option)

• The microphone cartridge capaci-tance [pF] (option).

Periodic Verification of Sound Measuring Instruments

• Measurements made in accord-ance with IEC 60 651, IEC 60 804, DIN/IEC 651, DIN/IEC 804, ANSI S1.4, ANSI S1.43, DIN 45 657, IEC 61 260. The periodic verifica-tion can be performed according to IOML 58 & 88.

• Accuracy of input range selector• Level linearity• Spectral weighting networks A, B,

C, Lin and Flat• F, S, and I time constants. Pulse

measurement capability• RMS detector capability• Overload detector• Inherent noise using microphone

equivalent• Integrating averaging functions

Leq and SEL

System DescriptionThe test system is based on a PC control-ling the signal generator, voltmeter, test unit and the DUT (Device Under Test).

The computer also reads the barom-eter, the hygrometer and the thermom-eter which all are included with the system in order to provide the required environmental data in a fast and accu-

rate manner for the calibration reports.The system permits the use of preci-

sion barometers, hygrometers and ther-mometers not equipped with digital interfaces. The values are then keyed in manually.

These parameters are also needed to refer the measurements to reference con-ditions.

A calibration is normally divided into three main parts; viz. electrical calibra-tion of the sound measuring instrument, calibration of the microphone and – if applicable – calibration of the sound cal-ibrator. An overall system calibration is made at the end in order to adjust the sound measuring instrument to the cor-rect microphone sensitivity.

Hardware System ConfigurationThe system is based on the following components:• The Stanford Research DS360

signal generator, generates all the necessary test signals. The genera-tor features a high accuracy atten-uator and an ultra-low distortion signal generator

• The Sound Measuring Equipment Calibration Unit Nor-483B. This serves as a control centre for all the equipment connected to it and fea-tures a 40 dB high voltage ampli-fier and an 800 VDC supply for the electrostatic microphone testing; a relay matrix; a 200 VDC refer-ence source and three microphone power supplies, out of which two are equipped with precision atten-uators, high and low pass filters and ultra low noise amplifiers

• The Hewlett Packard voltmeter 34401A

• Thermometer, hygrometer and barometer (optional)

• PC running Windows® 95/98 con-trolling the voltmeter, signal genera-tor, SME Calibration Unit Nor-483B and the device under test.

Calibrating Sound Measuring Instruments

The test is controlled by Windows® based software. The software controls all elec-trical tests in accordance with IEC 60 651 and 60 804, OIML R 58 & 88, ANSI S1.4, ANSI S1.43 and DIN 45 657. The three latter tests are optional.

The microphone is removed and the electrical signal is fed via a BNC to pream-plifier adaptor Nor-1447 in to the sound measuring instrument under test.

Computer

Oscilloscope (extra)

Voltmeter

Distortion meter

Sound isolating meas-urement chamber

Sound calibrator

Thermometer/hygrometer

Barometer

Signal generator

Test unit

Typical configuration of system components.

Depending on the instrument design one of three available control modes will be used.

Manual mode. The test equipment (signal generator, voltmeter etc.) is con-trolled by the computer. The user must set up the device under test and read the results from its display and key the results into the computer.

Semi-automated mode. The computer controls the test equipment. The user must set up the device under test. The measurement values are measured with the voltmeter connected to the DC output port on the device under test and then transferred via the IEEE interface to the computer.

Automated mode. The test equipment and the device under test are fully con-trolled by the computer. This requires an RS-232 or IEEE interface on the device under test.

The program proposes a full set of tests, but you may, of course, run just a few of these tests, if you so wish.

A total measurement report is pre-sented at the end of the test and may be printed out and/or stored for later use.

The user can add sound measuring instruments not included in the instru-ment libraries by writing new interface libraries or by modifying existing ones and then saving them under a new name to enable testing of new instru-ments. These library files contain all the necessary information like linearity range, number of gain steps, instrument accuracy class etc.

Calibrating Measuring MicrophonesThe calibration of a microphone is divided into three measurement tasks: the absolute sensitivity; the frequency response and – optionally – the car-tridge capacitance.

All measurements are controlled by Microsoft® Windows® based software. The operator simply clicks on the test to be performed (absolute sensitivity cali-bration, frequency response calibration

– electrostatic or acoustical method – or the optional cartridge capacitance). A calibration certificate is printed out once the tests selected are completed.

Absolute sensitivity is measured by using the insert voltage technique. This ensures a very high degree of calibra-tion accuracy. The calibration data is

corrected for the ambient pressure, the ambient air humidity and ambient tem-perature.

All data can be read directly into the computer via the digital interface and presented in the calibration certif-icate as well as stored together with the measurement results. The system is supplied with the insert voltage ampli-fier Nor-1203 for calibrating 1”, and 1⁄2” microphones and the insert voltage amplifier Nor-1205 for calibrating 1⁄4” microphones.

Frequency response is measured either by use of the electrostatic or the acousti-cal method. The electrostatical method is a fast and reliable method and hence used to the extent possible. However, there are cartridges to which the elec-trostatic method cannot be applied and there are cases where the conversion fac-tors needed to convert the electrostatic frequency response into the correspond-ing free field or diffuse field response remain unknown. In these cases the acoustical method is applied instead.

Screenshot of the software control panel for the Sound Measuring Calibration Unit Nor-483B. The radio buttons are used to control the Nor-483B directly from the PC.

The system measures the quality of the sound measuring instrument’s time-integration capabilitites. Measured values are compared to the theoretically correct values. The deviations recorded are listed along with the tolerances permitted.

Method selection is made in the menu in the program controlling the measure-ments.

ELECTROSTATICAL METHOD. The microphone cartridge protection grid is replaced by an electrostatic actuator and the microphone is placed in the sound isolat-ing measurement chamber. The signal generator generates a sine wave signal which is incremented in sixth-octave steps up to 20kHz or higher depending on cartridge type – the 1⁄4” microphones are normally calibrated up to 100kHz.

The upper and lower frequency limits are user-controllable. The sine wave is fed to the sound isolating measurement chamber via the test unit in which a high voltage amplifier boosts the signal and a 800 VDC is added to generate the elec-trostatic field. The microphone signal is fed via the microphone preamplifier into the test unit again where the signal is amplified by 40dB and spurious noise is removed by a bandpass filter. The filtered signal is measured by the volt-meter and the values transferred via the IEEE interface to the computer. The

frequency response measured is the electrostatic frequency response, which closely resembles the pressure response of the microphone cartridge. The free field correction is added to the actuator response curve. The measurement is fully controlled from the PC.

ACOUSTICAL METHOD. The software sup-ports two different ways of obtaining the acoustical frequency response:

• The first method measures the frequency response by use of a reference microphone. Then the microphone that shall be cali-brated replaces the reference micro-phone.

• The second method measures the reference microphone and the microphone to be calibrated simultaneously. This method is described in the IEC 61094-5 (for the time being this standard is at committee draft level only).

In both cases the signal generator gen-erates the sine wave signal. The pro-gram proposes both the start and stop frequencies. You may, however, select other start and stop frequencies. The sine wave signal is incremented in sixth- octave steps. The signal from the micro-phone is compared with the reference microphone to obtain the frequency response.

Cartridge Capacitance (optional) is determined by measuring the voltage division ratio when the microphone car-tridge forms a part of a capacitive volt-age divider. A special preamplifier is used for this.

Since the cartridge capacitance nor-mally depends on the polarisation volt-age applied, measurements are carried out using the polarisation voltage the car-tridge under test has been designed to work with – i.e. 200 V for non-polarised and 0 V for prepolarised cartridges.

Mini Anechoic Chamber Nor-1504A/3 Being a small anechoic chamber suita-ble for calibrating microphones in the frequency range 1 000–16 000 Hz, the Nor-1504A/3 provides a convenient alter-native whenever large anechoic cham-bers are not available. This is a complete chamber with a built-in speaker, a posi-tioning system for the microphone or sound level meter and an external ampli-fier for the speaker. A 1⁄2” preamplifier

Microphone cartridges are tested for their frequency response, sensitivity and car-tridge capacitance. The sensitivity has been compensated for the ambient pressure, temperature and humidity – the values used for this are stated in the lower left corner of the screen.

The system produces a hard copy of the microphone frequency response stating all the relevant information.

Nor-1201 and a 10 metre cable are also included. The external physical dimen-sions amount to 650×650×1100 [mm].

Testing the Low Frequency Response of a Microphone

The Nor-1504A/2-1 expands the system into a complete test device for the meas-urement of the low frequency response of a microphone. The free-field and random response of a microphone is either measured using the electrostatic actuator method or in an anechoic cham-ber – the acoustical method – where reflections from the walls are negligible. Unfortunately, none of these techniques can be applied at very low frequencies.

When the frequency is lowered, the frequency response of most measuring microphones will be flat until the vent-ing system for the microphone starts to limit the response. Most microphones are vented to the side or the rear of the cartridge. The distance between the normal acoustical port of the micro-phone (the diaphragm) and the port for the venting system is typically 10–30 mm. This means that for frequen-cies below 500 Hz, the distance between the different acoustic ports for the micro-phone corresponds to less than 5% of the wavelength. Therefore, the response of the microphone may be tested in a small coupler of similar size instead of being tested in a plane wave field.

The Principles Used The figure top right shows the cross section of the coupler arrangement for such testing. An adaptor is placed between the microphone cartridge and the normal preamplifier. This adaptor creates an airtight electrical connection to the preamplifier. The adaptor has a large vent connected to the rear side of the microphone. A sound pressure in the coupler will therefore be coupled to the rear side of the microphone and thereby expose the venting system.

One side of the coupler consists of the diaphragm of an electrodynamical loudspeaker. The coupler is constructed so that the diaphragm and rim is acting like a piston with a constant area within the frequency range of interest.

The loudspeaker is driven in a cur-rent-controlled mode by a built-in ampli-fier. This serves to limit the variations coming from changes in the electrical

impedance of the voice coil. Otherwise the response will depend on the previ-ous loading of the loudspeaker. A con-stant current leads to a constant force trying to move the diaphragm. The force acts against the spring holding the dia-phragm in position and against the air in the coupler being compressed by a movement of the diaphragm. In the implemented design the spring force

is less than 0,3% of the force from the air and will therefore not influence the response.

If a piston of known size is moved in the coupler by a known force, it will gen-erate a known pressure. The pressure will be the force divided by the area of the piston. This equation holds regard-less of the thermodynamic condition in the coupler. Hence the pressure will not

Cross section of the coupler arrangement for the testing of the low frequency response of microphone cartridges. The purpose of the sealing device is to prevent leakage of air through the preamplifer while at the same time keep the rear vent of the cartridge exposed to the same sound pressure as the diaphragm front.

Loudspeaker diaphragm

Diaphragm suspension

Microphone preamplifer

Coupler front

Diaphragm rim

Sealing deviceMicrophone cartridge

Hole to expose the car-tridge vent to the same sound pressure as the diaphragm front

Microphone and preamplifier

The Mini Anechoic Chamber Nor-1504A/3 is well-suited for the calibration of micro-phones within the frequency range 1 000–16 000 Hz. External physical dimensions amount to 650×650×1100 [mm].

A sound source is located inside here

A 10 metre microphone cable is a part of the system

Handles for easy trans-portation

depend on whether the compression is adiabatic or isothermal.

The lower frequency limit for the test-ing device will depend of how tight the coupler is. Any leakage will lead to larger movement of the piston and thereby be more influenced by the dia-phragm spring.

The upper frequency is mainly lim-ited by the resonance frequency of the moving mass and the spring force from the compressed air in the coupler.

Typical frequency range for the unit is 0,03–200 Hz for an accuracy of ± 0,5 dB.

Revealing Diaphragm HolesTesting the low frequency response of a measuring microphone will reveal any presence of physical holes in the dia-phragm. Although quite common, such defects can be hard to detect otherwise. Holes in the diaphragm influences the low frequency response of a microphone cartridge.

High Level LinearityThe unit may also be used to test the linearity of a microphone at high levels. Due to the very small amplitude of the movement of the loudspeaker dia-phragm, linearity is kept low even for levels in excess of 150 dB.

The software controls the measure-ment. The low frequency response may

be merged with the high frequency response which has been obtained either by means of the acoustical or the electro-statical method. This will provide you with a single frequency response curve for the microphone covering the entire frequency range of interest.

Calibrating CalibratorsCalibrators and pistonphones are cali-brated in accordance with the IEC 60942 by the use of a reference microphone, designed as a WS2P as defined by IEC01094.4. The frequency and level are adjusted, whenever applicable and the calibration data printed out and/or stored on disc. The ambient pressure, temperature and humidity are read via the RS-232 directly into the program, where the calibration data are normal-ised to the reference conditions as defined by the IEC 60942.

The complete measurement proce-dure, including verification of the ref-erence microphone and test system is controlled by software running on the Microsoft® Windows® platform.

Pistonphones are supplied with barometers in order to correct for the atmospheric pressure. This barometer is easily calibrated against the system barometer.

Acoustical ReferencesReference calibrators and microphones must be ordered separately. The Sound Level Calibrator Nor 1253 is a Type 0L calibrator – often referred to as The Electronic Pistonphone. It has shown a

All reference sources are controlled and verified before any calibration of sound level/microphone sensitivity takes place. Compensation for ambient pressure, tem-perature and humidity is made here.

After you have run through the calibration of a sound calibrator, a hard copy may be produced on the colour printer.

remarkable long time stability, with a typically change in the SPL level of less than 2/100 dB/year.

The applied principle of operation (patent pending) makes the Nor-1253 virtually independent of ambient con-ditions like temperature, atmospheric pressure and humidity within the speci-fied range of operation.

The Nor-1253 is available in a mul-titude of different versions. We recom-mend users to invest in two versions; one producing an SPL of 124dB @ 250 Hz and another producing an SPL of 124 dB @ 1 000 Hz.

The reference microphone Nor-1236/REF is a 1⁄2” pressure microphone with a nominal sensitivity of 12.5 mV/Pa. The reference microphones have been care-fully selected and measured thoroughly throughout a period of minimum nine months to document their stability.

Correction for Environmen-tal Parameters

Environmental parameters, such as tem-perature, humidity and atmospheric pressure may influence on the calibra-tion results. Hence, it is important to apply corrections to the reference micro-phones, sound calibrators and the meas-uring results from the device under test due the influence of ambient temper-ature, ambient humidity and ambient pressure.

The calibration system may be deliv-ered with or without one of the two options available for environmental con-ditions compensations.

Systems delivered without the

1504A Basic unit needed for all the below described items, consisting of:

• Test Unit 483B • HP voltmeter 34401A • Stanford Research DS360 Signal Generator• PC with IEEE card • Colour printer • Cables and adaptors

1504A/0 Installation & training course taking place at customer’s premises. Price includes all expenses except travel and accomodation expenses. This additional cost is covered by customer.

• Basic module requires 1,5 day •SLM calibration 1,5 day • Microphone calibration 1 day• Calibrator calibration 1 day

Prices are per day and are submitted on request. 1504A/1 Electric calibration of sound level meters accord-ing to IEC60651 and IEC60804, consisting of:

• Software for testing the sound measuring instru-ments

• Set of dummy microphones (1”, 1⁄2” and 1⁄4”)• Cables and adaptors

1504A/2 Electrostatic frequency calibration and level cal-ibration of microphones, consisting of:

• Software for calibration using electrostatic and the two acoustical methods,

• Set of electrostatic actuators (1”, 1⁄2” and 1⁄4”)• Sound Isolating Measurement Chamber Nor-1263A •Insert voltage preamplifier type 1203 (1” and 1⁄2” microphones) type 1205 (1⁄4”

microphones)• Cables and adaptors

1504A/3 Acoustic frequency calibration of microphones, mainly applicable to type 2 microphones that cannot be tested by electrostatic calibration. Consists of mini anechoic test chamber set including:

• Mini anechoic chamber• Sound source (loudspeaker)• Power Amplifier • Microphone preamplifier 1201 • Cables and adaptors

Note: Requires 1504A/2. 1504A/4 Calibration of calibrators in accordance with IEC 60 942, consisting of:

• Software module for sound calibrator calibration

Note: Requires 1504A/2, or: • Microphone preamplifier type Nor-1203•Cable Nor-1410•Sound isolating measurement chamber Nor-1263A.

Distortion meter is not included, but available on request. 1504A/5 Barometer, thermometer and hygrometer with direct data read-in to the program via RS-232. Accuracy corresponds to a pistonphone level estimation error of 1/1000dB. 1504A/5-1 Barometer, thermometer and hygrometer with direct data read-in to the program via RS-232. Accuracy cor-responds to a pistonphone level estimation error of 2/100 dB. 1504A/6 Filter test according to IEC 60 225 and IEC 61 260 (To be released August 2000). 1504A/7 Low frequency testing of 1⁄2” microphones (Requires 1504A/2), consisting of a low frequency coupler with amplifier and misc. adaptors.

Ordering information

optional compensation will require equipment from other manufacturers. The ambient pressure, humidity and temperature must then be keyed in man-ually every time the program prompts you to provide these data.

Norsonic offers two options for auto-mated inclusion of measured environ-mental parameters. The two differ on accuracy (and hence also the price) of the barometer.

The accuracy of sensitivity calibra-tions made on pistonphones and certain sound calibrators (those prone to sig-nificant ambient pressure sensitivity) is

closely connected to the accuracy of the system barometer itself.

This means that the barometer accu-racy contributes to the total uncertainty of the measured values of the calibra-tion laboratory.

Calibrating the System Itself The annual calibration cost of the system itself (to ensure it maintains its accu-racy) is low, since it will be necessary to calibrate just a few key components rather than the entire system. A self test

program has been designed to run at predefined intervals to test the electrical performance of the system. It calibrates the frequency response and the attenu-ator of the signal generator and the test unit.

The standard system is delivered as a system with traceable calibration, but it may be delivered with accredited cali-bration at additional cost.

P.O.Box 24, N-3421 Lierskogen, NorwayTel: +47 3285 8900 Fax: +47 3285 2208info@norsonic.com www.norsonic.com

Representative:

PD 1504A Ed. 2 Rev. 0 ENGLISH 01.00