The international standard ISO 1999-1990 indicates the noise-induced hearing loss (NIHL) to be expected at different audiometric frequencies as a function of noise exposure. It is based on statistical data of hearing loss due to exposure to broadband noise. Whereas industrial noise is most often of a broadband type, this is not the case for sounds produced by electro-acoustic systems and devices, such as warning signals or a howl-around of a sound reinforcement system. From the point of view of physiology, a concentration of the sound energy into one frequency band is equivalent to an increased load on small part of the basilar membrane. Compared to an exposure to a broadband signal of equal energy, this may increase the risk of hearing damage. Several cases of hearing loss or tinnitus (ringing) due to such narrow-band signals have been filed by Suva. For the assessment of such compensation cases, a tentative correction KNB (NB = narrow-band) based on the frequency spectrum in one-third octave bands has been developed. The main parameter is the number of frequency bands carrying 75 % of the total A-weighted sound energy.