Ans. is 'd' i.e., 4000 Hz Noise induced hearing loss Exposure to loud noise can lead to permanent hearing threshold shifts. This may happen immediately with extreme exposure (nearby explosion or gunfire), but more commonly occurs slowly over time with repeated exposure to industrial or environmental noise. Patients will often have a typical 4-KHz (4000 Hz) notch (dip at 4000 Hz) in their audiogram called acoustic dip. Auditory effects of noice are :? Auditory fatigue :- at 90 d B and 4000 Hz. Hearing loss Temporary :- At 4000 - 6000 Hz Permanent :- Repeated prolonged exposure to 100 d B or single exposure to 160 d B.