PXI Calibration System with equipment provided by
G.R.A.S.
PXI Calibration System uses a PXI 4461 board as standard equipment for generation/measurement, along with PXI chassis and embedded PXI computer provided by National Instruments. This System also uses some equipment and accessories provided by G.R.A.S., which is a company known by the reliability of the equipment it provides.
PXI 4461 has 24 bits resolution and is designed for sound and vibration applications. It comes with 2 output channels with 3 gains for generating signals and 2 input channels with 6 gains for measuring signals with very good reliability. PXI 4461 also includes AC/DC coupling and IEPE signal conditioning software configurable. This board includes all hardware and software required for precision measurements with microphones, accelerometers and other transducers characterized by very high dynamic ranges.
G.R.A.S. equipment and accessories that could be used on calibration systems are presented as follows.
G.R.A.S.
Quotation
G.R.A.S.
Technical Support
Old Sound Level
Meters (IEC 60651)
Noise Dosimeters
(ANSI S1.25)
Sound Exposure
Meters (IEC 61252)
For electrical tests carried out according to IEC 60651 (sound level meters), ANSI S1.25 (noise dosimeters) and IEC 61252 (sound exposure meters), it is necessary to replace the microphone of the equipment that will be calibrated by a capacitive adapter that has capacitance of the same magnitude of the capacitance of the replaced microphone. Therefore it is necessary that the laboratory has some adapters with different capacitances, since equipment of several manufacturers and models could use several microphones with different capacitances. Capacitive adapters provided by G.R.A.S . are presented as follows.
The RA0062 and RA0062-S1 are 20 pF and 50 pF input adapters for 1/2" preamplifiers. One end screws on to a 1/2" preamplifier and the other end has a BNC input connector. This enables connecting a signal generator to the input of the preamplifier. Short circuiting the BNC input results only in loading the preamplifier by 20 pF or 50 pF. For this purpose a BNC plug with a built-in 50 ohm resistor is enclosed. This plug is used when measuring the noise floor in the preamplifier.
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A cable with both BNC ends is required to make the connection between the BNC connector of the PXI 4461 card and the BNC connector of the capacitive adapter.
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A sound calibrator (GRAS 42AG or GRAS 42AB) is required to perform acoustic adjustment at the beginning of calibration on the sound level meter with its microphone. After this adjustment the microphone is removed from the sound level meter and then the capacitive adapter is connected to it so that the electrical signals are inserted into the equipment to be calibrated. The advantage of GRAS 42AG calibrator is that this equipment generates two levels (94 dB / 114 dB) at two frequencies (250 Hz / 1 kHz). GRAS 42AB calibrator generates only one level (114 dB) at one frequency (1 kHz).
New Sound Level Meters (IEC 61672-3)
For electrical tests carried out on sound level meters according to IEC 61672-3 two editions it is necessary to replace the microphone of the equipment that will be calibrated by a capacitive adapter that has capacitance of the same magnitude of the capacitance of the replaced microphone . Therefore it is necessary that the laboratory has some adapters with different capacitances, since sound level meters of different manufacturers and models could use different microphones with different capacitances. Following are presented capacitive adapters provided by G.R.A.S..
The RA0062 and RA0062-S1 are 20 pF and 50 pF input adapters for 1/2" preamplifiers. One end screws on to a 1/2" preamplifier and the other end has a BNC input connector. This enables connecting a signal generator to the input of the preamplifier. Short circuiting the BNC input results only in loading the preamplifier by 20 pF or 50 pF. For this purpose a BNC plug with a built-in 50 ohm resistor is enclosed. This plug is used when measuring the noise floor in the preamplifier.
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A cable with both BNC ends is required to make the connection between the BNC connector of the PXI 4461 card and the BNC connector of the capacitive adapter.
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A sound calibrator (GRAS 42AG or GRAS 42AB) is required to perform acoustic adjustment at the beginning of calibration on the sound level meter with its microphone. After this adjustment the microphone is removed from the sound level meter and then the capacitive adapter is connected to it so that the electrical signals are inserted into the equipment to be calibrated. The advantage of GRAS 42AG calibrator is that this equipment generates two levels (94 dB / 114 dB) at two frequencies (250 Hz / 1 kHz). GRAS 42AB calibrator generates only one level (114 dB) at one frequency (1 kHz).
The tests performed with sound level meter attached to its microphone are self-generated acoustic noise and acoustic frequency response.
Self-generated acoustic noise test (IEC 61672-3)
In the acoustic noise floor test, the laboratory must have a small acoustic chamber with enough insulation so that the background noise inside the device is preferably less than 20 dBA. It is desirable that background noise of the acoustic insulation chamber is around 15 dBA. In this camera the sound level meter with its microphone must be inserted and then the sound level must be measured.
According to IEC 61672-3 1st Edition (2006), the self-generated acoustic noise test is part of the conformity assessment. For this edition of the standard, the testing laboratory must have its own self-generated acoustic noise measurement system. This is necessary if the measurement of self-generated acoustic noise with the customer's sound level meter results in levels higher than the level stated by the instrument manual for self-generated acoustic noise measurements. In this case it is necessary that the laboratory prove with its own measurements that the ambient noise is not the cause of the measurement with higher levels than expected.
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According to IEC 61672-3 2nd Edition (2013), the self-generated acoustic noise test is for informational purposes only and is not part of the conformity assessment. According to this edition of the standard, the testing laboratory must have the acoustic chamber for the measurement of self-generated acoustic noise.
GRAS 40HL 1/2'' LEMO Low-noise Microphone System
The 40HL comprises a special high-sensitive 1/2" free-field measurement microphone and an integrated 1/2" low-noise preamplifier. In combination, they connect to most high-quality input modules with LEMO 1B connector. The preamplifier, a true 1/2" low-noise amplifIer with LEMO 1B connector, has a built-in compensation filter for free-field microphones. The 1/2" microphone is an externally-polarized free field microphone with a specially reduced inherent noise floor in order to achieve a high dynamic range and wide frequency range. Its diaphragm is specially tuned to yield high sensitivity coupled with low internal noise.
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GRAS 12HF 1-Channel Power Module for Low-noise Systems is the ideal power supply to use on self-generated acoustic noise tests.
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This is the best option G.R.A.S. provides for measuring very low noise level. Whit this equipment (40HL and 12HF-1) it is possible to achieve levels down to 6,5 dBA. If the laboratory budget is not enough to purchase this set (microphone + preamplifier + power supply) it is possible to use another set, cited as follows. Despite another measurement set may have noise floor above 10 dBA, its noise floor could be low enough to perform measurements of self-generated noise of the majority of sound level meters.
IEPE/CCP G.R.A.S. preamplifiers and G.R.A.S. pre-polarized microphones type WS2P.
The simplest set for measuring self-generated acoustic noise can be composed by IEPE preamplifier and pre-polarized microphone.
IEPE preamplifiers (Integrated Electronic Piezo-Electric) can be connected directly to PXI 4461 board input channels , since this device provides such type of signal conditioning. These preamplifiers have at their output a BNC connection, which means they can be connected to PXI 4461 board input channel using only one cable with both BNC ends.
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CCP (Constant Current Power) is the same as IEPE (Integrated Electronic Piezo-Electric) and CCLD (Constant Current Line Drive) and is compatible with many other constant current driven products such as Deltatron® (Brüel & Kjaer), ISOTRON® (Endevco Corp.), ICP® (PCB Group, Inc.).
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Microphones used with IEPE preamplifiers must be pre-polarized, which means their bias voltage must be 0 Volt. For the measurement of self-generated acoustic noise only Working Standard ½’ Pressure field microphones (WS2P according to IEC 61094-4 specifications) will be presented, which are transducers intended for use in cavities, such as couplers and sound calibrators . Thus, these microphones can also be used to perform calibration of sound calibrators by the comparison method, but they have to be used with an adapter ring to transform their front geometry into the front geometry of a Laboratory Standard ½’ Pressure field microphone (LS2P).
G.R.A.S. power supplies, preamplifiers with LEMO connection and 200 Volts polarized microphones
For measurement of self-generated acoustic noise, a set consisting of a preamplifier with LEMO termination and LS2P or WS2P microphone (with 200V polarization voltage) can also be used. To use this set obviously it is needed to supply the 200V voltage for the microphone bias and therefore an external power supply must be used. A LEMO cable must be used for connection between power supply and preamplifier.
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In the list of power supplies presented below, the G.R.A.S 12AK-S3 power supply is not present. This device used along with G.R.A.S. 26AG preamplifier, can be used to perform insert voltage technique, which could be used to determine level generated in the calibration of sound calibrators and also to determine open circuit sensitivity in the calibration of measurement microphones. Metroacustica recommends that you purchase this set previously mentioned, since these same equipment can be used in other measurement sets. The specifications of this source cannot be found at G.R.A.S. website and to obtain detailed information about this device it is necessary to contact G.R.A.S. representative in your country or G.R.A.S. technical support in Denmark by clicking here.
Following are presented some power supplies G.R.A.S. provides.
Traditional preamplifiers require an external power supply that provides ± 15 to ± 60 VDC or +28 to +120 VDC voltage supply. This type of preamplifier is usually connected to a suitable power supply (eg G.R.A.S. 12AK) through a 7 pin LEMO plug.
Among many types of preamplifiers available from G.R.A.S., it will be cited only two models: G.R.A.S. 26AG that allows performing of insert voltage technique and G.R.A.S. 26AK which is intended for general use. Metroacustica recommends the use of G.R.A.S. 26AG along with the GRAS 12AK-S3 power supply, since these equipment can be used in other measurements, such as in the determination of generated level in calibration of sound calibrators and also in the determination of open circuit sensitivity in the calibration of measurement microphones.
Following are presented two preamplifiers options that can be used with 200 Volt polarized microphones that G.R.A.S. provides.
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G.R.A.S. 200V polarized pressure field microphones are used with standard preamplifiers, such as G.R.A.S. 26AK, which has a 7-pin LEMO connector. The preamplifier must be connected to a power source to supply preamplifier with the power required for 200 V bias to the microphone.
For the measurement of self-generated acoustic noise, will be presented only working standard and laboratory standard pressure field microphones (WS2P according to the specifications of IEC 61094-4 or LS2P according to the specifications of IEC 61094-2), which could be used in measurements such as couplers and sound calibrators. Therefore, these microphones can also be used to perform the calibration of sound calibrators.
Acoustic frequency response test (IEC 61672-3)
This test must be performed with the microphone installed on the sound level meter. G.R.A.S. provides equipment for this test to be performed using the electrostatic actuator method.
This method uses ½ "inch electrostatic actuator and an 800 Vdc power supply. The method is standardized by IEC 61094-6 for frequency response calibration of measurement microphones and its great advantage is that electrostatic actuators and the DC power supply can also be used to perform calibrated frequency response calibration of measurement microphones. This method can only be used if the microphone protection grid could be removed. In this method it is necessary to have corrections of electrostatic pressure field for free field for the microphone used.
G.R.A.S provides 14AA source that supplies the 800 Vdc voltage for the ½ "inch RA0014 electrostatic actuator. G.R.A.S. support AL0010 (microphone calibration stand) is used to keep the microphone upright and also to connect the 14AA source to the RA0014 electrostatic actuator.
Sound Calibrators (IEC 60942)
Sound calibrators can be calibrated by two techniques: insert voltage or sequential comparison. The insert voltage technique can be used to perform calibrations on sound calibrators of classes LS, 1 and 2. If the sequential comparison technique is employed, it will be possible to calibrate only class 1 and 2 sound calibrators.
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For the calibration of laboratory standard sound calibrators (class LS) it must be used the insert voltage technique and therefore it is necessary to use the following instrumentation:
- Laboratory Standard microphone (LS1P or LS2P)
- Pre-amplifier with insert voltage (G.R.A.S. 26AG)
- 200V polarization source with insert voltage (G.R.A.S. 12AK-S3)
- Class LS Pistonphone
If the laboratory intends to calibrate only Class 1 or Class 2 sound calibrators (usually for half-inch microphones) it may be used insert voltage technique (as quoted above) or sequential comparison technique. The instrumentation used to perform sequential comparison tests usually has lower cost, then some laboratories choose this technique and in this case it will be necessary to use one of the following instrumentation options:
a) With pre-polarized microphones
- Pre-polarized microphone (WS2P with G.R.A.S. adapter ring part number GR0606)
- Pre-amplifier with IEPE supply
- Class LS or class 1 sound calibrator
b) With 200V polarized microphones
- Microphone (LS2P or WS2P with G.R.A.S. adapter ring part number GR0606)
- Traditional preamplifier with LEMO termination
- Power supply for microphone bias
- Class LS or class 1 sound calibrator
Standard laboratory microphones (LS2P or LS1P) are G.R.A.S 40AU and G.R.A.S 40EU microphones of ½" and 1" respectively. In the nomenclature "LS2P" LS comes from "Laboratory Standard", P from "Pressure field" and 2 which corresponds to a half-inch microphone. Laboratory standard microphones complies with IEC 61094-2 and IEC 61094-4 recommendations.
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Working standard microphones (WS1P and WS2P) must be used with and adapter ring instead their protection grid. This adapter ring (G.R.A.S. part number GR0606 for 1/2" microphones) transforms front geometry of the working standard microphone into front geometry of a laboratory standard microphone, type LS1P and LS2P respectively. In nomenclature WS2P "WS" comes from "Working Standard", P comes from "Pressure field" and 2 corresponds to a half-inch microphone. Working standard microphones complies with IEC 61094-4 recommendations.
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Following will present instrumentation options that can be used for the calibration of sound calibrators.
Instrumentation needed to perform
INSERT VOLTAGE TECHNIQUE
G.R.A.S power supply for microphone polarization that has insertion voltage option is G.R.A.S. 12AK-S3, but specification of this equipment cannot be found at www.gras.dk. To get more information about this device it is need to contact G.R.A.S. representative in your country or G.R.A.S. technical support in Denmark by clicking here.
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Metroacustica recommends the acquisition of GRAS 26AG preamplifier and GRAS 12AK-S3 power supply, as this same equipment can be used in other measurement sets, such as in determination of microphone open-circuit sensitivity in calibration of measuring microphones.
Instrumentation needed to perform
SEQUENTIAL COMPARISON TECHNIQUE
with pre-polarized WS2P microphone
The lowest cost set for calibration of sound calibrators can be composed of pre-polarized microphone, preamplifier with IEPE supply and class 1 standard sound calibrator and G.R.A.S. provides some options for this set.
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Preamplifiers with IEPE (Integrated Electronic Piezo-Electric) power supply can be connected directly to input channels of PXI 4461 board, since this device provides such type of signal conditioning. These preamplifiers have at their output a BNC connection, which means they can be connected to PXI 4461 board input channel using only one cable with both BNC ends.
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CCP (Constant Current Power) is the same as IEPE (Integrated Electronic Piezo-Electric) and CCLD (Constant Current Line Drive) and is compatible with many other constant current driven products such as Deltatron® (Brüel & Kjaer), ISOTRON® (Endevco Corp.), ICP® (PCB Group, Inc.).
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Microphones used with IEPE preamplifiers must be pre-polarized, so their bias voltage must be 0 Volt. For the calibration of sound calibrators it could be used working standard pressure field microphones (WS2P according to IEC 61094-4 standard), which are transducers intended for use in cavities, such as couplers and sound calibrators. These microphones should be used with their protective grid replaced by an adapter ring (GRAS part number GR0606 for 1/2” microphones) to transform the front geometry of the WS2P microphone into the front geometry of a laboratory standard pressure field (LS2P) microphone. Following will be presented G.R.A.S. options for this type of microphone.
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Sound calibrators used as standard for level calibration by sequential comparison technique shall be class LS or class 1.
G.R.A.S. 42AP and G.R.A.S. 42AA are the most accurate standard equipment and can also be used in measurement of open circuit sensitivity on calibration of measuring microphones, since this measurement is performed at 250 Hz, which is the same frequency generated by these pistonphones. G.R.A.S. 42AA can be an LS class device if it is used together with a barometer with accuracy better than 0.3%.
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Sound calibrators G.R.A.S. 42AG and G.R.A.S. 42AB shown below can also be used as standard for calibration of the level generated by sequential comparison, since they are class 1 equipment. The advantage of using G.R.A.S. 42AG calibrator is that this equipment generates two levels (94 dB and 114 dB) at two frequencies (250 Hz and 1000 Hz) and for this reason it can be used both for acoustic adjustment in the calibration of sound level meters and for sensitivity measurement on measurement microphones calibration.
Instrumentation needed to perform
SEQUENTIAL COMPARISON TECHNIQUE
with 200 Volts polarized microphones
Traditional preamplifiers require an external power supply that provides a supply voltage of ± 15 to ± 60 VDC or +28 to +120 VDC. This type of preamplifier is usually connected to a suitable power source (eg G.R.A.S. 12AK) through a 7 pin LEMO plug.
Among many types of preamplifiers provided by G.R.A.S. will be cited only two models: G.R.A.S. 26AG which could also be used to perform insert voltage technique and G.R.A.S. 26AK which are intended for general use, both for use with microphones built in accordance with IEC 61094-2 (LS1P / LS2P) and IEC 61094-4 (WS1P / WS2P) standards. Metroacustica recommends acquisition of G.R.A.S. 26AG (together with power supply G.R.A.S. 12AK-S3), since this same equipment could also be used in other measurements, such as on determination of open-circuit sensitivity in calibration of measurement microphones.
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Following are presented two preamplifiers options provide by G.R.A.S. for 200 V polarized microphones.
G.R.A.S. 200V polarized pressure field microphones are used with standard preamplifiers, such as G.R.A.S. 26AK, which has 7-pin LEMO connector. A power source must be connected to the preamplifier in order to supply it with the power required for 200 V bias to the microphone.
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Following will be presented pressure field microphones according to the specifications of IEC 61094-4 (WS2P) and according to the specifications of IEC 61094-2 (LS2P), which are transducers intended for use in cavities, such as couplers and sound calibrators. WS2P type microphones should be used with their protective grid replaced by an adapter ring (G.R.A.S. part number GR0606 for ½” microphones) to transform the front geometry of this microphone into the front geometry of a laboratory standard pressure field microphone (LS2P).
Sound calibrators used as standard for level calibration by sequential comparison technique shall be class LS or class 1.
G.R.A.S. 42AP and G.R.A.S. 42AA are the most accurate standard equipment and can also be used in measurement of open circuit sensitivity on calibration of measuring microphones, since this measurement is performed at 250 Hz, which is the same frequency generated by these pistonphones. G.R.A.S. 42AA can be an LS class device if it is used together with a barometer with accuracy better than 0.3%.
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Sound calibrators G.R.A.S. 42AG and G.R.A.S. 42AB shown below can also be used as standard for calibration of the level generated by sequential comparison, since they are class 1 equipment. The advantage of using G.R.A.S. 42AG calibrator is that this equipment generates two levels (94 dB and 114 dB) at two frequencies (250 Hz and 1000 Hz) and for this reason it can be used both for acoustic adjustment in the calibration of sound level meters and for sensitivity measurement on measurement microphones calibration.
To provide the power required for the 200 Volts bias to the microphone, an external source must be used in addition to a LEMO cable for connection between the source and the preamplifier.
In the list of power supplies presented below, the G.R.A.S 12AK-S3 power supply is not present. This device used along with G.R.A.S. 26AG preamplifier, can be used to perform insert voltage technique, which could be used to determine level generated in the calibration of sound calibrators and also to determine open circuit sensitivity in the calibration of measurement microphones. Metroacustica recommends that you purchase this set previously mentioned, since these same equipment can be used in other measurement sets. The specifications of this source cannot be found at G.R.A.S. website and to obtain detailed information about this device it is necessary to contact G.R.A.S. representative in your country or G.R.A.S. technical support in Denmark by clicking here.
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Following will be presented some power supply provided by G.R.A.S. for 200V polarized microphones.
Measurement Microphones (IEC 61094-6)
In measurement microphones calibration, two characteristics must be checked: frequency response and sensitivity.
Frequency response calibration is performed by using the electrostatic actuator method. This method is defined by IEC 61094-6 standard, which determines dimensions and characteristics of electrostatic actuators. The electrostatic acoustic field that is generated by electrostatic actuators resembles the pressure field. Correction curves must be used to transform electrostatic acoustic field into free field response. Electrostatic actuators must be supplied with 800V DC voltage plus an AC sinusoidal signal that should be applied at some frequencies for measuring frequency response of the microphone under test. In order to measure its frequency response, the microphone under calibration could be connected to a general purpose preamplifier and so to a power supply to provide bias voltage to the microphone (0V or 200V). However, for open circuit sensitivity determination, it must be used the insert voltage technique.
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Therefore, it is preferable to acquire the set composed by the G.R.A.S. 26AG with source G.R.A.S. 12AK-S3, since this set may be used to determine the frequency response and open-circuit sensitivity.
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Following are presented instrumentation provided by G.R.A.S. for determination of frequency response and open circuit sensitivity.
This method uses ½" inch electrostatic actuator and an 800 Vdc power supply. The method is standardized by IEC 61094-6 for frequency response calibration of measurement microphones and its great advantage is that electrostatic actuators and the DC power supply may also be used to perform acoustic measurements of sound level meters with their microphones according to IEC standard 61672-3. Calibration by electrostatic actuator method can only be used if the microphone protection grid can be removed. In this method it is necessary to have corrections in order to transform electrostatic field response into free field response for the microphone under calibration.
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G.R.A.S provides 14AA actuator power supply and ½ "inch RA0014 electrostatic actuator. G.R.A.S. AL0010 stand (microphone calibration stand) may be used to keep the microphone upright and also to connect the 14AA power supply to RA0014 electrostatic actuator.
Instrumentation needed to measure microphone
OPEN CIRCUIT SENSITIVITY
For microphone open circuit sensitivity measurement, which is the ratio between output voltage (mV) and input dynamic pressure (Pa) when the microphone is not loaded by a preamplifier, the insert voltage technique must be used. For determination of microphone sensitivity when it is loaded by a preamplifier, a general purpose one could be used.
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The insert voltage technique could be performed by using G.R.A.S. 26AG preamplifier and G.R.A.S. 12AK-S3 power supply. For performing this method it should be used a G.R.A.S. 42AP pistonphone, or a G.R.A.S. 42AA pistonphone or even a G.R.A.S. 42AG sound calibrator. The G.R.A.S. 42AG sound calibrator may be used to generate the acoustic signal at 250 Hz for the determination of the open circuit sensitivity of microphones in the pressure field.
To keep the preamplifier upright firmly attached to the piston, ensuring stability, it could be used Pistonphone Calibration Stand G.R.A.S. AL0017. The specifications of this accessory are not present at www.gras.dk and for detailed information contact must be made with G.R.A.S. representative on your country or with G.R.A.S. technical support by clicking here.
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Additional Measurement Systems
The generation/measurement set consisting of the PXI chassis, PXI embedded computer and PXI 4461 board, as well as being used in all calibration systems described above, can also be used in other systems that require both generation and measurement of standard signals . These systems may be, for example:
- Calibration of audiometers;
- Calibration of artificial mastoids;
- Calibration of accelerometers;
- Acoustic testing;
- Testing of hearing aids;
- Reverberation time measurements;
- Acoustic insulation measurements;
Etc...
Metroacustica will also be able to develop the measurement systems mentioned above.