To date, the functional and physiological impact on the mature brain of moderate-level environmental noises that do not cause noticeable peripheral deficits remains largely unstudied. Here we show that exposing adult rats to structured noise at a sound pressure level of 65 dB, which is markedly below the broadly accepted safety level standard, results in behavioural impairments and substantially impairs the function of the auditory cortex. The strong deterioration in cortical processing of acoustic inputs is independent of the modulation rates of structured noises. Almost equally strong effects result from 10-h daily versus 24-h daily exposure regimens. These results indicate that there can be substantial negative consequences for the auditory system documented at the cortical level, attributable to environmental exposure to structured noises delivered under conditions that do not directly impact hearing sensitivity. These noises are deemed to be 'safe' and are often present in modern human environments.