Single Number Rating is a rating system set up by the European Union (EU). Tests are conducted by independent testing laboratories with no direct participation by manufacturers. The independent testing laboratories meet all of the regulatory requirements as set out by the EU. The test results serve as a guideline to indicate the amount of potential protection a hearing protection device will give in a noisy environment. All Howard Leight hearing protection devices are tested according to the requirements established under EN 352.2 1993 (ear plugs) and EN 352.1 1993 (ear muffs). All tests are performed ant independent testing laboratories located at either B.I.A., Germany, or by Inspec Laboratories LTD, England at the University of Salford, Department of Acoustics.