[. . . ] Ambient noise, such as engine noise on long flights, is a big contributor to stress and fatigue. [. . . ] NoiseGardTM against ambient noise NoiseGardTM active noise compensation is based on the principle of cancelling out sound in the low frequency range using counter-sound (inverse phase sound). The NoiseGardTM electronics then create a sound wave, which is phase modified by 180° compared to the noise - which results in the two sound waves almost completely cancelling one another out. ATTENTION: Avoid using headphones with the volume turned up for an extended period of time. If, by any chance, during NoiseGard operation, you can hear crackling or sizzling, or if dropouts occur, this can be due to the battery contact areas. [. . . ] If this happens, clean the battery contact areas by rubbing them on a rough surface and re-insert the batteries. [. . . ]