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New testing procedures for software in sound level meters in legal metrology


Sound level meters serve the purpose of measuring sound levels and, thereby, in particular, to measure and check the observance of legally prescribed maximum or reference values (for example, in accordance with the "Technical Instructions on Noise Abatement" ["TA Lärm"]). Sound level meters can be found in private life (e.g. in home building), in environmental protection, and at work (in the form of worker protection). In order to ensure a correct and verifiable measuring process, all equipment used in official transactions must undergo a type examination that is carried out according to the Measures and Verification Act. Now, also a software test must be included in this type examination.

To harmonize acoustic metrology and the procedures that are required for a type examination, technical standards have been developed which lay down the requirements for measuring instruments (e.g. DIN EN 61672[2014], DIN 45657[2014]). To fulfill these requirements, a metrological evaluation of microphone signals is necessary which, however, can be realized today only by means of sophisticated software.

Since the new Measures and Verification Act and the Measures and Verification Ordinance took effect, in type examinations that are to be carried out by PTB's Working Group 1.63 on sound level meters, also WELMEC's requirements for software security (Software Guide 7.2, Version 5) have to be complied with. The aim of these requirements is to prevent users from manipulating sound level meters. Such manipulations can lead to intentional or unintentional changes of the measurement results during measurement or after the storing of the measurement data. Such changes might be, for example, changes in microphone calibration, changes to the internal clock, or changes to the saved measurement data. Also, the use of unapproved equipment software or the changing of metrology-relevant software by means of updates must be prevented.

In cooperation with Working Group 8.51 ("Metrological Software") at PTB, a range of requirements has been developed for sound level meters. These requirements are intended to prevent a user from influencing the measurement results. For example, updates of the operating software should only be carried out by authorized persons, and calibrations should only be made possible by the user in the maximum level range of ±1.5 dB and should be automatically documented in the internal data memory.

A complete list of requirements can be found on the website of Working Group 1.63 at:

The implementation of the required measures and compliance with the requirements will be checked during type examination.

Figure: Sound level meters as they are lined up for type examination.


Contact person:

Ingolf Bork, FB 1.6, AG 1.63 „Geräuschmesstechnik“, E-Mail: Opens window for sending emailingolf.bork(at)ptb.de