Low-frequency noise generated by engines, motors and fans is very difficult to attenuate because the noise waves are long, can travel great distances and can penetrate passive barriers, even cement walls. Active Noise Reduction (ANR) is the only effective method of attenuating low-frequency noise. Active Noise Cancelling Headphones This technology is also commonly termed Electronic Noise Cancellation (ENC), Active Noise Canceling (ANC) or “anti-noise”. ANR is achieved by electronically coupling a noise wave with its exact mirror image, thereby canceling the noise.