DNV TR-107 PDF

This Technical Report describes an alternate background noise correction to the standard background noise correction in ECMA-74 and ISO 7779. The alternate background procedure may permit the reporting of lower noise emissions for quiet products than those reported using the standard background noise correction. The alternate background correction depends not only on mean background noise proximity to measured source levels, like the standard background noise corrections of ISO 3741, ISO 3744, ISO 3745, and ISO 11201, but also on the steadiness of the background noise. Background noise fluctuates over time because of uncontrollable acoustic affecting the acoustic environment and uncontrollable electronic and electromagnetic sources measurement instrumentation. Like the standard background noise corrections, the alternate background noise correction tends to increase as the background noise level approaches the measured source level, but the alternate background noise correction also increases with the steadiness of the background noise, unlike the standard background noise corrections.

For low level sources relative to the background, the alternate background noise correction provides background corrected source levels lower than those provided by the standard background noise correction, when background noise is within 6 dB of measured source level for engineering Grade 2 methods and within 10 dB of measured source level for precision Grade 1 methods, because of caps in the standard background noise correction. For the precision Grade 1 method with a steady background, background corrected source levels are approximately 2 dB lower when the measured source level is 3 dB above background level because of equal source and ambient contributions; for the engineering Grade 2 method they are about 1 dB lower. For source contributions below a steady background, the alternate background correction provides background corrected source levels more than 1 to 2 dB lower, depending on the Grade, and as much as 10 dB lower than those provided by the standard background correction.