To perform professional acoustic measurements, you need kit which meets a high standard of performance, after all, legal metrology has consequences. For example, if a noise limit is imposed on a construction site, you need to know with a demonstrable level of confidence, that your measurement is correct.
In the sound level meter world, the performance standard is IEC 61672:2013. The earlier standards are IEC 6651:1994 & IEC 60804:2001 which are now withdrawn. All three standards have many similar elements, such as frequency response, weightings, detectors, linearity, etc but there the similarity ends. The classification and tolerances of accuracy are completely different.
IEC 61672 refers to two ‘classes’ of accuracy, Class 1 and Class 2. IEC 60651/804 refer to four classes of accuracy, Type 0 to Type 3.
There is no equivalence between the two classes. Class 1 is not the same as Type 1.
Many meters out in the field were built to the old standards, and therefore older measurement standards still allow their use. However, new and current measurement standards (such as BS 5228:2019 for construction sites) will require meters built to the new standard, or the use of older meters with greater uncertainties.
IEC 61672 consist of three parts: Specification, Pattern Evaluation and Periodic Testing.
Part 1 covers all the electrical and acoustical performance, but also include EMC testing, environmental effects to the nth degree of detail. If a meter is developed to this standard, it must even include a display and a detailed manual!
Part 2 covers pattern evaluation, which means sending the instrument(s) to an independent laboratory for checking the specification. In most mainland European countries, this is compulsory, and it is illegal to sell an instrument if the pattern evaluation is not done. This is similar to the car industry, where each model of vehicle is pattern evaluated before it can go on the road. OK, you can’t crash a sound level meter, but you get the drift.
Part 3 covers the periodic checking of the instrument in a local laboratory, and ongoing calibration. It’s quite a stiff process but covers the basic electronic and acoustical parameters. However, calibration to Part 3 of the standard is impossible if any required information is not available in the instruction manual. With the introduction of this part of the standard, the size of instruction manuals doubled overnight!
Note that this is not the same as UKAS calibration – this simply means that the measurements are done by a laboratory accredited by UKAS to do the Part 3 measurements and certification.
So, what about Type Approval in the UK? Well, in this country, we take the caveat emptor approach, i.e. buyer beware, so it is up to the purchaser to satisfy themselves that the claimed performance of the instrument is as it should be.
However, there is no doubt that if an instrument is Type Approved in Europe, then you can be assured it meets the requirements of the standard. If not, then you’re on your own.
By the way, some say that one country’s type approval is better than others, e.g. PTB approval in Germany is better than LNE approval in France. Not so – there is constant dialog between measurement authorities to ensure standards are equivalent, and in fact, Part 2 of IEC 61672 closes down any possible variation.
One key feature of the current standard is that you can’t cherry-pick the specifications. The instrument either meets the standard or it doesn’t. In the past, we see claims on websites that “the instrument meets relevant parts of IEC 61672 or IEC804”, “the microphone is a Class 1 microphone” or “the frequency response meets IEC61672 Type 1 (sic)”
There is no such thing as “relevant parts”. The standard is always taken in its entirety and cannot be quoted otherwise.
There is no such thing as a Class 1 microphone.
There is no such thing as IEC 61672 Type 1.
Finally, the instrument must be paired with a single or published list of sound level calibrators. The type of calibrator will be specified in the manual, and if that type of calibrator is not available, then you are not meeting IEC61672.
This all might sound complicated, and if you want to dig down into the detail, then download a copy of “A Guide to Sound Level Meters”, published by the IOA in 2019, or request a copy.
However, if you have a day job, then follow these rules of thumb:
- Always use an instrument which meets Class 1 of IEC 61672:2013. Use a Type 1 instrument to IEC 60651/60804 if you can’t find anything else but be aware of increased uncertainty.
- Make sure the information required by IEC 61672 is provided by the manufacturer in the manual and data sheet. In writing. If not, you will never be able to perform periodic calibration at a minimum, or UKAS calibration if required.
- Steer towards instruments which have been Type Approved in other European countries or at least a manufacturer who has a track record of type approving their instruments. Remember PTB approval is good, but it’s no better than any other European type approval.
- If the instrument has not been type approved anywhere, because it’s too new (type approval takes time) for example, steer towards a manufacturer who has had other instruments type approved.
Remember, if your noise limit is 65dBA, and you’re measuring 64.5dBA, and you don’t know your uncertainties, a smart barrister will easily show you’ve exceeded the limit, with consequent penalties, unless you can show you’ve followed the standards!
AcSoft is a multidisciplinary company offering solutions to large multinational businesses and small consultancies alike, along with applications advice, to ensure maximum return on investment. Contact us or call 01234 639550 to find out more about the class 1 sound level meters we can supply.