FEATURE CALIBRATION
Calibrating noise measurement equipment I
Calibration of noise measurement instruments is an essential part of their maintenance to ensure efficiency and compliance, as James Tingay of Cirrus Research explains
f you are using your noise measurement instruments to meet any standards, regulations
or guidelines, it is essential that your equipment is measuring accurately. Calibration is an essential part of undertaking noise measurements.
WHY IS CALIBRATION IMPORTANT? There are numerous examples of why the calibration of noise measurement instruments is important. For example, when the information collected is to be used for legal purposes (to show compliance with occupational noise standards) or where the measurement data is to be used to select hearing protection or noise control products. This demonstrates why it is important that
noise measurement instruments are calibrated regularly and you are confident that they are measuring accurately. A sound level meter should be calibrated
otherwise it can be very difficult to show traceability when the measurements are challenged now or in the future. If there is no evidence that the equipment has been calibrated before the measurements were made and that all of the equipment had been calibrated to a suitable standard, the measurements could be called into question.
WHAT IS CALIBRATION? When we are talking about the calibration of a sound level meter there are effectively two types. The first, field calibration, is what you would carry out before (or possibly after) each measurement using a reference acoustic source such as an acoustic calibrator. The second is what we will refer to as
recalibration or, more correctly, periodic verification. This is the process that your instrument will undergo when it is returned to a calibration laboratory.
WHAT IS FIELD CALIBRATION? Each time your instrument is used it should be calibrated using an acoustic calibrator before and after each measurement to check that it is working correctly. This is field calibration. A sound level meter or noise dosimeter is a
precision instrument and so should be treated with care. Any damage may not be immediately obvious and so using an acoustic calibrator before you make any measurements helps to check that the instrument will measure correctly. However, using an acoustic calibrator only checks one level and one frequency. Therefore, regular calibration (or periodic verification) is essential to ensure that all of the features and functions of your instrument are working as intended by the manufacturer. Field calibration and periodic verification are very different and provide different information.
18 MARCH 2018 | INSTRUMENTATION
If you are sending your equipment to a calibration laboratory, always check that they are capable of providing the level of calibration that you need. Modern sound level meters are sophisticated instruments and so require a level of understanding from a calibration lab that is often not found in labs that are not noise specialists. Some noise measurement instruments require
FIELD CALIBRATION A field calibration is a simple check of the instrument that checks it against a known level (usually 94dB or 114dB) at a known frequency (usually 1kHz) using an acoustic calibrator. The limitations of a field calibration are that it is
only checking the performance of your instrument at a single point, whereas when you are undertaking a real noise measurement, your instrument will be recording a wide range of frequencies and levels which are combined to provide the noise parameters that you need. Your sound level meter will often be measuring a number of acoustic parameters such as LAeq and LCPeak (and possibly many more such as octave bands and Ln values if you are measuring environmental noise). A field calibration does not check all of these functions and it does not check the performance of the the most critical component on your sound level meter, the microphone, at a range of frequencies. Therefore, a field calibration is simply a way of
confirming that the instrument is functioning at that specific reference point. Typically, the only information recorded is the time, date, level and any offset from the previous calibration.
PERIODIC VERIFICATION OR RECALIBRATION Periodic verification is very different to field calibration as it requires a much more extensive range of tests and performance checks to be made and is usually only carried out in a calibration laboratory that has the correct equipment and expertise. Please be aware of low cost calibration that
some companies provide that may only be carrying out the same checks that you would do during a field calibration. This is not sufficient to meet the demands of standards such as the Control of Noise at Work Regulations and would not be acceptable evidence of calibration if your measurements were ever challenged. Always check what you are getting when you are
paying for the calibration of your equipment and ensure that it meets the requirements of any standards or regulations that you are working to.
specialist tools or software and these are often not found outside of the manufacturer or a few, specialised calibration laboratories. The periodic verification (or recalibration) of a sound level meter should be done, where possible, by removing the microphone capsule and testing the instrument electrically. The microphone capsule should be tested using
either a multi-frequency acoustic calibrator or an electrostatic microphone calibration system. When both the sound level meter and the
microphone capsule have been tested and verified, they should be recombined and the final calibration carried out. This may sound like a long process but it is the only way to confirm that the entire instrument is working as it was when it was new. To give an example of what happens at Cirrus,
when an Optimus sound level meter comes into the calibration lab, the microphone capsule is removed and checked using the company’s reference electrostatic calibration system. This tests not only the frequency response of the microphone over a wide range, it also tests a range of different levels and the absolute sensitivity of the microphone (using an external reference source). Only if the microphone capsule passes all of these tests is it deemed acceptable to be used with the sound level meter. The whole instrument is then passed into the
calibration area where it is subject to a battery of tests to check that it still meets all of the performance points that it did when it was new. These tests check many aspects of the instrument such as the accuracy of frequency and time weightings as well as the linearity over a wide range of levels. This checks if the instrument measures with the same level of accuracy at low levels as it does at high levels, something that is essential if you are using your instrument for both environmental and occupational noise measurements. Only when all of these tests are completed do
Cirrus consider the instrument to be calibrated and fit to go back into use, accompanied with a certificate of calibration. This may seem a lot of information to consider
but it is vital that your noise measurement instruments are measuring correctly and that you are getting reliable, accurate results.
Cirrus Research
www.cirrusresearch.co.uk
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