What type of noise-induced hearing loss (NIHL) is first observed at 4000 Hz?

The type of noise-induced hearing loss first observed at 4000 Hz is characterized as a notched audiogram configuration, which is why this choice is correct. Notched NIHL typically presents with a distinct dip in hearing sensitivity specifically at the 4000 Hz frequency, often recovering at frequencies above and below this notch. This pattern is associated with exposure to loud sounds, particularly hazardous or repetitive noise environments, where the 4000 Hz range tends to be particularly vulnerable due to the mechanics of how sound affects the cochlea and the hair cells within it.

In contrast, sloping NIHL generally shows a gradual decline in sensitivity at higher frequencies and may not specifically reveal a pronounced notch at 4000 Hz. Flat NIHL is characterized by a consistent level of hearing loss across all frequencies, meaning there is no specific frequency deterioration. Lastly, transient NIHL refers to temporary changes in hearing sensitivity that typically resolve after the exposure to noise has ceased and does not correspond to a fixed frequency such as 4000 Hz. Thus, the unique characteristics of notched NIHL make it distinct in its presentation at the 4000 Hz mark, reinforcing why this is the appropriate answer.