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| Quantity - sound pressure. |
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| Unit - Pa. |
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| Tasks of the laboratory | ||||||||||||||||
| The main task of the
laboratory is the realisation and development of the Sound
Pressure National standard which based on
reciprocity method for calibration of standard
microphones. |
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| International comparisons | ||||||||||||||||
| The
intercomparison of laboratory standard microphones have
shown mutual deviation of less than 0.04 dB between SMU
and NPL England, PTB Germany, VNIIFTRI Russia and DTU
Denmark. The laboratory
participates in EUROMET, COOMET and DUNAMET acoustic
projects. |
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Available metrological services | |||||||||||||||
| Verification of measuring microphones with nominal diameter 24 mm, 12 mm, 6 mm (IEC 61094-1, IEC 61094-4). Calibration of sound calibrators (IEC 60942). Verification of sound level meters, classes 0, 1, 2, 3 (IEC 60651). Calibration of acoustic couples and artificial aers (IEC 60318-3, IEC 60318-1). Verification of audiometers, classes 1, 2, 3, 4, 5 (IEC 60645-1). Within an additional acoustic instruments: verification of electronic filters (classes 1, 2, 3 - IEC 61260), calibration of generators, analysers, noise analysers, noise dose meters, level recorders, testing of hearing aids, personal aound exposure meters (IEC 61252). | ||||||||||||||||
| Methods of measurement: | ||||||||||||||||
| - a pressure
sensitivity of a laboratory standard microphone (typr LS
IEC 61094-1), reported in terms of an open-circuit
voltage per unit of sound pressure applied uniformity to
the diaphragm, is obtained through a reciprocity method
with two microphones and an auxiliary sound source (an
active coupler). Microphone electrical terminals are
attached to a groung-shield configuration in according
with the IEC 61094-1 standard. Pressure calibration of
type LS microphones is performed within a range from 63
Hz to 2.5 kHz with an uncetrainty of approximately 0.04
dB; - lumped paremeters, representating acoustic impendance are obtained by an indirect method based on the measurement of the electrical admittance of the microphone acoustically terminated with a closed quarter-wavelength tube. Another method used for frequency and loss factor by exciting the diaphragm with an electrostatic actuator while terminating the diaphragm with the quater-wavelength tube; - a front cavity volume and the equivalent volume is evaluated using an acoustical method for comparison of a microphone under test with a number of cavities with known volumes; - calibration of working standard microphones is performed using a computer controlled comparison method which uses an active coupler with a laboratory standard microphone. This microphone, in turn, is calibrated by the reciprocity technique. The frequency response of a microphone is determinated by the electrostatic actuator method; - methods of verification and calibration of instruments meet technical standards the IEC 61094-2, PNU 181-2, PNU 1811-2 and PNU 1803-2 respectively. |
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| Ranges and uncertainties of calibrated and verified instruments. Click for details! | ||||||||||||||||
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Instruments in use | |||||||||||||||
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The instrument equipment consists of a National Standard of Sound pressure based on a computer controlled reciprocity method for determination of open circuit pressure sensitivity of condenser microphones. The device development at the SMU uses an active measuring coupler of nominal l volume 4cm3 and a special microphone preamplifier, anabling microphone function as receiver and transmitter without a mechanical change during the measurement. The open circuit sensitivity of standard microphones can be evaluated in a range of 63 Hz - 2500 Hz with an uncertainty less than 0.04 dB. | |||||||||||||||
| A set of equipment of Brüel & Kjaer completed with calibrated attenuators and digital voltmeters is used for verification of sound level meters, audiometers, filters and other electro-austic instruments testing. | ||||||||||||||||
| An instrument development at the SMU will be used for a determonation of acoustic impendance of microphones. | ||||||||||||||||
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