These are our Electroacoustics Calibration Systems
Find here general information regarding the calibration services provided by Metroacústica. All technical requirements that are verified in the calibration services by Metroacústica System follow the recommendations of guideline document "DOQ-CGCRE-052: guidance for calibration of acoustic measurement devices", of the General Coordination of Accreditation of INMETRO (CGCRE), which is the Body for the accreditation of conformity assessment bodies recognized by the Brazilian Government. This document presents the minimum characteristics that should be calibrated by a Brazilian laboratory that performs the conformity assessment (calibration) of noise measurement equipment, such as sound level meters, noise dosimeters, sound exposure meters, sound calibrators and microphones.
Calibration of Old Sound Level Meters
The conformity assessment of the sound level meter, also known as "sonometers", is performed according to its construction standard. For the calibration of sound level meters manufactured prior to the publication of IEC 61672-1 (2002) - "Sound Level Meters - Specifications", IEC 60651 (1979) and IEC 60804 (1985) are used. It is important to note that the tests of the ANSI S1.4 standard for sound level meters are practically the same as the tests of the IEC standards cited above. Therefore a meter that conforms to IEC specifications, by similarity will also conform to the specifications of the ANSI S1.4 standard.
Sound Level Meters are equipment manufactured in accordance with internationally applicable technical standards, such as International Electrotechnical Commissions (IEC) standards, or national standards such as those of the American National Standards Institute (ANSI - United States), British Standards Institute (BSI - United Kingdom). And the Association Française de Normalization (AFNOR - France). The Brazilian Association of Technical Standards (ABNT) does not yet have standards related to sound level meters, so in Brazil the IEC standards are used.
IEC 60651 and IEC 60804 standards were widely used during the 1980s and 1990s for the production of noise meters. As the life span of good metrological quality equipment can be a few decades, still today there are in perfect condition many sound meters that were built according to these two technical recommendations.
As the calibration of these equipments must be carried out according to their respective construction standard, Metroacústica Calibration Systems also perform the calibration of the sound level meters according to international standards:
IEC 60651 (1979) - Sound Level Meters
IEC 60804 (1985) - Sound Level Integrators.
In 2002 these standards became obsolete for the production of equipment with the publication of the first of a series of three standards IEC 61672. The production of sound level meters from 2002 should be carried out in accordance with the requirements established by this Standards (IEC 61672-1), model approval (IEC 61672-2) and periodic verification (IEC 61672-3) of sound level meters, and for this reason the production of sound meters shall be based on the specifications of this standard and no longer in the specifications of IEC 60651.
For this reason the Metroacústica Systems performs calibration of old sound level meters through the following standard tests:
- Level Linearity
- Frequency Weighting Curves (A, C and LIN)
- Time weighting (Fast, Slow and Impulse)
- RMS Detector
- Equivalent Sound Level (LAeq)
Calibration of New Sound Level Meters
The conformity assessment of the sound level meter, also known as "sonometers", is performed according to its construction standard. For the calibration of sound level meters produced after the publication of IEC 61672-1 (2002) - "Sound Level Meters - Specifications", the two editions of the IEC 61672-3 standard are used (1st Ed. and 2nd Ed 2013 ).
As the calibration of these equipments must be carried out according to their respective construction standard, the Metroacústica Calibration Systems also performs the calibration of the sound level meters according to the two editions of the IEC 61672-3 standard.
Therefore, Metroacústica Systems performs the calibration of new sound level meters through the following standard tests:
- Self-generated acoustic noise
- Linearity level in the reference range
- Linearity Level including range control
- Tone pulses response (Fast, Slow, SEL and LAeq)
- Frequency weighting (A, C and Z)
- Frequency and time weightings at 1 kHz
- Overload indication
- C-weighted peak level
- Long-term stability (2nd Dec 2013)
- Stability at high level (2nd Dec 2013)
- Acoustic test with the microphone attached
Calibration of Noise Dosimeters
Noise dosimeters are produced mainly according to technical standards of the American National Standards Institute (ANSI - United States). The Brazilian Association of Technical Standards (ABNT) does not have yet standards regarding noise dosimeters.
Metroacústica's compliance assessment in the noise dosimeter is performed in accordance with ANSI S1.25 and ANSI S1.4 standards. It is important to note that the dosimeter is calibrated according to class 2 specifications of ANSI S1.4 in the tests for the parts of the meter. These tests of the ANSI S1.4 standard are practically the same as the tests of the IEC 60651 and IEC 60804 standards. Thus, a dosimeter that conforms to the ANSI standard specifications, by similarity will also comply with IEC standards For class 2 sound level meters.
IEC is an international normative organization that defines constructive characteristics and related tests for measuring equipment of various physical quantities. In particular, IEC 60651 deals with sound level meters and defines various constructive requirements for the manufacture of these equipment. IEC 61252 defines the constructive requirements for the production of sound exposure meters, which can also be used to perform noise dose calibration.
In the United States ANSI standards define the constructive characteristics of equipment for noise measurement. ANSI S1.4 defines the constructive requirements of sound level meters and is equivalent to IEC 60651 in relation to specifications. ANSI S1.25 defines the characteristics of noise dosimeters for the measurement of noise exposure. These are equipment with objectives similar to sound exposure meters (IEC 61252). However, noise dosimeters, as the term itself suggests, measure "DOSE", rather than measuring sound exposure. The noise dose expressed as a percentage has become a very common parameter for the evaluation of occupational noise in the United States.
In Brazil, the characterization of an unhealthy activity in relation to the noise level during a working day is carried out by regulation NR-15: Unhealthy activities and operations. In accordance with this standard, it is possible for companies and trade unions of the professional categories concerned to request the Ministry of Labor to carry out an expertise in an establishment or sector thereof, in order to characterize and classify or determine unhealthy activity. In the skills required of the Regional Labor Stations, provided that the insalubrity is proven, the expert of the Ministry of Labor shall describe in the report the technique and equipment used.
Annex 1 of NR-15 defines the tolerance limits for continuous or intermittent noise. Continuous or intermittent noise, for the purposes of applying tolerance limits, is understood to mean noise other than impact noise.
According to NR-15, continuous or intermittent noise levels shall be measured in decibels (dB) with a sound pressure level instrument operating in "A" compensation circuit and slow response circuit (SLOW). Readings should be made close to the worker's ear. The exposure times for noise levels shall not exceed the tolerance limits stipulated in Annex 1 of NR-15.
The NHO-01 - "Occupational exposure to noise assessment" establishes criteria and procedures for the occupational noise assessment that implies occupational risk. This standard establishes the exchange rate of 3 dB for doubling the noise dose.
The noise dosimeters constructed in accordance with the ANSI S1.25 standard are widely used equipment to evaluate worker exposure to noise according to the criteria that are defined by both NR-15 and NHO-01.
In order for the dosimeter to be used effectively, some adjustments to the device settings must be made:
- The Metering Criterion must be set to 8 hours;
- The Dosimeter Level Criterion should be set to 85 dB;
- The dosimeter exchange rate should be adjusted to 5 dB for evaluation according to NR-15 standard or 3 dB for assessment of occupational noise according to NHO-01.
A dosimeter that is measuring the noise with these previously determined settings will be evaluating the exposure of the worker to noise, but as a measurement parameter "DOSE".
Metroacústica calibration systems were developed based on the ANSI technical standards, since they are normative documents accepted and used in several countries of the world, including Brazil. As the calibration of these equipment must be carried out according to their respective construction standard, Metroacústica systems perform the calibration of noise dosimeters according to the norms:
ANSI S1.25: 1991 - "Specification for pesonal noise dosemeters"
ANSI S1.4: 1985 - "Specification for sound level meters".
Calibration of Sound Exposure Meters
Noise dose meters defined by the ANSI S1.25 standard are often designed to indicate the noise dose as a percentage value of a legal limit. The limit and its setting vary from country to country and are subject to change. In order to make easier international comparison of sound exposure records with numerical values of convenient magnitude, IEC 61252 specifies an instrument that indicates sound exposure in Pascal-Square Hours (Pa2h).
This standard also allows for an indication of sound exposure with a unit other than pascal-square hours when the manufacturer specifies a procedure to convert the indication into pascal-square hours, for example in a "dose" indication as a fraction or a Percentage of a sound exposure specified in pascal-square hours.
The main application for an individual sound exposure meter is the measurement of the sound exposure in the proximity of a person's head; That is, to assess the potential for hearing loss according to standards such as ISO 1999. The microphone of an individual sound exposure meter can be used on the shoulder, neck or other location near one ear.
Metroacústica calibration systems for sound exposure meters were developed based on the IEC technical standards, since they are normative documents accepted and used in several countries of the world, including Brazil. Most of the requirements defined by the standard are only checked at the model approval stage, but some of them must also be checked periodically.
According to the DOQ-CGCRE-052 the Metroacústica systems perform the generation of the normalized signals for the following tests:
- Linearity response to stationary signals;
- Frequency weighting;
- Response to short-term signals;
- Response to unipolar pulses.
Calibration of Sound Calibrators
The calibration system of sound calibrators verifies if constructive characteristics of the equipment meet the established requirements. To perform the periodic verification, which can also be called "calibration", Metroacústica has developed an exclusive system to accomplish this purpose with accuracy and efficiency.
Calibration systems are based on the use of generation / acquisition cards (PXI and USB) from the National Instruments manufacturer . Proprietary software was developed in LabVIEW language and for programs to run correctly it is necessary to use a Microsoft Excel file, which is a spreadsheet template. In this worksheet, before starting the calibration, some characteristics of the equipment to be calibrated must be selected, such as its generated levels with the respective frequencies, the edition of the standard and the accuracy class of the calibrator to be calibrated.
The International Electrotechnical Commissions (IEC) is an international normative organization that defines constructive characteristics and related tests for measuring equipment of various physical magnitudes. Particularly the standard IEC 60942 deals with sound calibrators and defines several constructive requirements for the manufacture of these equipments. The calibration systems were developed based on the IEC technical standards, since they are normative documents accepted and used in several countries of the world, including Brazil. Most of the requirements defined by the standard are only checked at the model approval stage, but some of them must also be checked periodically.
According to the DOQ-CGCRE-052 Metroacústica systems measure the standard signals for the following tests:
1. Generated Sound Level
2. Signal Frequency
3. Harmonic or Total Distortion
Calibration of Measurement Microphones
The calibration of measuring microphones is being developed by Metroacústica. The electrostatic actuator method described in IEC 61094-6 will be used to determine the frequency response of the microphone.
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