Metroacúsica USB System
The USB System is still under development
Metroacústica USB system will use software developed in LabVIEW language that will work in conjunction with the hardware for generation and acquisition of signals, consisting of a board designed specifically for sound and vibration applications.
The USB system offers proprietary software for calibration of the following equipment:
- old sound level meters - software "IEC60651usb.exe"
- new sound level meters - software "IEC61672usb.exe"
- sound exposure meters - software "IEC61252usb.exe"
- noise dosimeters - software "ANSI_S125usb.exe"
- sound level calibrators - software "IEC60942usb.exe"
- measuring microphones - software "IEC61094usb.exe"
The development of the USB Calibration System was selected by FAPERJ. In this project will be implemented the optical character recognition technique where the reading of the indication on the display of the equipment will be automatically performed by a webcam.
The NI USB-4431 is a five-channel dynamic signal acquisition module formaking high-accuracy measurements from IEPE sensors. The USB-4431delivers 100 dB of dynamic range and incorporates software-selectable IEPE (2.1 mA constant current) signal conditioning for accelerometers and microphones. The module consists of four analog input channels for reading from IEPE sensors with a single analog output. The four analog input channels simultaneously acquire at rates from 2 to 102.4 kS/s. In addition, each channel includes built-in anti-aliasing filters that automatically adjust to your sampling rate. The USB-4431 is ideal for a wide variety of portable test applications such calibration of noise dosimeters, sound exposure meters and sound calibrators.
The maximum amplitude generated by the USB 4431 device may not be high enough to perform the calibration of the higher levels for linearity level test of some sound level meter models. The performance of the overload indication, high level stability and C peak level (IEC 61672-3) tests, where the reference level for the test depends on the upper limit of linearity of the sound level meter, may be compromised .
Considering for example two microphones with different sensitivities:
A) microphone with a sensitivity of 50mV / Pa - the USB 4431 card can generate signals up to approximately 127 dB.
B) microphone with sensitivity of 12.5mV / Pa - the 4431 USB card can generate signals up to approximately 140 dB.
Despite its limitations, it is also possible to calibrate several models of sound level meters with this device.
The guideline document DOQ-CGCRE-052 of the General Coordination of Accreditation (Cgcre) of the National Institute of Metrology (INMETRO), provides a list with the recommended minimum requirements to be checked when calibrating some electroacoustic equipment in Brazil.
Software "IEC60651usb.exe" generates the standard signals for the following calibration tests of old sound level meters:
1. Linearity Level
2. Frequency Weighting (A, C and LIN)
3. Time Weighting (Fast, Slow and Impulse)
4. RMS Detector
5. Time Average (LAeq) - IEC 60804 test
The software "IEC61672usb.exe" performs generation / measurement of the standard signals for the following calibration tests of new sound level meters:
1. Linearity of level in the reference range
2. Linearity Level including range control
3. Tonal pulse Response (Fast, Slow, SEL and LAeq)
4. Frequency weighting (A, C and Z)
5. Frequency and time weightings at 1 kHz
6. Overload indication
7. C-weighted peak level
8. Long-term stability
9. Stability at high level
10. Acoustic test with the microphone attached
Software "IEC61252usb.exe" generates the normalized signals for the following calibration tests of sound exposure meters:
1. Linearity Level
2. A Frequency weighting
3. Response to short-duration signals
4. Response to unipolar pulses
Software "ANSI_S125usb.exe" generates the normalized signals for the following tests of the calibration of noise dosimeters:
1. Linearity Level
2. Frequency weighting (A, C and Lin)
3. RMS Detector - ANSI S1.4 Test
4. Time weighting (Fast and Slow) - ANSI S1.4 test
5. Time Average (LAvg)
6. DOSE (%) - Stationary Signal
7. DOSE (%) - Pulse Signal
Software "IEC60942usb.exe" performs the measurement of the standard signals for the following calibration tests of sound level calibrators:
1. Level
2. Frequency
3. Harmonic or Total Distortion
Software "IEC61094usb.exe" performs the measurement of the standard signals for the following calibration tests of measuring microphones:
1. Frequency response
2. Sensitivity
For the elaboration of the System Metroacústica USB the following documents were used as reference:
4431 USB Card Manual
NBR ISO / IEC 17025: 2005 - General Requirements for Competence of Testing and Calibration Laboratories.
IEC 60651: 1979 - "Electroacoustics - Sound Level Meters".
IEC 60804: 1985 - "Electroacoustics - Integrating-averaging Sound Level Meters".
IEC 60942: 2003 - "Sound Calibrators".
IEC 61672-1: 2013 - Sound level meters - Specifications
IEC 61672-3: 2013 - Sound level meters - Periodic tests
IEC 61252: 1993 - Sound Exposure Meters
ANSI S1.25: 1991 - "Specification for personal noise dosemeters".
ANSI S1.4: 1983 - "Specification for sound level meters".
Guide for the expression of measurement uncertainty - GUM, 2003, Third edition in Portuguese language.
VIM International Vocabulary of Metrology - Fundamental and General Concepts and Associated Terms, 1st edition Luso-Brazilian, 2012
DOQ-CGCRE-052 - Guidance for calibration of acoustic measurement devices.