A noise-exposed worker's audiogram shows a characteristic notch at 4000 Hz with relatively better thresholds at 8000 Hz. This 4 kHz notch in noise-induced hearing loss (NIHL) occurs because sound at 1000–2000 Hz (dominant speech frequencies) preferentially damages hair cells at the:

• A Apical turn corresponding to 1 kHz, and resonance shifts damage to 4 kHz
• B Basal turn at 3–6 kHz, worsened by the resonant frequency of the outer ear canal amplifying around 3–4 kHz ✓
• C Middle turn corresponding to 2 kHz, and the notch is simply delayed presentation
• D Apex of cochlea corresponding to 4 kHz due to maximum fluid turbulence there

Explanation

The 4 kHz notch in NIHL occurs because: (1) the outer ear canal and pinna have a resonant frequency that amplifies incoming sound by 15–20 dB around 3–4 kHz, increasing the energy delivered to the basal turn hair cells at this frequency; (2) the basal turn hair cells (tuned to high frequencies, especially 3–6 kHz) are the most metabolically active and most vulnerable to noise trauma; and (3) the forward-traveling wave peaks slightly basal to the stimulating frequency, so 1–2 kHz noise causes peak damage at 3–4 kHz on the basilar membrane. This explains why 4 kHz is affected first and most severely in occupational NIHL.

Reference: Dhingra Diseases of Ear, Nose and Throat, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.