If you are using your noise measurement instruments to meet any Standards, Regulations or Guidelines, it is essential that your equipment is measuring accurately. Most modern standards, regulations of guidelines will clearly state the standards to which the equipment must comply.
For example, ISO 11202:2010 states:
“The instrumentation system, including the microphones, cables, and windscreen if used, shall meet the requirements of IEC 61672-1:2002, class 1, and the filters, if relevant, shall meet the requirements of IEC 61260:1995, class 1.”
and similarly, BS 4142:2014 states:
“Select systems for measuring sound pressure levels, including microphone(s), cable(s), windscreen(s), recording devices and other accessories which conform to BS EN 61672-1, Class 1, for free-field application, as appropriate. Filters, where used, should conform to BS EN 61260, Class 1, and sound calibrators to BS EN 60942, Class 1.”
If you are going to continue to use your noise measurement instruments over a long period, it is vital that they continue to meet these same standards and this is why calibration, both Field Calibration and Periodic Verification, are an essential part of undertaking noise measurements.
Why is Calibration Important?
Examples of why the calibration of noise measurement instruments is important include:
This is not an exhaustive list but 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 (or any noise measurement instrument) should be calibrated otherwise it can be very difficult to show traceability when the measurements are challenged now or in the future.
If I was looking at your noise measurements, the first thing I would ask (even before looking at the measured levels) would be “Did you calibrate the equipment before and after the measurement?” and then “When were the sound level meter AND the acoustic calibrator last calibrated by a suitable calibration laboratory?”
If there’s 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 noise measurement instrument, a sound level meter in this instance, there are effectively two types.
The first, field calibration, is what you would carry out before (as 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 (when you send your equipment to Cirrus for example).
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 as correctly. This is field calibration.
We also often get asked why an instrument needs to be calibrated every time it is used when you have a certificate of 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.
What is the Difference between Field Calibration and Periodic Verification?
These two things are very different and provide you with very different information.
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, nothing more.
Periodic Verification or Recalibration
This 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.
Note: 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 or subject to scrutiny.
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.
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.
What do we mean by this? 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 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 (a very expensive item) or an electrostatic microphone calibration system (also rather expensive!).
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 and it is but it’s 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 our calibration lab, the microphone capsule is removed and checked using our 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 (essentially the same tests that we do when we made the instrument) 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 (and there are lots of them) do we 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 measurements instruments are measuring correctly and that you are getting reliable, accurate results.
If you have any questions about the calibration of your equipment, please contact us and we will be pleased to help.
