Noise-induced hearing loss is sensorineural damage from exposure to loud sound — occupational, recreational or both. It is one of the most common sensorineural losses, and almost entirely preventable, which makes its early audiometric recognition worthwhile.

The audiometric signature: the 4 kHz notch

The hallmark is a notch — a sensorineural dip centred around 4000 Hz (sometimes 3000 or 6000 Hz), with relatively better thresholds both below it and, importantly, above it at 8000 Hz.¹⁶ That recovery at 8 kHz is what distinguishes a noise notch from the steady downward slope of presbycusis.

Normal hearing

Noise-induced hearing loss

O Right ear, air
X Left ear, air
< Right ear, bone
> Left ear, bone

Signature: Sensorineural notch at 4 kHz with partial recovery at 8 kHz — the classic noise notch.

Figure 1. Noise-induced hearing loss versus normal hearing. A sensorineural notch at 4 kHz with partial recovery at 8 kHz — the classic noise notch. There is no air–bone gap.

Trainee The notch is usually bilateral and roughly symmetrical, because both ears share the exposure. With continued exposure the notch deepens and widens; the recovery at 8 kHz is eventually lost, and the audiogram comes to resemble a steep high-frequency loss that can be hard to distinguish from presbycusis on shape alone. The 4 kHz notch is most informative early, before that happens. The 4 kHz vulnerability is thought to reflect a combination of the ear canal’s resonance and the mechanics of cochlear damage.

Why the audiogram is not enough

A 4 kHz notch is suggestive of noise damage but it is not pathognomonic. A substantial minority of people with a notched audiogram have no history of significant noise exposure, and there is no single agreed numerical definition of an audiometric notch.¹⁶ The diagnosis rests on the configuration and a credible exposure history together, not on the shape of the curve alone.

ClinicianTwo practical points. First, otoacoustic emissions can reveal outer-hair-cell dysfunction before the pure-tone audiogram shows a notch, so a normal audiogram does not fully exclude early noise damage. Second, because noise-induced loss has clear medico-legal and occupational-health consequences, the four-frequency average that includes 4000 Hz is often the reported figure, and serial audiometry at fixed intervals is the basis of hearing conservation programmes. The audiogram’s value here is as much in monitoring as in diagnosis.

Special tests

Noise-induced loss is cochlear, the damage falling on the outer hair cells. Recruitment is therefore present and the SISI is typically positive in the notched region, while tone decay is not a feature. As with the other cochlear losses, the special tests confirm the site rather than make the diagnosis — the exposure history does that.