## Noise-Induced Hearing Loss: Characteristic Audiogram Pattern **Key Point:** Noise-induced sensorineural hearing loss (NIHL) classically presents with a **notch at 4000 Hz** on the audiogram, with relative preservation of thresholds at 2000 Hz and 8000 Hz. This is called the **4 kHz notch** and is pathognomonic for NIHL. ### Frequency-Specific Patterns in Sensorineural Loss | Frequency Range | Pattern in NIHL | Clinical Significance | | --- | --- | --- | | 250–500 Hz | Preserved (normal) | Low frequencies spared | | 1000–2000 Hz | Relatively preserved | Speech frequencies initially spared | | **3000–6000 Hz (4 kHz peak)** | **Notch/maximum loss** | **Hallmark of NIHL** | | 8000 Hz | Partial recovery | Characteristic dip pattern | **High-Yield:** The 4 kHz notch is: - Most sensitive indicator of early NIHL - Occurs before speech frequencies are affected - Reversible in early stages (temporary threshold shift) - Becomes permanent with chronic noise exposure **Mnemonic:** **"4 kHz is the Culprit"** — The resonant frequency of the external auditory canal and middle ear amplification peaks around 2–4 kHz, making this region most vulnerable to noise trauma. **Clinical Pearl:** As NIHL progresses, the notch widens and deepens, eventually merging into a broad high-frequency sensorineural loss affecting speech intelligibility. Early detection at 4 kHz allows intervention before irreversible damage to speech frequencies (2000–3000 Hz) occurs. 
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