Investigation of the 4,000-Hertz dip by detailed audiometry

It is well known that a 4,000-Hz dip is often observed in the initial stage of noise-induced hearing loss and acoustic trauma, but it does not always occur as a result of these causes, and quite often the cause is unknown. Pure tone audiograms of 159 patients (76 cases of noise-induced hearing loss, 62 cases of hearing loss of unknown cause, 21 cases of familial hearing loss) with a 4,000-Hz dip were studied by detailed audiometry to clarify this configuration. The types of dip were classified into 11 groups; however, the basic types are the cone type, bowl type, and dish type, which show the smooth line of the hearing threshold. There were no clear differences in dip configuration among the 3 causes of hearing loss. It was thought that the dips displayed similar configurations regardless of the cause.